Department of Cell Biology and Physiology, Washington University School of Medicine, St. Louis, Missouri 63110
To explore the role of GTPases in endocytosis, we developed an assay using Xenopus oocytes injected with recombinant proteins to follow the uptake
of the fluid phase marker HRP. HRP uptake was inhibited in cells injected with GTPS or incubated with aluminum fluoride, suggesting a general role for GTPases
in endocytosis. Injection of Rab5 into oocytes, as well
as Rab5:Q79L, a mutant with decreased GTPase activity, increased HRP uptake. Injection of Rab5:S34N, the
dominant-negative mutant, inhibited HRP uptake. Injection of N-ethylmaleimide-sensitive factor (NSF)
stimulated HRP uptake, and ATPase-defective NSF
mutants inhibited HRP uptake when coinjected with
Rab5:Q79L, confirming a requirement for NSF in endocytosis. Surprisingly, injection of Rab7:WT stimulated both uptake and degradation/activation of HRP.
The latter appears to be due to enhanced transport to a
late endosomal/prelysosomal degradative compartment
that is monensin sensitive. Enhancement of uptake by
Rab7 appears to function via an Rab5-sensitive pathway in oocytes since the stimulatory effect of Rab7 was
blocked by coinjection of Rab5:S34N. Stimulation of
uptake by Rab5 was blocked by Rab5:S34N but not by
Rab7:T22N. Our results suggest that Rab7, while functioning downstream of Rab5, may be rate limiting for endocytosis in oocytes.
Gtp binding proteins are versatile molecular switches
(Bourne, 1988 Among the monomeric small GTPases, the Rab family
is well characterized as regulators of intracellular trafficking during endocytosis and secretion (Zerial and Stenmark, 1993 In transient transfection experiments and in vitro fusion
assays using enriched membrane preparations, Rab5 has
been shown to play a role in the regulation of endocytosis
(Grovel et al., 1991 Both fluid phase and receptor-mediated endocytosis occur through vesicle fusion and transfer of endocytic materials from one compartment to another in a highly regulated fashion. Depending on the receptor system, ligands
and receptors are internalized and sorted in the early endosomal compartment (Goldstein et al., 1985 mAbs used in this study include: 4F11, a mouse 1 g G2aK mAb that recognizes mouse Rab5, generously provided by A. Wandinger-Ness (Northwestern University, Evanston, IL), and A46, a mouse IgM mAb that recognizes native NSF, kindly provided by J.E. Rothman (Sloan-Kettering
Memorial Hospital, New York). Irrelevant IgG and IgM mAbs were purchased from Sigma Chemical Co. (St. Louis, MO) and used as control. Recombinant NSF wild-type and mutant proteins were generously provided
by S.W. Whiteheart (University of Kentucky, Lexington). HRP-conjugated antibodies and enhanced chemiluminescence detection reagents were
from Amersham Corp. (ECL; Arlington Heights, IL). All other reagents
were from Sigma Chemical Co.
Preparation of Oocytes
Ovaries were dissected from adult female Xenopus laevis (Nasco, Fort Atkinson, WI) anesthetized with 3-aminobenzoic acid ethyl ester (1 g/liter)
in ice water. The ovaries were agitated gently for 2 h in modified Barth's
saline (MBS) containing no calcium with 2 mg/ml type 1 collagenase at
room temperature (Swick et al., 1992 Expression and Purification of Recombinant Rabs
and Mutants
The Rab5 fusion proteins used in this study including Rab5:Q79L, Rab5:
S34N, and Rab5: Microinjection of Test Proteins in Xenopus Oocytes
and HRP Uptake
Before microinjection, healthy oocytes (stage V or VI) were visually selected. Oocytes were injected with the indicated concentrations of test molecules in 50 nl of buffer A (20 mM Hepes, pH 7.4, containing 20 mM NaCl,
1 mM MgCl2, and 100 µM ATP), and then allowed to recover in MBS at
18°C for 2 h. Finally, healthy oocytes were selected for HRP uptake. The
injected oocytes were incubated in the presence of HRP (2 mg/ml) for 1 h
at 18°C. HRP uptake by the oocytes was stopped by washing the cells
three times with MBS containing 5% BSA. Individual oocytes were then
lysed in 200 µl PBS containing 0.1% Triton X-100 and centrifuged, and
the lysate was assayed for HRP. The enzyme assay was conducted in a 96well plate (Costar Corp., Cambridge, MA) using O-phenylenediamine as
the chromogenic substrate (Wolters et al., 1976 Western Blot Analysis of Rab5 and Mutants
Oocytes were injected with Rab5:Q79L or Rab5: Degradation Assay Using Preloaded HRP in Oocytes
To monitor the inactivation/degradation of the marker protein, oocytes
were preloaded with HRP (2 mg/ml) in MBS for 1 h at 18°C. Subsequently, cells were washed three times with ice-cold MBS to remove unbound HRP. HRP-preloaded cells were injected with the test proteins and
incubated for 2 h at 18°C. The amount of HRP activity present after the
incubation was determined after lysis of the oocytes.
