* Vollum Institute, Oregon Health Sciences University, Portland, Oregon 97201; and Department of Pharmacology,
The University of Texas Southwestern Medical Center, Dallas, Texas 75235
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Abstract |
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The regulated sorting of proteins within the trans-Golgi network (TGN)/endosomal system is a key determinant of their biological activity in vivo. For example, the endoprotease furin activates of a wide range of proproteins in multiple compartments within the TGN/endosomal system. Phosphorylation of its cytosolic domain by casein kinase II (CKII) promotes the localization of furin to the TGN and early endosomes whereas dephosphorylation is required for efficient transport between these compartments (Jones, B.G., L. Thomas, S.S. Molloy, C.D. Thulin, M.D. Fry, K.A. Walsh, and G. Thomas. 1995. EMBO [Eur. Mol. Biol. Organ.] J. 14:5869-5883). Here we show that phosphorylated furin molecules internalized from the cell surface are retained in a local cycling loop between early endosomes and the plasma membrane. This cycling loop requires the phosphorylation state-dependent furin-sorting protein PACS-1, and mirrors the trafficking pathway described recently for the TGN localization of furin (Wan, L., S.S. Molloy, L. Thomas, G. Liu, Y. Xiang, S.L. Ryback, and G. Thomas. 1998. Cell. 94:205-216). We also demonstrate a novel role for protein phosphatase 2A (PP2A) in regulating protein localization in the TGN/endosomal system. Using baculovirus recombinants expressing individual PP2A subunits, we show that the dephosphorylation of furin in vitro requires heterotrimeric phosphatase containing B family regulatory subunits. The importance of this PP2A isoform in directing the routing of furin from early endosomes to the TGN was established using SV-40 small t antigen as a diagnostic tool in vivo. The role of both CKII and PP2A in controlling multiple sorting steps in the TGN/endosomal system indicates that the distribution of itinerant membrane proteins may be acutely regulated via signal transduction pathways.
Key words: furin; endosome; PP2A; sorting; PACS-1 ![]() |
Introduction |
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TWO compelling and fundamental questions in cell
biology are to identify the mechanisms by which
proteins are routed to their correct intracellular
compartments and how these sorting steps are regulated.
The broad importance and complexity of the trans-Golgi network (TGN)/endosomal sorting system (for review see
Robinson et al., 1996) has been highlighted by recent studies of such diverse processes as delivery of vacuolar proteins in yeast (Cowles et al., 1997a
,b; Piper et al., 1997
;
Voos and Stevens, 1998
), transcytosis in polarized cells
(Apodaca et al., 1996
; Odorizzi et al., 1996
; Zacchi et al.,
1998
), the mobilization of major histocompatability complex class 2 and Glut-4-containing endosomes (for review
see Keller and Simons, 1997
), and synaptic vesicle biogenesis (Cameron et al., 1991
; West et al., 1997
; Partoens et
al., 1998
). Despite their central role to the physiology of
cells, the regulation of these complex trafficking systems
remain largely undetermined.
One factor that contributes to the dynamic capacity of
the TGN/endosomal sorting system is regulation by phosphorylation. The link between protein trafficking and
phosphorylation provides a means by which cells can rapidly and reversibly alter the distribution and function of a
variety of transmembrane proteins. Kinase and phosphatase activities have been shown to control both general and cargo-specific trafficking. Phospholipid kinases and
phosphatases (for reviews see Stack et al., 1995; De Camilli et al., 1996
; Shepherd et al., 1996
; Woscholski and
Parker, 1997
) modulate membrane dynamics throughout
the TGN/endosomal system, including budding from the
TGN (Simon et al., 1996
; Chen et al., 1997
; Jones et al.,
1998
) and recycling from endosomal compartments (Cardone and Mostov, 1995
; Spiro et al., 1996
; Chung et al.,
1997
; Luo and Chang, 1997
; Malide and Cushman, 1997
).
Although protein kinase activities have long been recognized as important modulators of receptor transduction
complexes at the cell surface and in signaling endosomes
(Bevan et al., 1995
; Wang et al., 1996
; Grimes et al., 1997
),
there is growing evidence that protein kinases and phosphatases also control the sorting of itinerant membrane
proteins (for review see Seaman et al., 1996
). Regulation
of protein traffic by phosphorylation can occur via both
general e.g., modification of adaptin binding to clathrin
(Wilde and Brodsky, 1996
), and TGN export (Ohashi and
Huttner, 1994
; Austin and Shields, 1996
) and specific,
cargo-directed mechanisms. Examples of the latter include
the transcytosis of the polymeric immunoglobulin receptor
(Apodaca and Mostov, 1993
; Okamoto et al., 1994
), internalization of T cell receptors (CTLA-4 [Bradshaw et al.,
1997
] and CD4 [Pelchen-Matthews et al., 1993
]), as well as
the TGN localization of the endoprotease furin (Jones et
al., 1995
; Takahashi et al., 1995
; Dittié et al., 1997
; Wan et
al., 1998
).
Whereas a few of the itinerant membrane protein-directed
kinases involved in regulation of protein trafficking have
been identified (e.g., CKII, Jones et al., 1995), both the
phosphatases and the machinery responsible for the differential sorting of phosphoproteins remain largely uncharacterized. Furin is an excellent model for defining the cellular
machinery involved in phosphorylation state-dependent protein sorting within the TGN/endosomal system. The
endoprotease is routed through multiple proprotein processing compartments by virtue of defined trafficking signals within its cytosolic domain (cd)1 (Jones et al., 1995
;
Schäfer et al., 1995
). Although the steady-state localization of furin is predominantly in the TGN (Bosshart et al.,
1994
; Molloy et al., 1994
; Schäfer et al., 1995
; Shapiro et
al., 1997
), the protease cycles between this compartment
and the cell surface via an endosomal pathway (Molloy et
al., 1994
; Jones et al., 1995
; Liu et al., 1997
).