To follow degradation, oocytes were incubated with 125I-HRP (500 µg/ml;
2 × 105 cpm/µg) for 1 h at 18°C. The cells were then washed to remove unbound radioactivity. The preloaded cells were injected either with control
buffer or Rab proteins and incubated in MBS for 2 h at 18°C, the media
were collected, and the cells were lysed. TCA-soluble and -precipitable radioactivity both in the cell lysates and in the medium was determined by
addition of 10% TCA followed by centrifugation.
Confocal Microscopy
For confocal microscopy, oocytes were injected with Rab7 or mutant proteins and allowed to recover for 1 h at 18°C. The cells were then incubated
with HRP-FITC for 1 h at room temperature under slow rotation in the
dark. Subsequently, the cells were washed three times with MBS and incubated in the dark for one additional hour. The labeled oocytes were then
observed with an MRC 600 laser confocal microscope (Bio Rad Microscience, Cambridge, MA) using a ×63 plan apochromat oil immersion objective (Carl Zeiss, Inc., Thornwood, NY).
Role of GTPases in Endocytosis in Xenopus Ooocytes
To determine the function of GTPases in the endocytic
pathway, we used HRP as a marker to follow endocytosis
and transport from the cell surface to a lysosomal compartment. The results presented in Fig. 1 show that HRP
uptake is linear with time when the cells are incubated
with HRP at 2 mg/ml. All subsequent uptake experiments
were carried out for 1 h with 2 mg/ml HRP. Many experiments were also carried out at a higher ligand concentration (5 mg/ml) with identical results.
To determine the role of GTPases in intracellular trafficking, we measured HRP uptake by oocytes after injection of GTP Effect of Injected Rab GTPases on Oocyte Endocytosis
To explore the role of Rab GTPases in endocytosis in Xenopus oocytes, we microinjected different Rab proteins
(100 ng) and followed HRP uptake. Both Rab5 and Rab7
significantly stimulated HRP uptake by oocytes by 1.5- to
twofold (Fig. 2). This result is consistent with the finding
that Rab5 stimulates endocytosis during transient expression in vivo and endosome-endosome fusion in vitro (Li
and Stahl, 1993
Role of NSF in Rab5-Stimulated Endocytosis of HRP
NSF is required for in vitro endosome fusion (Diaz et al.,
1989
Effect of Rab7 and Its Mutants on HRP Uptake
As indicated above, injection of Rab7:WT (100 ng) into
oocytes produced a marked stimulation in HRP uptake
(Fig. 2). To characterize the effect of Rab7 on endocytosis,
we examined the effect of Rab7 on the uptake HRP-FITC
by confocal microscopy. The results presented in Fig. 6
show substantial increases in fluorescence when the cells
were injected with Rab7:WT protein. Cells injected with Rab7:T22N appeared very similar to control cells. In fact,
injection of Rab7:T22N showed significant inhibition of
HRP-FITC uptake (data not shown). In control and Rab7:
T22N injected cells, most of the staining was observed
near the plasma membrane, presumably in early endosomal compartments. In Rab7:WT-injected cells, it is evident
that HRP-FITC is localized to larger vesicles and much deeper into the cells, possibly reflecting the transport of
the marker proteins into later compartments. To further
confirm the effect of Rab5 and Rab7 on the endocytic
pathway at the EM level, oocytes were injected with Rab5:
WT and Rab7:WT, and then allowed to internalize WGA-
gold particles for 60 min. The cells were then fixed and
prepared for EM. Oocytes injected with Rab5 had substantially more early endosome-like vesicular structures,
whereas a significant labeling of large multivesicular endosomes (1-1.5 µm) deeper inside the cells was observed
when cells were injected with Rab7:WT. These data suggest that Rab7 induced the transport to a late endosome/
prelysosome compartment. About 180-200 large multivesicular compartments were observed per square millimeter only in oocytes injected with Rab7:WT protein (data
not shown).
Differential Kinetics of Rab5 and Rab7 on Endocytosis
in Oocytes
Rab5 is an early endosomal Rab that regulates endosome
fusion, whereas Rab7 is localized to the late endosomal
compartment (Wichmann et al., 1992
To examine the degradation of internalized HRP, oocytes were incubated with 125I-HRP for 60 min, after which
they were injected with control buffer or Rab7. The data
presented in Fig. 7 b show the distribution of internalized
125I-HRP in oocytes after injecting Rab7:WT protein into
the cells. 43% of the HRP was degraded by the Rab7:WT
protein-injected cells. In contrast, only 25% of the preloaded HRP was degraded when the cells were injected
with control buffer. In both sets of cells, the same amount
of the TCA-precipitable radioactivity (50%) was recovered in the medium, suggesting that a substantial amount of recycling occurs in this preparation. However, Rab7 did
not induce or enhance recycling or regurgitation of the
HRP to the medium.