Internalization of furin from the cell surface and export
from the TGN are directed by canonical tyrosine and/or
dileucine based clathrin-coated pit recruitment motifs interacting with the clathrin sorting machinery (Ohno et al.,
1996; Schäfer et al., 1995
; Wan et al., 1998
). Localization of
furin to the TGN, however, requires a cluster of acidic residues (AC)1 that constitute a CKII phosphorylation site
(Bosshart et al., 1994
; Jones et al., 1995
; Schäfer et al.,
1995
). Phosphorylation of this AC motif by CKII regulates
the TGN localization of the protease by promoting its retrieval from immediate post-TGN compartments (Wan et
al., 1998
). The phosphofurin acidic cluster-sorting protein (PACS-1) directs this TGN retrieval step by linking the
phosphorylated furin-cd to the clathrin sorting machinery
(Wan et al., 1998
).
Despite their importance for establishing processing
compartments within endosomes and at the cell surface
(Liu et al., 1997), the factors which control sorting of furin
in peripheral compartments have not been well characterized. Phosphorylation of the furin-cd has, however, been
implicated in the trafficking of the protease in the endosomal system (Jones et al., 1995
). This observation indicates
dual roles for both CKII and AC motifs in protein sorting. Furthermore, the ability of a phosphatase inhibitor (tautomycin) to alter the routing of internalized furin suggested
that dephosphorylation is a critical determinant of furin
sorting in early endosomes (Jones et al., 1995
). The identity of the furin phosphatase, and the mechanism(s) by
which these various factors act to regulate endosomal sorting, however, are not known.
The emerging complexity of the protein phosphatase
(PP) 1 and 2A families suggests a myriad of roles for these
enzymes. Indeed, isoforms of PP1 containing catalytic and
regulatory or targeting subunits have been shown to control glycogen metabolism and myosin dephosphorylation
(for review see Hubbard and Cohen, 1993). PP2A has
been linked to the regulation of mitogen-signaling pathways, microtubule dynamics, and control of gene expression in the cell cycle (for reviews see Hubbard and Cohen,
1993
; Mayer-Jaekel and Hemmings, 1994
; Barford, 1996
).
The active form of PP2A in vivo is predominantly a heterotrimer which consists of a catalytic moiety (C subunit),
as well as an A subunit which mediates the binding of variable regulatory subunits. Although neither phosphatase has been shown to direct protein trafficking, the demonstrated importance of subunit composition in determining
their function in vivo suggests such a role is feasible.
A variety of PP2A regulatory subunit families genes,
and splice variants have been reported (McCright et al.,
1996; Zolnierowicz et al., 1996
), yet specialized functions
for these phosphatase isoforms remain to be established.
Three unique PP2A regulatory subunit gene families have
been identified to date in mammals based upon the characterization of tissue-specific isoforms and homology cloning. The B/PR55 family includes
,
, and
gene products,
whereas the B'/B56 family contains 5 genes (
,
,
,
, and
)
some of which express multiple splice variants (
1-5,
1-3).
The PR72/130 regulatory subunits are the product of differential splicing of a single gene. Although some of these
regulatory subunits are associated with isoforms of PP2A
highly expressed in particular tissues or cell types (Strack
et al., 1998
), their roles in vivo are largely undetermined.
Here we report the identity of a furin phosphatase and
the importance of furin-cd phosphorylation state in regulating multiple steps in the trafficking of the endoprotease
in vivo. We demonstrate that furin undergoes phosphorylation-dependent local cycling between early endosomes
and the cell surface. This peripheral cycling loop mirrors
that reported for the TGN localization of the protease (Wan et al., 1998), and requires the phosphorylation state-dependent sorting protein PACS-1. Analyses in vitro and
in vivo show that the movement of furin between early endosomes and the TGN is regulated by specific PP2A isoforms containing B family regulatory subunits. These findings demonstrate a novel role for PP2A and reveal the importance of phosphorylation/dephosphorylation in the
acute regulation of protein sorting within the TGN/endosomal system.
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Materials and Methods |
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Antibodies and Reagents
Antibodies against PP2A regulatory subunits have been described previously (Kamibayashi et al., 1994; Tehrani et al., 1996
). PP2A catalytic subunit antibody was obtained from B. Wadzinsky (Vanderbilt University,
Nashville, TN). mAb M1 was from Eastman Kodak Co. (Rochester, NY).
PP1 and PP2A catalytic subunits were provided by A.A. DePaoli-Roach
(Indiana University, Indianapolis, IN). All chemical reagents were obtained from Sigma Chemical Co. (St. Louis, MO) unless otherwise noted.
Cell Culture and Immunofluorescence Analyses
BSC-40 and PACS-1 control (C1) and antisense (AS19) cells were cultured as previously described (Thorne et al., 1989; Wan et al., 1998
). HeLa
cells expressing TS-Dyn I under control of Tet-suppressing elements were
obtained from S. Schmid (The Scripps Research Institute, La Jolla, CA),
and maintained as described (Damke et al., 1995
). For immunofluorescence analyses, cells were cultured directly on glass coverslips to a density
of 50-80% confluence before experimental manipulation as described in
figure legends. Cells were fixed in 4% paraformaldehyde and processed
for immunofluorescence as previously described (Molloy et al., 1994
).
mAb M1 was detected using either FITC- or TXR-conjugated goat anti-
mouse IgG2b-specific secondary antibodies (Fisher Scientific Co., Pittsburgh, PA).
Cloning and Expression Vectors
Epitope-tagged furin (fur/f), the CKII phosphorylation site point mutants
(fur/f-DDD and fur/f-ADA), and the glutathione-S-transferase (GST)-
Furcd fusion protein were generated previously (Molloy et al., 1994; Jones
et al., 1995
). Dynamin I WT and K44E cDNAs were each excised from
pSVL (Herskovits et al., 1993
) using EcoRI. The inserts were then
blunted with Klenow and cloned into the vaccinia recombination vector
pZVneo cut with StuI. SV-40 small t WT and the truncated small t mut 3 (Sontag et al., 1993
) were excised from pCMV5 using EcoRI and BamHI,
and then blunted with Klenow and inserted into pZVneo cut with StuI.