Effects of Rab5- and Rab7-negative Mutants on
Rab5-stimulated Endocytosis
We next examined the effects of the dominant-negative
mutants of Rab5 and Rab7 to determine which acts first in
the sequence. Injection of Rab5:WT into oocytes stimulated HRP uptake (Fig. 8 a) by about twofold. Coinjection
of the Rab7 dominant-negative mutant (Rab7:T22N) with
Rab5 had no effect on the Rab5-mediated stimulation of
uptake. However, coinjection of the Rab5 dominant-negative mutant (Rab5:S34N) with Rab5:WT significantly attenuated the Rab5:WT effect. On the other hand, injection
of Rab7:WT produced a threefold increase in uptake, as
shown in Fig. 8 b, and coinjection of the Rab7 dominantnegative mutant with Rab7:WT blocked the Rab7:WT effect. Interestingly, Rab5:S34N inhibited Rab7:WT-induced
endocytosis. Moreover, Rab7:T22N and Rab5:S34N inhibited uptake when injected alone. Lastly, we examined the
additive effects of Rab5 and Rab7 on endocytosis when
coinjected. Injection of Rab5 and Rab7 separately increased endocytosis. Coinjection of Rab5 and Rab7 produced an additive effect on uptake of HRP (Fig. 9).
The Xenopus laevis oocyte is an excellent cell model for
investigating the mechanisms and the regulation of endocytosis (Opresko et al., 1980 In the last few years, it has become apparent that GTP
binding proteins play an important role in endocytosis,
and that a series of GTPases act in concert to effect transport from cell surface to lysosomes or the TGN. A large
number of studies have shown that GTP Numerous reports indicate that several small GTPases
regulate endocytosis. Most of these studies have employed
transient overexpression of proteins using recombinant virus encoding the proteins of interest. This approach has
been extremely useful in studying transport but is somewhat limited because usually only one variable can be
changed at a time. We have developed an in vivo assay by
microinjection of purified proteins into Xenopus oocytes
followed by endocytosis of HRP. This approach takes advantage of the high endocytic capacity of oocytes and allows one to examine the effect of one or more proteins
that can be injected simultaneously. These studies have led
to some unexpected findings.
Two experiments, one with GTP Rab4 and Rab5 are associated with early endosomes,
and Rab5, in particular, has been shown to play a role in
the regulation of endocytosis. As shown in Fig. 3 a, Rab5:
Q79L, a GTPase-defective mutant, enhances HRP uptake.
This result suggests that the GTP form of Rab5 is the molecular switch for endocytosis in oocytes as reported for
other cells (Barbieri et al., 1994 The COOH-terminal motif of Rab proteins serves as a
recognition site for isoprenylation that is important for
membrane attachment and biological function (Li et al.,
1994 To further characterize endocytosis in microinjected oocytes, we examined NSF. NSF fusion protein is required
for efficient fusion of endosomes in vitro. Mutants of NSF
that fail to bind or to hydrolyze ATP are effective inhibitors of endosome fusion in vitro (Colombo et al., 1996 Recently, it has been shown in mammalian cells that
Rab7 regulates transport between the early and late endosomes (Feng et al., 1995 The results with the Rab coinjection experiments suggest that Rab7 mediates endocytosis through the early endosome and that the effect of Rab7 is downstream of
Rab5. Similar results have been reported in the yeast
where Ypt51p, Ypt52p, and Ypt53p, functional homologues of mammalian Rab5, mediate the degradation of
the yeast pheromone ; Bourne et al., 1990
), and during
the past few years, there has been an explosion of
interest in unraveling the role of these proteins, particularly low molecular weight GTPases, in membrane traffic
(Balch, 1990
; Barr et al., 1992
). Evidence from a variety of
sources, including the temperature-sensitive secretion mutants in yeast and in vitro reconstitution experiments in
permeabilized and broken cell preparations, has established that at least two subfamilies of GTP binding proteins, namely the Rab and ADP ribosylation factor subfamilies, are required for membrane traffic (Pfeffer, 1994
;
Rothman, 1994
). Heterotrimeric GTPases (Colombo et
al., 1992
) and ras (Barsagi and Feramisco, 1986
) also appear
to play significant roles.
). Rab proteins are specifically localized to the
cytoplasmic surface of the intracellular compartments that
they subserve, such as ER (Rab1 and Rab2) (Chavrier et al.,
1990
; Tisdale et al., 1992
), cis-Golgi (Rab1 and Rab2) (Chavrier et al., 1990
; Tisdale et al., 1992
), medial Golgi
(Rab6) (Goud et al., 1990
), trans-Golgi (Rab9) (Gravotta
et al., 1990
; Lombardi et al., 1993
), early endosomes (Rab4
and Rab5) (Chavrier et al., 1990
; van der Sluijs et al., 1991),
late endosomes (Rab7 and Rab9) (Chavrier et al., 1990
;
Lombardi et al., 1993
; Feng et al., 1995
), and plasma membrane/endosomes (Rab5) (Chavrier et al., 1990
).
; Li and Stahl, 1993
). Overexpression of
Rab5 increases the rate of endocytosis and the recycling of
the transferrin receptor. Moreover, Rab5-specific antiserum and Rab5 dominant-negative mutants specifically inhibit endosome fusion in vitro (Bucci et al., 1992
; Li et al.,
1994
; Barbieri et al., 1994
). While it has been shown that
Rab5 regulates endocytosis at the level of early endosome fusion, the mechanism by which Rab5 exerts its function
remains to be established. Moreover, transport of macromolecules along the endocytic pathway requires a series of
highly coordinated and specific fusion events involving
multiple cytosolic and membrane-bound factors including
the N-ethylmaleimide-sensitive factor (NSF)1 (Diaz et al.,
1988
; Rothman, 1994
) and a membrane-bound trypsin-sensitive factor (Colombo et al., 1991
). Protein phosphorylation
has also been shown to regulate the fusion of endosomal
vesicles (Toumikoski et al., 1989
). Certain lipids may also
play a role in membrane docking and fusion. Both PLA2
(Mayorga et al., 1993
) and PI3 kinase (Li et al., 1995
) are intimately involved in the regulation and/or action of Rab5.
However, the exact relationship among all these factors in
the overall mechanism of intracellular trafficking during
endocytosis is largely unknown.
), often requiring acidic pH, and selected molecules are recycled back
to the plasma membrane. Some are transported to the late endosomal compartment that may communicate with the
trans-Golgi network or the lysosomal compartment (Yamashiro et al., 1984
). The late endosome represents the
convergence point of several pathways such as (a) the targeting of newly synthesized lysosomal enzymes and lysosomal membrane proteins to the lysosome (Griffiths et al.,
1990
; Riederer et al., 1994
), (b) the killing of internalized microorganisms by phagocytes (Ludwig et al., 1991
; Rabinowitz et al., 1992
; Desjardins et al., 1994
; Beron et al.,
1995
), and (c) the processing and presentation of antigens
where major histocompatibility complex class II molecules
are recycled to the plasma membrane after the formation
of peptide-major histocompatibility complex class II complex in the late endosomal compartment (Cresswell, 1994
;
Germain, 1994
; Qui et al., 1994
). It is now clear that multiple Rab proteins, including Rab7, Rab9, and Rab24, and
perhaps others to be identified, are associated with the late
endosomal compartment (Chavrier et al., 1990
; Lombardi
et al., 1993
; Olkkonen et al., 1993
). Rab9 has been shown
to regulate the traffic from the late endosome to the TGN
(Lombardi et al., 1993
; Riederer et al., 1994
), and Rab7 has
been shown to regulate the traffic from early to late endosomes (Wichmann et al., 1992
; Feng et al., 1995
) or from late endosomes to vacuoles (Schimmoller et al., 1993; Hass
et al., 1995
). Considerable progress has been made toward
understanding the regulation of transport by specific Rabs,
each with its own family of effector proteins. However, the
relation of one Rab to another and the regulation of intracellular pathways made up of a series of relays is poorly
understood. In the present investigation, we have exploited the Xenopus oocyte as a model system to study the
regulation of endocytosis by multiple Rabs. Our data suggest that the action of one Rab may influence the function
of a second Rab acting downstream.
Materials and Methods
). After collagenase treatment, the
oocytes were washed six times in MBS containing 5% BSA. Cells obtained in this manner were incubated overnight in MBS with BSA and
gentamycin (50 µg/ml) at 4°C.
C4 were described previously (Li and Stahl, 1993
; Li et al.,
1994
). Rab7 wild type (WT) and Rab7:T22N were expressed and purified
as glutathione-S-transferase (GST) fusion protein as described for Rab5
(Li and Stahl, 1993
; Li et al., 1994
). To prepare fusion proteins, cDNAs were
amplified by 25 cycles of PCR and subcloned into unique BamH1/EcoR1
restriction sites of the bacteria expression vector pGEX-3X (Pharmacia
Biotech Inc., Piscataway, NJ). Recombinant proteins were expressed in large quantities as GST fusion proteins in Escherichia coli strain JM 101. GST fusion proteins were affinity purified with glutathione-Sepharose.
). Briefly, the reaction was
initiated by adding 5 µl of the lysate to 100 µl of 0.05 N sodium acetate buffer, pH 5.0, containing O-phenylenediamine (0.75 mg/ml) and H2O2
(0.006%). After incubation (5 min at room temperature), the reaction was
stopped with 0.1 N H2SO4. The products were quantified by measuring the
OD 490 nm in a microplate reader (Bio Rad Laboratories, Hercules, CA).
Results were expressed as ng of HRP uptake per oocyte.
C4 (100 ng in 50 nl of
buffer A) and incubated for 2 h at 18°C. The cells were then lysed, and
crude membrane and cytosolic fraction were prepared by centrifuging at
70,000 rpm for 15 min in a TL-100 ultracentrifuge (Beckman Instruments,
Inc., Palo Alto, CA). Each fraction (5 µg) was separated by SDS-PAGE
and transferred onto nitrocellulose membranes, and Western blot analysis
was carried out using polyclonal anti-Rab5 antibody and developed by enhanced chemiluminescence.
Results
Fig. 1.