Recombinant vaccinia expressing the various constructs were generated
by standard methods (VanSlyke et al., 1995
).
Surface Labeling and Uptake Analysis
HeLa cells expressing the temperature-sensitive dominant-negative dynamin I were cultured in 35-mm plates for 3 d at permissive temperature
in the absence of tetracycline. The cells were then infected with vaccinia
virus expressing fur/f with either the serine to alanine (fur/f-ADA) or
serine to aspartic acid (fur/f-DDD) substitutions within the cd CKII site
(multiplicity of infection [m.o.i.] = 10) and allowed to express at nonpermissive temperature, 37°C. At 6 h postinfection the cells were placed on
ice, rinsed with cold PBS and surface proteins were labeled for 1 h using 0.5 mg/ml EZ-link NHS-SS-biotin (Pierce Chemical Co., Rockford, IL) in
PBS. After labeling, the cells were rinsed three times with PBS containing
50 mM glycine to quench unreacted biotin, and then refed with prewarmed medium (except for 0 time points) and placed at permissive temperature (31°C) for 10, 20, 30, or 40 min. At each time point, cells were
transferred to ice, rinsed with cold PBS, and then either harvested directly
in modified RIPA buffer (mRIPA, 50 mM Tris-HCl, pH 8, 150 mM NaCl,
1% NP-40 [Calbiochem-Novabiochem Corp., La Jolla, CA] and 1% sodium deoxycholate) for immunoprecipitation of total labeled furin using
mAb M1 as previously described (Molloy et al., 1994), or were stripped of remaining surface biotin by washing three times for 10 min in Tris-buffered saline (TBS; 10 mM Tris-HCl, pH 8, 150 mM NaCl) with MesNa (50 mM). The surface-stripped samples were rinsed with PBS supplemented
with Hepes (10 mM, pH 7.5) and harvested in mRIPA for immunoprecipitation of internalized labeled furin with mAb M1. The immunoprecipitates were resolved by SDS-PAGE on 8% gels, transferred to nitrocellulose membranes and probed with avidin HRP (1:2,000 dilution of 1 mg/ml
in TBS + 0.05% Triton X-100) and developed by chemiluminescence
(Renaissance; NEN Life Science Products, Boston, MA) to detect biotinylated furin. In double-strip analyses, cells processed as described above after the initial 20-min uptake period were then incubated for a second 20-min period at permissive temperature to allow for recycling of the internal pool to the cell surface. After the second chase period, cells were placed
on ice and either harvested directly (control) or subjected to a second
MesNa strip as described above before harvesting and analysis by Western blot.
PP2A Expression and Assays
Baculoviruses expressing PP2A catalytic, A, B, and B
subunits have
been described previously (Kamibayashi et al., 1994
). The recombinant
baculovirus encoding the B'
subunit was generated as described (Tehrani et al., 1996
). For expression experiments, Sf9 cells growing in log
phase were seeded in 25-cm flasks (3 × 106 cells/flask) and infected with
baculovirus recombinants (m.o.i. = 2). The cells were harvested at 64-72 h
postinfection by trituration and then pelleted and washed with PBS. The
washed cell pellet was then resuspended in 0.5 ml of ice-cold harvest
buffer (50 mM Tris, pH 7.0, 1 mM EDTA, and 2 mM DTT with pepstatin,
leupeptin, PMSF, aprotinin, and E64) and the cells broken open by passage (10×) through a 25-gauge needle. The lysates were clarified by low
speed centrifugation at 500 g and brought to 10% glycerol before performing phosphatase assays (see below).
Phosphatase Assays
Substrates for phosphatase assays were prepared by phosphorylating GST-Furcd fusion protein and phosphorylase b in vitro with CKII or phosphorylase kinase, respectively. The phosphorylation reactions (100 µl) contained 50 µg of substrate, and 100 µM of 32P-labeled ATP (3,000 cpm/ pmol). After phosphorylation for 1 h at 30°C, the reactions were run over two G25 spin columns to remove free ATP. Dephosphorylation assays were conducted in a 50-µl reaction containing lysates, buffer (25 mM Tris, pH 7.0, 0.2 mM MnCl2, 1 mM DTT, and 0.2 mg/ml BSA) and ~10 µM phosphorylated substrate (either phosphorylase a or GST-Furcd). Aliquots (5 µl in triplicate) of each reaction were removed at 15 and 30 min of incubation and applied to P81 filter paper (Whatman Inc., Clifton, NJ) squares that were washed three times with 75 mM phosphoric acid to remove free phosphate and then dried and subjected to scintillation counting. In some cases, the aliquots were mixed with SDS sample buffer, resolved by SDS-PAGE, and then analyzed using a PhosphoImager (model 445SI; Molecular Dynamics Inc., Sunnyvale, CA). Both measurement techniques gave identical results.
Microcystin Affinity Chromatography
Affinity chromatography for purification of endogenous furin-directed
phosphatase from BSC-40 cell extract was performed essentially as described (Moorhead et al., 1994). In brief, microcystin (MC)-LR (LC Laboratories, Woburn, MA) was derivatized with aminoethanethiol to generate a primary amine group for coupling to NHS-activated sepharose
(Pharmacia Biotech Inc., Piscataway, NJ). Approximately 0.25 mg of derivatized MC was linked to 1 ml of Sepharose resin. For the preparation of
extracts, three 15-cm plates of confluent BSC-40 cells were rinsed three
times with cold PBS and one time with harvest buffer (see above). The
cells were then scraped from the plates and lysed by passage (10×)
through a 25-gauge needle. The extract was then clarified by low-speed
centrifugation (10 min at 500 g) and brought to 10% glycerol. The clarified extract was then cycled over the MC column (0.5 ml/min) for 1 h at
4°C to allow binding of the phosphatase. The column was then washed with 10 vol of harvest buffer and eluted with 3M NaSCN. Each eluted fraction (10 ml) was dialyzed over night against harvest buffer before assaying for phosphorylase a and GST-Furcd-directed phosphatase activities
as described above.
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Results |
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Importance of Furin-Cd Dephosphorylation for Early Endosome to TGN Sorting
Phosphorylation of the furin-cd by CKII is necessary for
maintaining the steady-state distribution of the endoprotease in the TGN and also for the localization of cell surface-internalized furin to early endosomes. By contrast,
sorting of furin from early endosomes to the TGN requires
dephosphorylation of the furin-cd (Jones et al., 1995).
These findings were confirmed by antibody uptake studies
(Fig. 1). Incubation of cells expressing epitope (FLAG)-
tagged furin (fur/f) with mAb M1 demonstrated the retrieval of cell surface fur/f to the TGN (Fig. 1 A). However, treatment of cells with tautomycin, a potent inhibitor
of PP1 and PP2A (Takai et al., 1995
), resulted in the accumulation of internalized fur/f in peripheral punctate structures characteristic of early endosomes (Fig. 1 B). The
effect of tautomycin is specific for a furin-directed phosphatase since (a) this drug has no effect on the internalization of a nonphosphorylatable furin construct, fur/
fS773,775A (ADA) (Jones et al., 1995
), and (b) internalization of a phosphorylation mimic furin construct, fur/
fS773,775D (DDD), also resulted in accumulation in the peripheral punctate compartments (Fig. 1 C). Double-labeling experiments showed that internalized fur/f-DDD and
transferrin colocalized within these peripheral vesicles
(Fig. 1, D and E). Together these results demonstrate that
after internalization, phosphorylated furin localizes to
early endosomes and sorting of the endoprotease from this
compartment to the TGN requires the activity of a tautomycin-sensitive furin phosphatase.
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Surface Recycling of Furin
The dynamic nature of the early endosomal sorting system
suggests that the phosphorylation state-dependent accumulation of furin in early endosomes is not likely due to
static retention in these structures. Therefore, we tested
the possibility that this accumulation reflected a directed
cycling of phosphorylated furin between the cell surface
and early endosomes. To facilitate quantitative analyses of
internalization and recycling, we first investigated the
mechanism of furin endocytosis as a means by which to increase the surface pool of the protease. The presence of
functional tyrosine- and dileucine-based internalization
signals within the furin-cd suggested that endocytosis occurs via a clathrin-dependent pathway (Schäfer et al.,
1995; Ohno et al., 1996
). Therefore, we expressed a dominant-negative form of the GTPase dynamin I, K44E, to effectively block clathrin-dependent endocytosis (Herskovits et al., 1993
; van der Bliek et al., 1993
) and examined its
effect on furin trafficking. Immunofluorescence analysis of
furin internalization from the cell surface (Fig. 2) showed
that although overexpression of native dynamin I had no
effect on uptake (compare Fig. 2 A with Fig. 1 A), the
K44E dominant-negative mutant blocked uptake and resulted in accumulation of the enzyme at the cell surface (Fig. 2 B). This effect on furin internalization mimicked
the block in TfR endocytosis as observed with TRITC-transferrin uptake (Fig. 2, C [Dyn I] and D [K44E]).
|
Based on this result, quantitative analyses of furin trafficking were performed in stably transfected HeLa cells
expressing a temperature-sensitive and reversible dominant-negative form of dynamin I (TS-Dyn I). Incubation
of cells expressing fur/f with TS-Dyn I at nonpermissive
temperature resulted in a dramatic increase in the amount
of furin detected by surface biotinylation when compared
with control cells at permissive temperature (data not
shown). Therefore, we used this system to compare the
trafficking of fur/f-DDD, to fur/f-ADA (Fig. 3). After surface labeling, cells expressing either fur/f-DDD or fur/f-ADA were chased for the increasing times to allow endocytosis/recycling. At each time point duplicate samples
were either processed to detect internalized (I) or total (T)
surface labeled furin (Fig. 3 A). The ratio of protected/total furin was plotted quantitatively as a percentage of surface furin internalized versus time at the permissive temperature (Fig. 3 B). At time points up to 20 min the
internalization of fur/f-DDD and fur/f-ADA were similar.
However, at longer incubation times the percentage of fur/
f-DDD in internal compartments decreased while the percentage of internalized fur/f-ADA continued to increase. Additional double-strip experiments showed that a significant portion (~50%) of the fur/f-DDD found in internal
compartments at the 20-min time point was reexpressed at
the cell surface during a subsequent 20-min chase (Fig. 3
B, inset). Reexpression of substantial amounts of fur/f-DDD at the cell surface within 20-30 min is consistent with the recycling time of transferrin receptor in HeLa
cells (Bleil and Bretscher, 1982), and suggests that the
phosphorylated form of furin undergoes a similar local cycling between early endosomes and the cell surface.