Time-dependent uptake of HRP by the oocytes. Oocytes were incubated with 2 mg/ml of HRP for different periods
of times. The cells were washed three times with MBS and each
oocyte was lysed in 200 µl of lysis buffer. Aliquots of the lysate
were used to measure the amount of HRP present in each cell.
The results, expressed as ng of HRP per oocyte, represent the average of eight independent observations.
[View Larger Version of this Image (11K GIF file)]
S, a nonhydrolyzable analogue of GTP. About
80% inhibition of HRP uptake was obtained when GTP
S
(20 µM; 50 nl per cell) was injected, suggesting that one or
more GTPases are involved in endocytosis in oocytes (data
not shown). HRP uptake was also inhibited by aluminum
fluoride (data not shown) but was not significantly affected when the cells were microinjected with equivalent
concentrations of aluminum sulfate or potassium fluoride.
These results provide evidence for a role for heterotrimeric
GTPases in endocytosis in oocytes (Mayorga et al., 1989;
Chabre, 1990
; Colombo et al., 1994
).
; Bucci et al., 1992
; Li et al., 1994
; Barbieri
et al., 1994
; Stenmark et al., 1994
). The stimulation of HRP
uptake by Rab7 was an unexpected but interesting observation. Injection of Rab4 had no effect on HRP endocytosis. We studied four different constructs of Rab5: Rab5:
S34N, a dominant-negative mutant (Li and Stahl, 1993
;
Grovel et al., 1991
; Burstein et al., 1992
); Rab5:
C4 where
the isoprenylation motif is deleted; Rab5:Q79L, a GTPasedefective mutant (Li et al., 1994
; Burstein et al., 1992
); and
Rab5:wild type. All the proteins were prepared as GST fusion proteins and the purified proteins were injected directly into oocytes. As shown in Fig. 3 a, both Rab5:WT
and Rab5:Q79L enhanced uptake of HRP. Rab5:S34N, on
the other hand, markedly inhibited HRP uptake by the oocytes (Fig. 3 a). Rab5:
C4 failed to associate with membranes and did not significantly affect HRP uptake. Rab5: DC4 was present exclusively in the cytosolic fraction (Fig.
3 b). These results strongly suggest that frog oocytes have
the ability to carry out the required posttranslational modification of Rab proteins, and that COOH-terminal isoprenylation is required for the binding of the Rab GTPase to
the target membrane and for biological activity. Moreover,
we have also found that Rab proteins are readily prenylated
in vitro using oocyte cytosol in the presence of labeled
substrate (data not shown). To confirm that the stimulatory effect of Rab5:Q79L was due to the injected Rab protein, coinjection experiments were carried out with the
Rab fusion protein and an anti-Rab5 antibody. Coinjection of the fusion protein with anti-Rab5 antibody, but not
with a control antibody, dramatically inhibited Rab5:
Q79L-enhanced uptake of HRP (Fig. 4).
Fig. 2.
Effect of different Rab proteins on HRP uptake in oocytes. Oocytes were injected with 100 ng of the indicated Rabs in
50 nl of microinjection buffer and incubated for 2 h at 18°C. The
cells were then incubated with HRP (2 mg/ml) for 1 h at 18°C to
determine the uptake rate. Results were expressed as percentage
of uptake per oocyte with respect to control (220 ng per oocyte) ± SD (average of eight experiments).
[View Larger Version of this Image (15K GIF file)]
Fig. 3.
Effect of Rab5 and
its mutants on endocytosis in
oocytes. (a) Oocytes were injected with 100 ng each of the
indicated proteins, and the
uptake was carried out as described in Fig. 1. The results were expressed as percentage of control (200 ng/oocyte) ± SD. (b) Oocytes were injected
with 100 ng of Rab5:Q79L or Rab5:C4 in 50 nl of microinjection buffer and incubated for 2 h at 18°C. The cells were lysed,
and crude membrane and cytosolic fractions were prepared by
centrifuging at 70,000 rpm for 15 min. Each fraction (5 µg) was
separated by SDS-PAGE and transferred onto nitrocellulose, and Rab5 protein was localized using polyclonal Rab5-specific antibodies (Alvarez-Dominguez et al., 1996
).
[View Larger Versions of these Images (15 + 17K GIF file)]
Fig. 4.
Inhibition of HRP uptake by anti-Rab5 antibody. The
oocytes were injected with Rab5:Q79L (100 ng) alone or with
anti-Rab5 antibody (200 ng). HRP uptake was followed as described in Fig. 1. Results are expressed as percentage of control
uptake (150 ng per oocyte) from the average of eight experiments.