|
PACS-1 Is Required for Early Endosome Localization of Phosphorylated Furin
The cell surface/early endosome recycling of furin requires
multiple sorting motifs within the enzyme's cd. Internalization of furin from the cell surface is directed by the canonical tyrosine and di-leucine-based endocytosis signals
(Schäfer et al., 1995). Recycling from the early endosome
to the cell surface, however, requires the CKII-catalyzed
phosphorylation of the furin-cd AC (Fig. 3). Whereas the
hydrophobic endocytosis signals bind directly to the clathrin adaptor AP-2 (Ohno et al., 1996
), association of the
phosphorylated furin-cd AC with the clathrin sorting machinery requires a connector protein, PACS-1 (Wan et al.,
1998
). A requirement for PACS-1 for the early endosomal
localization of phosphorylated furin was demonstrated by
monitoring furin internalization in PACS-1-deficient cells
(Fig. 4). Parallel plates of control or PACS-1-deficient
cells expressing fur/f were incubated with mAb M1 for 1 h
in the presence of tautomycin. In agreement with Fig. 1,
tautomycin treatment caused the accumulation of internalized fur/f in transferrin-containing early endosomes
(Fig. 4 A). By contrast, in the absence of PACS-1, internalized fur/f failed to localize to early endosomes and instead
showed a faint and dispersed staining pattern throughout
the cytosol (Fig. 4 C). Early endosomal localization of internalized transferrin, however, was unaffected by PACS-1
depletion (Fig. 4, compare B with D). The inability of internalized fur/f to localize to early endosomes in the
PACS-1-deficient cells was not the result of defective furin endocytosis since after short times of antibody uptake
both the control and PACS-1-deficient cells showed similar punctate staining patterns (Fig. 4, E and G). Rather,
the faint staining pattern in panel B reflects a shared requirement for PACS-1 in the localization of phosphorylated furin both to early endosomes and to the TGN (Wan
et al., 1998
). Indeed, mAb M1 uptake in the absence of
tautomycin showed that in control cells fur/f is retrieved to
the TGN and to a peripheral endosome/lysosome population (Fig. 4 F). In PACS-1 antisense cells, however, internalized fur/f fails to localize to either the TGN or to the
peripheral endosome/lysosome population (Fig. 4 H). Together, these observations show that the sorting, but not
the internalization, of furin requires PACS-1.
|
Identification of Furin Phosphatase
After internalization into early endosomes, phosphorylated furin may either (a) undergo a PACS-1-dependent
recycling to the cell surface (Figs. 3 and 4) or (b) be directed to the TGN by a tautomycin-sensitive furin phosphatase (refer to Fig. 1) (Jones et al., 1995). The sensitivity
to tautomycin suggested the furin phosphatase is either
PP1 or PP2A. Therefore, the ability of both PP1 and PP2A
catalytic (C) subunits to dephosphorylate the phosphorylated furin-cd in vitro was determined (Fig. 5). Surprisingly, neither phosphatase was able to dephosphorylate
the furin-cd, whereas both enzymes efficiently dephosphorylated a control substrate, phosphorylase a. Similar results
were obtained for PP2B (data not shown).
|
Whereas neither the PP1 nor the PP2A catalytic subunits were active against the phosphorylated furin-cd, both
cell extracts and bovine brain cytosol contained a tautomycin- and okadaic acid-sensitive furin phosphatase activity (Fig. 6 A). Because of the high total protein concentrations required to detect furin phosphatase activity, we
were unable to identify the phosphatase by differential
sensitivity to these inhibitors. Therefore, an affinity chromatography procedure was used to isolate the furin phosphatase. Cell extract containing furin phosphatase activity
was applied to a microcystin (a potent PP1/PP2A inhibitor) affinity column (Moorhead et al., 1994, 1995
). Although microcystin normally covalently binds to both PP1
and PP2A (MacKintosh et al., 1995
), derivatization for
coupling to the column matrix blocks the reactive group
and changes the reagent to a tight-binding reversible inhibitor. Bound material was then eluted with NaSCN, dialyzed and assayed for phosphatase activity using either the
phosphorylated furin-cd or phosphorylase a as substrate
(Fig. 6 B). The microcystin affinity column efficiently bound both the phosphorylase a and furin-cd phosphatase
activities from the load sample. However, only the phosphorylase a activity could be recovered from the eluate.
The elution conditions were not responsible for the loss
of furin-directed activity since treatment of extract with
NaSCN followed by dialysis did not inactivate the furin
phosphatase. Furthermore, neither the pooling of all eluted
fractions from the column before dialysis nor the addition of the column flow-through to the assay was able to restore
furin-directed phosphatase activity (data not shown). These
results suggested that the depletion of furin phosphatase
activity was not simply the result of separating the catalytic
moiety from a required cofactor during purification.
|
The enhanced total phosphorylase a-directed activity
recovered in the eluate fractions is consistent with previous reports (Moorhead et al., 1994, 1995
), and reflects the
purification of PP1 away from endogenous inhibitors. Interestingly, despite the ability of microcystin to bind and
inhibit both PP1 and PP2A, only PP1 isoforms from muscle myofibrils and glycogen particles have been successfully isolated using this affinity technique (Moorhead et
al., 1994
, 1995
). It is possible, therefore, that exposure of
bound PP2A to the high concentrations of chaotropic agent required to dissociate it from the resin leads to an
irreversible inhibition of the enzyme. These results, together with those in Fig. 5, indicate that the furin phosphatase is indeed a PP1/PP2A-like enzyme (tautomycin-sensitive, binds to the microcystin column). Furthermore,
the inability to recover the furin phosphatase activity from
the affinity column implicates PP2A.
Role of PP2A Regulatory Subunits in Determining Specificity for Furin
Although the PP2A C subunit is active towards numerous
substrates in vitro, the predominant form of the enzyme in
vivo is a trimeric complex with additional A and B subunits (for reviews see Barford, 1996; Hubbard and Cohen,
1993
; Mayer-Jaekel and Hemmings, 1994
). The A subunit
promotes association of the C subunit with one of a variety
of regulatory B subunits. Recent cloning studies reveal
several unique families of regulatory subunits with multiple members derived from both separate genes and alternative splicing. Although the role of complex formation
was originally envisioned as restricting the otherwise
broad substrate specificity of the isolated C subunit, recent
studies show that the B subunits can act as positive regulators to enhance catalytic activity toward particular substrates both in vitro and in vivo (Sontag et al., 1996
).