[View Larger Version of this Image (17K GIF file)]
). To delineate the role of NSF in endocytosis in oocytes, we injected anti-NSF antibody into the cells and
measured HRP uptake. The results presented in Fig. 5 a
demonstrate that anti-NSF antibody inhibits the stimulation of HRP uptake in cells injected with Rab5:Q79L when
anti-NSF antibodies and Rab5:Q79L were coinjected. Injection of anti-NSF antibodies in control oocytes resulted
in a modest reduction in uptake (data not shown). To confirm the participation of NSF in Rab5 function, we coinjected oocytes with Rab5:Q79L and His6NSF wild-type
protein. The results in Fig. 5 b show that Rab5:Q79L-stimulated HRP uptake is enhanced in the presence of NSF
protein. NSF is a heterotrimer whose polypeptide subunits
are made up of three distinct domains: an amino-terminal domain and two homologous ATP (D1 and D2) binding
domains (Whiteheart et al., 1994
). The ability of the D1
domain to hydrolyze ATP is required for NSF-mediated
membrane fusion. The D2 domain is required for trimerization, but its ability to hydrolyze ATP is not absolutely required for NSF function. Two distinct mutations in the
first ATP binding site (D1), i.e., D1-K266A and D1-E329Q,
are known to affect ATP binding and hydrolysis, resulting
in inactive forms of NSF (Whiteheart et al., 1994
). To determine the role of the ATP binding domains of NSF in
Rab5-mediated endocytosis, we coinjected Rab5:Q79L and
different NSF mutant proteins into oocytes and followed HRP uptake. The results in Fig. 5 b demonstrate that His6
D1-E329Q and His6 D1-K266A significantly inhibit HRP
uptake mediated by Rab5:Q79L, indicating that ATP hydrolysis and binding are required for NSF activity and
Rab5-mediated endocytosis.
Fig. 5.
Effect of NSF on HRP uptake by oocytes. (a) Cells
were injected with either 100 ng of Rab5:Q79L with or without
anti-NSF (200 ng) antibody. Uptake was carried out as described
in Materials and Methods. Results were expressed as percentage
of control uptake (180 ng/oocyte) from the average of eight experiments. (b) Oocytes were injected with 75 ng of Rab5:Q79L
alone or with 100 ng of NSF and its mutants as indicated. Uptake
was determined as described, and the results are expressed as
percentage of control (220 ng per oocyte) uptake.
[View Larger Versions of these Images (16 + 16K GIF file)]
Fig. 6.
Rab7-mediated uptake of HRP by oocytes. Oocytes
were injected with 100 ng of the indicated proteins and uptake
was carried out with HRP-FITC for 1 h in the dark (1 mg/ml).
Subsequently, the cells were washed three times with MBS, incubated in the dark for another 1 h, and then observed under the
confocal microscope.
[View Larger Version of this Image (22K GIF file)]
; Schimmoller et al.,
1993; Feng et al., 1995
). Our data indicate that both proteins stimulate HRP uptake in oocytes, suggesting an effect of both proteins on the early endocytic pathway. To
check the effect of Rab5 and Rab7 on later events, we developed a more direct biochemical assay to measure the
degradation of internalized HRP after injection of the test proteins, as a measure of transport to a degradative or late
compartment. In this assay, oocytes were preloaded with
HRP for 1 h at 18°C. HRP activity was then followed over
time after injecting Rab5 or Rab7 proteins. The results
presented in Fig. 7 a show that ~80% of the HRP activity
is lost within 2 h when the cells were injected with Rab7:
WT protein. In contrast, most of the HRP activity is retained when the cells were injected with Rab5:WT protein, much like control cells. Thus, Rab7 not only promotes the
internalization of HRP but also mediates apparent rapid
degradation possibly by transferring internalized molecules to a late compartment. Moreover, when the Rab7:
WT-injected/HRP-preloaded cells were incubated in the
presence of monensin (100 µM), ~80% of the HRP inactivation was prevented. We speculate that monensin prevents the inactivation and degradation of HRP either by
blocking the transport to the later compartment or by increasing the pH of the lysosome-like compartment, which
in turn inhibits the inactivation and/or degradation of HRP.
Fig. 7.
Rab7-mediated inactivation/degradation of HRP by
oocytes. (a) Oocytes were preloaded with HRP by incubating
with HRP (2 mg/ml) for 1 h at 18°C. Preloaded cells were washed
and incubated with 100 µM monensin or control buffer as indicated for 1 h at 18°C. The cells were then injected with 100 ng of
the respective fusion proteins. HRP activity present in the cells
was measured after a 2-h incubation at 18°C. Results are expressed as percentage of control (110 ng HRP/oocyte) from the
average of eight experiments. (b) Oocytes were incubated with
125I-HRP (0.5 mg/ml; 2 × 105 cpm/µg) for 1 h at 18°C and washed
three times to remove unbound radioactivity. Subsequently, the
cells were injected with 100 ng of Rab7:WT protein or control
buffer, and nine oocytes were incubated in 300 µl of MBS for 2 h
at 18°C. Oocytes were washed and solublized in PBS containing
0.1% Triton X-100, and TCA-soluble and -precipitable radioactivity was determined in cell lysates and media. Results are expressed as ng of HRP associated with or degraded by the cells.
[View Larger Versions of these Images (17 + 26K GIF file)]
Fig. 8.
Rab5 regulation of endocytosis is upstream of Rab7. (a)
Rab5 proteins were injected alone (50 ng) or in combination with
both mutants (100 ng) of Rab5 and Rab7. (b) Rab7 proteins (50 ng)
were injected alone or in combination with both mutants (100 ng)
of Rab5 and Rab7. Results are expressed as percentage of control uptake by uninjected oocytes (average of eight experiments).