Moreover, the distinct subcellular localization of the different B subunits could act to target isoforms of PP2A to
particular locations within the cell (Sontag et al., 1995
; McCright et al., 1996
; Okamoto et al., 1996
). Together, these
findings suggest a potentially high degree of isoform-specific PP2A characteristics.
To test the possibility that such isoform-specific composition could modulate the activity of PP2A toward furin,
we adopted a strategy of phosphatase isoform reconstitution by baculovirus expression (Kamibayashi et al., 1994).
Sf9 cells were coinfected with baculovirus recombinants
expressing combinations of the C, A, and one of the various B subunits (Fig. 7 A). Furin phosphatase activity was
strictly dependent on B subunit composition. Whereas expression of C subunit alone or coexpression of C and A
subunits failed to generate increased furin phosphatase activity, coexpression of the C and A subunits with B family
regulatory subunits (B
and B
) resulted in a selective
increase in furin phosphatase activity (two- to fourfold).
Importantly, the furin phosphatase activity was B family-
specific since coexpression of C and A subunits with a
member of the B' family, B'
, failed to stimulate furin
phosphatase activity. Indeed, B'
selectively inhibited the
basal furin-directed activity. This effect is most likely due
to displacement of endogenous Sf9 cell B family regulatory subunits from holoenzyme complexes by the overexpressed B'
, which has a higher affinity for the AC complex in vitro (Tehrani et al., 1996
). As expected, both the
phosphorylase a and furin phosphatase activities were inhibited completely by submicromolar concentrations of
okadaic acid and tautomycin. Furthermore, Western blot
analyses of the Sf9 extracts using subunit-specific antisera
show that (a) the appropriate regulatory subunits were indeed expressed by the recombinant baculoviruses and (b)
similar levels of catalytic subunit were expressed in each
combinatorial infection (Fig. 7 B). This latter observation
indicates that the dramatic changes in furin dephosphorylation are not the result of differential stabilization of the
catalytic subunit, but rather reflect the inherent ability of
the regulatory subunits to facilitate the recognition of furin by PP2A holoenzyme.
|
PP2A Regulates Furin Trafficking In Vivo
The in vitro data described above clearly implicate PP2A
isoforms containing B family subunits in the dephosphorylation of furin. The characterization of SV-40 virus-transforming factors has also provided a mechanism through
which to probe the importance of these particular PP2A
isoforms in vivo. The small t antigen of SV-40 contributes
to the transforming capacity of the large T antigen, at
least in part, by disrupting the mitogen-activated protein
(MAP) kinase signaling pathway. Recent studies have
shown that this effect of small t derives from the ability of
the SV-40 protein to displace B family regulatory subunits
from PP2A, resulting in a change in PP2A substrate specificity and hyperphosphorylation of several members of the
MAP kinase cascade (Sontag et al., 1993). We used this
characteristic of small t antigen to examine directly the importance of PP2A for the correct routing of furin in vivo.
To test this possibility, replicate plates of BSC-40 cells
were coinfected with vaccinia virus recombinants expressing fur/f and either small t or an inactive small t mutant
(mut3) lacking the PP2A binding site (Sontag et al., 1993).
The effect of small t expression on the recycling of furin
from the cell surface was then assessed in immunofluorescence studies as described above (Fig. 8). As seen previously, antibody uptake showed that recycling furin is primarily localized to the TGN/late endosome in control cells
(Fig. 8 A). Coexpression of small t, however, resulted in an
accumulation of the internalized furin in peripheral endosome-like structures (Fig. 8 C) as seen in cells treated with
tautomycin (Fig 8 B). This effect required the PP2A-binding function of small t since the inactive mut3 construct
had no discernible effect on furin trafficking (Fig. 8 D).
These results complement the in vitro data described in
Fig. 7 and show that PP2A isoforms containing B family
regulatory subunits control the dephosphorylation dependent transfer of furin from early endosomes to the TGN.
|
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Discussion |
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The mechanisms by which the endosomal system achieves
and controls its complex sorting functions are largely unknown. Studies of furin recycling, however, demonstrate
several factors that affect sorting within the early endosomal system. Here we report the identification of cellular
machinery that directs the phosphorylation state-dependent
trafficking of furin through endosomal compartments. Cell surface furin is endocytosed in a clathrin-/dynamin
I-dependent step by direct interaction of the tyrosine- and
dileucine-like internalization motifs with the AP-2 adaptor
(refer to Fig. 2) (Schäfer et al., 1995; Ohno et al., 1996
).
After localization to transferrin containing early endosomes (refer to Fig. 1), furin undergoes a phosphorylation
state-dependent sorting step that is controlled by the activities of CKII, and PP2A isoforms containing B family regulatory subunits (Jones et al., 1995
) (refer to Fig. 7). Phosphorylated furin molecules bind the connector protein PACS-1 and are placed in a local cycling loop between the
early endosome and the cell surface (refer to Figs. 3 and
4). By contrast, furin molecules dephosphorylated by
PP2A are sorted to the TGN (refer to Figs. 5-8).
Several findings indicate that phosphorylation/dephosphorylation of the furin-cd by CKII and PP2A directly affect furin trafficking in the TGN/endosomal system (Jones
et al., 1995; Dittié et al., 1997
; Wan et al., 1998
). For example, the accumulation of furin in early endosomes observed upon treatment of cells with tautomycin (refer to
Fig. 1) (Jones et al., 1995
) is replicated by the fur/f-DDD point mutant designed to mimic phosphorylation at the
CKII site. Furthermore, this effect of tautomycim requires
an intact, phosphorylatable CKII site in the furin-cd
(Jones et al., 1995
). Similarly, the effect of depletion of
the phosphorylation state-dependent furin-binding protein PACS-1 upon trafficking is selective for furin and proteins containing related AC motifs (Wan et al., 1998
). Although these results suggest direct effects on furin sorting,
it seems likely that CKII and PP2A also have additional
global roles in controlling membrane traffic within the
TGN/endosomal system.