[View Larger Versions of these Images (16 + 13K GIF file)]
Fig. 9.
Rab5- and Rab7-coinjected cells produced additive effects. Oocytes were injected with 50 ng of Rab5 and Rab7 alone
or in combination and incubated for 2 h at 18°C. The cells were
incubated with 2 mg/ml of HRP for 1 h at 18°C to determine the
rate of HRP uptake. Results are expressed as percentage of uptake per oocyte with respect to control (205 ng per oocyte) ±SD
(average of eight experiments).
[View Larger Version of this Image (15K GIF file)]
Discussion
). Several characteristics of
receptor-mediated endocytosis have been explored in the
frog oocyte (Wall and Patel, 1987
), and ultrastructural and
biochemical studies have shown that intracellular transport of internalized ligands occurs via fusion between
early endocytic vesicles and consecutively enlarged endosomes (Busson et al., 1989
). In brief, these studies show
that vitellogenin, the precursor to stored yolk protein, is
taken up by receptor-mediated endocytosis. Internalized
vitellogenin receptor-ligand complexes are selectively proteolyzed and sequestered from the normal endocytic pathway with transport to yolk platelets. Selective targeting to
the storage pathway appears to be dependent both on
receptor occupancy and on selective ligand proteolysis
(Opresko and Karpf, 1987
). Fluid phase markers, on the
other hand, in the absence of vitellogenin, are routed to
the degradative pathway. In the work described in this paper, we have used HRP as a fluid phase marker.
S can impair
transport by interfering with budding or the fusion events,
consistent with the conclusion that a large repertoire of
GTPases is required for efficient endocytosis (Gomperts and Fernandez, 1985
; Mayorga et al., 1989; Goda and Pfeffer, 1988
; Damke et al., 1995
).
S and another with
aluminum fluoride, indicate that one or more GTPases are
involved in oocyte endocytosis and that GTP hydrolysis by
at least one GTPase is required. Aluminum fluoride is a
potent and reversible activator of the heterotrimeric GTP
binding proteins (Chabre, 1990
; Colombo et al., 1994
), although recent experiments (Mittal et al., 1996
) indicate
that, under certain circumstances, low molecular weight GTPases can be affected by AlF4. Heterotrimeric GTPases
have been implicated in transport along the secretory
pathway (Burgoyne, 1992
) and in endocytosis (Colombo
et al., 1992
).
; Stenmark et al., 1994
).
Rab5:Q79L has been shown to induce HRP uptake in
BHK cells (Li et al., 1994
) and to enhance fusion between
early endosomes in an in vitro reconstitution assay (Stenmark et al., 1994
; Barbieri et al., 1994
). Thus, enhanced uptake of HRP by oocytes is probably due to enhanced fusion between early endosomes. In contrast, Rab5:S34N, the dominant-negative mutant, inhibited HRP uptake.
This is in agreement with the observation that Rab5:S34N
inhibits both endocytosis, when overexpressed in mammalian cells, and in vitro endosome fusion. The inhibition of
HRP uptake by Rab5:S34N may be due to competition for
the Rab5-specific exchange factor, as suggested for rasspecific GEF (Feig and Cooper, 1988
; Hwang et al., 1993
;
Schweighoffer et al., 1993
).
; Burstein et al., 1992
). It has been shown that deletion of three or four residues from the COOH-terminal
domain of Rab5 completely abolishes prenylation. The
data presented in the Fig. 3 a show that Rab5:
C did not
significantly affect HRP uptake in oocytes. This is probably due to the fact that this protein is not prenylated and
unable to bind to target membranes. This was further confirmed by the Western blot analysis (Fig. 3 b). A band corresponding to Rab5 was detected in the membrane preparation of oocytes injected with Rab5:Q79L but not Rab5:
C4 where most of the protein was detected in the cytosol.
In vitro prenylation experiments confirmed that oocyte cytosol has the capacity, in the presence of added substrate,
to prenylate Rab GTPases (data not shown). These results
indicate that the oocyte has the ability to carry out prenylation, and that COOH-terminal isoprenylation is required
for the binding of the Rab proteins and for their biological
activity.
).