Regulation of Isoform-specific Phosphatase Function
The regulation of furin trafficking by the combined activities of CKII and PP2A indicates a link between signaling
pathways and control of protein localization within the
TGN/endosomal system. Together with their known function in cell cycle progression, our studies support a broad
role for CKII and PP2A in regulating diverse cellular processes. Although modulation of CKII activity in vivo has
not been demonstrated (Allende and Allende, 1995), PP2A can be regulated in several ways. The importance of
holoenzyme composition in generating a furin-directed
phosphatase (refer to Fig. 7) illustrates the high level of
substrate specificity determined by subunit composition.
Similarly, the B family is required for the PP2A-catalyzed
dephosphorylation of tau both in vitro and in vivo. Although a selective role for B' family subunits has not been
demonstrated in mammalian cells, deletion of the B' homologue in yeast, Rts1, results in a temperature-sensitive growth defect that can be rescued by rabbit B' family subunits (Zhao et al., 1997
). Together, these studies show that
the PP2A regulatory subunits can act as positive effectors
for select substrates.
Regulatory subunit composition may also modulate
PP2A by targeting the phosphatase to select compartments. For example, B' subunits differentially partition
between the nucleus and cytoplasm (McCright et al.,
1996), whereas B
subunits target a population of PP2A to
microtubules where they are positioned for efficient dephosphorylation of tau (Sontag et al., 1995
). The microtubule-associated PP2A also undergoes cell cycle-dependent
modulation of its activity, indicating an additional level of
regulation. The observation that both the catalytic and
regulatory subunits of PP2A are phosphoproteins (Mayer-Jaekel and Hemmings, 1994
; McCright et al., 1996
) introduces the possibility that this second tier of regulation represents rapid, reversible changes in PP2A function linked
to second messenger signaling pathways.
Role of PP2A in Protein Sorting
The effect of SV-40 small t expression on furin sorting
(Fig. 8) provides an unequivocal demonstration of PP2A's
role in directing trafficking in the endocytic pathway. The
selective displacement of B family regulatory subunits by
small t offers a sensitive and direct diagnostic tool for delineating the role of PP2A isoforms in vivo. Our data do
not, however, exclude a potential contribution by PP1 to
the regulation of furin sorting. At limiting concentrations in vitro, PP1 is at least an order of magnitude more sensitive to tautomycin whereas PP2A is more sensitive to okadaic acid (Takai et al., 1995). Initial studies of the phosphorylation state-dependent sorting of furin, however,
showed that, at low concentrations (100 nM), tautomycin
affected sorting while similar concentrations of okadaic
acid had no effect. Okadaic acid at high concentrations (e.g., 1 µM) not only affected furin trafficking, but also
caused a dramatic dispersal of the paranuclear staining,
consistent with disruption of the microtubule network
and fragmentation of the TGN and Golgi (Lucocq, 1992
;
Reaven et al., 1993
; Horn and Banting, 1994
), precluding
the use of this inhibitor in evaluating protein sorting. This
preferential inhibition of furin trafficking by tautomycin
could reflect cell type differences in the permeability of
the inhibitors and/or their relative ability to penetrate the cellular compartments associated with PP2A-dependent
sorting. Although the empirical determination of effective
concentrations by examination of residual PP1 and PP2A
activities in treated cells can facilitate the application of inhibitors as diagnostic tools (Favre et al., 1997
), the possibility of resistant or sensitive pools of the enzymes remains
a complicating factor.
TGN and Endosomal Protein Sorting Share Common Machinery
The requirement of PACS-1 for the sorting of phosphorylated furin in early endosomes points to several commonalties between the early endosome/cell surface furin cycling loop and the phosphorylation- and PACS-1-dependent
localization of the endoprotease to the TGN (Wan et al.,
1998). In the model shown in Fig. 9, the cell surface/early endosome local cycling loop represents a mirror image of
a TGN/endosome cycling pathway. As for endocytosis of
cell surface furin to early endosomes (Schäfer et al., 1995
),
efficient export of TGN-localized furin to a post-TGN endosomal compartment requires the canonical tyrosine-
and/or dileucine-based internalization signals (Wan et al.,
1998
). These hydrophobic sorting signals bind directly to
the clathrin adaptor AP-2 at the cell surface, whereas budding from the TGN uses AP-1 (Alconada et al., 1996
;
Honing et al., 1996
; Ohno et al., 1996
; Wan et al., 1998
).
Although furin in the peripheral recycling pathway colocalizes with TfR, demonstrating its presence in early endosomes, the identity of the post-TGN recycling compartment is not established. The recovery of phosphorylated furin from immature secretory granules in neuroendocrine
cells (Dittié et al., 1997
), however, provides an analogy for
the TGN recycling loop. PACS-1 directs the retrieval step
of both cycling loops by linking phosphorylated furin to
the clathrin sorting machinery. Binding assays in vitro
show PACS-1 connects the phosphorylated furin-cd to
AP-1, which is consistent with a role for this adaptor in the
TGN localization of membrane proteins. This finding,
however, does not exclude the possibility that other adaptors (e.g., AP-3) may also mediate retrieval. The composition of the adaptor species used in the PACS-1-directed
recycling between early endosomes and the plasma membrane has yet to be established. Interestingly, the prevalence on endosomes of clathrin-coated buds which contain neither
or
adaptin (Stoorvogel et al., 1996
) indicates
that unique adaptor species may mediate the retrieval step
within the peripheral cycling loop.
|
As reflected in Fig. 9, depletion of PACS-1 leads to sorting defects at both the level of endosomal trafficking and
TGN localization. This model, however, does not address
why nonphosphorylatable furin, fur/f-ADA (which does
not bind PACS-1), displays some trafficking characteristics unique from fur/f in PACS-1 antisense cells (compare Fig. 4 and Wan et al., 1998 with Takahashi et al., 1995
and
Jones et al., 1995
). Most notable is the finding that fur/f-ADA mislocalizes from either the TGN or early endosomes to larger punctate membrane structures whereas
fur/f in PACS-1 antisense cells shows very fine, dispersed
vesicular staining. There are several possible explanations
for these results, including (a) the presence of additional
phosphorylated-furin binding proteins, (b) multiple roles
for PACS-1 in organizing endosomal sorting compartments, and (c) the presence of binding proteins which interact selectively with the nonphosphorylated furin cd but
not with fur/f-ADA. This latter possibility implies that
nonphosphorylated AC motifs constitute positive sorting
signals for trafficking within the TGN/endosomal system.
Phosphorylation-dependent Sorting Regulates Furin Processing Compartments
While PACS-1 directs the retrieval of phosphorylated furin in both the TGN and peripheral cycling loops, the
transport of furin between these loops requires dephosphorylation by PP2A. This dephosphorylation-dependent
trafficking step provides a mechanism by which cells may
control the distribution of furin between the biosynthetic
and plasma membrane/endosomal processing compartments. Furin at the cell surface is tethered by interaction
with the actin binding protein ABP-280 (Liu et al., 1997).
This interaction modulates the rate of furin internalization
and may also act to generate processing sites at the cell
surface (e.g., bacterial toxin activation) (Gordon et al.,
1995
). The cycling of furin between the cell surface and endosomes may reflect a requirement for the formation of
peripheral processing compartments in which the enzyme
can be concentrated with its substrates. In addition, the internal milieu of the early endosomes (e.g., acidic pH) facilitates the processing of some substrates such as pseudomonas and diphtheria toxins (Gordon et al., 1995
). Thus, due
to the highly dynamic and transitory nature of the endosomal system, continuous cycling of phosphorylated furin
between the plasma membrane and early endosomes
could be the best mechanism for maintaining functional concentrations of the protease within the peripheral processing compartments. Similarly, the TGN cycling loop
might function to optimize processing of substrates in the
biosynthetic pathway. This concept is consistent with the
observation that, in some cells, endogenous furin can be
predominantly localized to a post-TGN processing compartment (Sariola et al., 1995
).
A Broad Role for PACS-1/PP2A in Protein Sorting
The PACS-1-directed trafficking of furin suggests that the
localization of additional proteins containing acidic cluster
motifs may be regulated via the phosphorylation state-dependent sorting machinery. Indeed, a number of itinerant secretory pathway membrane proteins including the
cation-independent (Meresse et al., 1990) and cation-dependent (Mauxion et al., 1996
) mannose 6 phosphate receptors, carboxypeptidase D (Xin et al., 1997
), and sortilin (Petersen et al., 1997
) have CKII phosphorylatable
ACs within their cytosolic domains. In the case of the
MPRs this phosphorylation has been linked to the efficient function of the receptor, however, it is not clear how
this modification effects trafficking. Interestingly, many
herpes virus envelope glycoproteins (e.g., VZV-gE, HSV-1-gE, and HCMV-gB) have phosphorylatable AC sorting
motifs on their cds (Edson et al., 1987
; Norais et al., 1996
;
Yao et al., 1993
). Recent data indicate that the phosphorylation of these sites can influence trafficking of the envelope proteins (Alconada et al., 1996
; Fish et al., 1998
) and
that PACS-1 binds VZV-gE cytosolic domain (Wan et al.,
1998
), raising the possibility that the CKII-mediated sorting system is used in viral biogenesis and/or spread.
Our studies of the regulation of furin sorting between
the early endosome and plasma membrane also reveal several parallels with the cellular machinery that controls the
phosphorylation-dependent resensitization of G protein-
coupled receptors. The trafficking of both furin and AR
depend upon (a) the phosphorylation state of the cytosolic
domain (Ferguson et al., 1995
, 1996
), (b) the presence of a
phosphorylation state-dependent connector protein (either PACS-1 or
arrestin) (Goodman et al., 1996
) that
provides a link to the clathrin-sorting machinery, and (c)
dephosphorylation by specific endosome-associated PP2A
isoforms (Pitcher et al., 1995
).
In summary, our results reveal the integrated roles of identified components of the endosomal trafficking machinery that direct the phosphorylation state-dependent sorting of furin. The importance of kinase (CKII) and isoform-specific phosphatase (PP2A) activities in regulating protein routing also indicates that these sorting events are likely under the control of second messenger systems. Future studies, therefore, will focus on how this dynamic endosomal sorting system responds to intracellular signaling pathways in order to control the distribution and activity of membrane proteins in vivo.
![]() |
Footnotes |
---|
Received for publication 19 June 1998 and in revised form 19 August 1998.
Address all correspondence to G. Thomas, Vollum Institute, Oregon
Health Sciences University, Portland, OR 97201. Tel.: (503) 494-6955. Fax: (503) 494-4534. E-mail: thomasg{at}ohsu.edu
C. Kamibayashi's present address is Simmons Cancer Center, University of Texas Southwestern Medical Center, Dallas, TX 75235.
We thank B.G. Jones (University of Sheffield, Sheffield, UK) for contributions to the early part of this study. We also wish thank S.L. Schmid for the TS-Dyn I cell line, A.A. DePaoli-Roach for purified PP1 and PP2A, Wadzinsky for PP2A catalytic subunit antibody, R. Valle (Worcester Foundation, Shrewsbury, MA) for dynamin I and K44E plasmids, and P. Cohen (University of Dundee, Dundee, UK) for helpful discussions.
This work was supported by National Institutes of Health grants to G. Thomas (DK-44629 and DK-37274).
![]() |
Abbreviations used in this paper |
---|
AC, acidic cluster; cd, cytosolic domain; GST, glutathione-S-transferase; MC, microcystin; m.o.i., multiplicity of infection; PACS-1, phosphofurin acidic cluster-sorting protein; PP2A, protein phosphatase 2A.
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