NSF and associated SNAP molecules (Wilson et al., 1992
;
Sollner et al., 1993
) are required in multiple intracellular
vesicle fusion events (Becker et al., 1989
; Diaz et al., 1989
;
Sztul et al., 1993
; Sogaard et al., 1994
). The results presented in Fig. 5 a show that anti-NSF antibody inhibits
Rab5:Q79L-induced HRP uptake in oocytes. (Western blot
experiments not shown indicate that the mAb used recognizes a frog protein of the same apparent molecular weight
as NSF.) Injection of anti-NSF into control cells produced
a modest decrease in uptake. The role of NSF was further confirmed by coinjecting wild-type NSF or NSF mutants
(D1E-Q, D1K-A) along with Rab5:Q79L. The results show
that NSF activates endocytosis of HRP mediated by Rab5
and suggest that NSF may be rate limiting in Rab5-sensitive pathways. This is the first report of activation of a
transport pathway by NSF in vivo. To confirm the role of
NSF, we used two NSF mutants that are known to block NSF function. These mutant proteins inhibited uptake, suggesting that ATP binding and hydrolysis by NSF is required for function in oocytes as in mammalian cells (Whiteheart et al., 1994
). However, why anti-NSF antibodies
effectively block Rab-induced endocytosis and yet only modestly inhibit control uptake is unclear. It is likely that endogenous NSF and endogenous Rab GTPases are more efficient than the injected mammalian counterparts. This
might explain why antibody inhibition of uptake is only
modest in control cells since the antibody would be competing with endogenous factors for binding to NSF. Similarly, exogenous Rabs might be more easily competing for
interaction with the docking and fusion machinery than
endogenous Rabs by the anti-NSF antibody.
). Similar results were also obtained in yeast using Ypt7, a yeast homologue of mammalian Rab7. Schimmoller and Riezmann (1993) have shown
that Ypt7 regulates the transport between the late endosome to the vacuole. Moreover, it has been shown that Ypt7 is also involved in the homotypic fusions between the
vacuole in the yeast (Hass et al., 1995
; Mayer et al., 1996
).
All these studies have shown that Rab7 in mammalian
cells or Ypt7 in yeast initiates the degradation of the
marker proteins by enhancing transport to a late endosome/prelysosome or lysosomal compartment. Our data
demonstrate that injected Rab7 WT stimulates HRP uptake. Rab7:T22N, the dominant-negative mutant of Rab7
(Feig et al., 1988; Nuoffer et al., 1994
; Riederer et al., 1994
),
was found to be mildly inhibitory for HRP uptake when
injected alone but was fully able to block the stimulatory
effect of Rab7. As outlined above for inhibition by antiNSF antibodies, it is likely that the exogenous mammalian
Rab7-negative mutant would more effectively antagonize
injected mammalian wild-type Rab7 than the corresponding endogenous Rab. The results presented in Fig. 7 a
show that when Rab7 was injected into HRP preloaded
cells, only 20% of the injected HRP activity was recovered
at the indicated times in Rab7:WT-injected cells compared
with controls. This rapid inactivation of HRP may be due
to loss of enzymatic activity, actual degradation, or both. In Fig. 7 b, we show that Rab7-injected cells degrade 125IHRP more rapidly than control cells, but the degradation
was not commensurate with the loss of HRP activity in
Rab7-injected cells. Thus, the loss of HRP activity in Rab7injected cells is probably due to inactivation and degradation of the marker protein. The results in Fig. 7 b also confirm that the loss of activity in Rab7-injected cells is not
due to the recycling or regurgitation of HRP. Rab7 probably mediates the rapid degradation of the marker protein
by stimulating transport to a late endosome/prelysosomal compartment (Fig. 7). Thus, kinetically, Rab7 function is
different from that of Rab5 because the latter does not
stimulate inactivation. The rapid inactivation of HRP in
Rab7-injected cells is effectively blocked by monensin.
Monensin may block the transport of the marker protein
from an early compartment to the degradative compartment, or it may inhibit the degradation of the ligand by increasing the pH of the lysosome-like compartment. The
latter is consistent with the finding that monensin blocks
the transport of markers to a later compartment described
by Opresko and Karpf (1987)
. An effect of Rab7 on transport to late compartments is further strengthened by localization of WGA-gold in large multivesicular endosomes
deep inside the oocyte when the cells were injected with
Rab7:WT. These data are consistent with that of Wall and colleagues (Wall and Patel, 1987
), who demonstrate that
multivesicular bodies are found along the endocytic pathway. One issue that remains unresolved is whether the degradation of the marker protein occurs in late endosomes or
lysosomes. GTP
S has been shown to block the transport
from late endosomes to lysosomes, implicating GTPases in
this process (Mullock et al., 1994
). It is also possible that Rab7 is involved in both processes. Interestingly, homotypic fusion of the vacuole has been shown to require Ypt7
(Hass et al., 1995
), and recently Rab7 was found, in part,
to be localized in lysosomes (Meresse et al., 1995
).
-factor via delivery to the vacuole.
A null Ypt7 mutant delayed the onset of
-factor degradation threefold, while a triple mutation Ypt51,Ypt52,Ypt53
delayed
-factor degradation more than sixfold (SingerKruger et al., 1994; Wichmann et al., 1992
). The fact that
-factor is accumulated in the triple mutant in the early
endosome, whereas the Ypt7 mutant accumulated in the
late endosome, suggested that Ypt51 function is upstream
of Ypt7 (Schimmoller and Riezman, 1993
). These conclusions are similar to our findings in oocytes where we show
that Rab7-mediated uptake is blocked by the Rab5-negative mutant, but the Rab5-mediated uptake remains unaffected in the presence of the Rab7-negative mutant. We
speculate that Rab5 fills an intracellular compartment, perhaps an early endosomal sorting compartment. Internalized solute present in the early endosomal compartment
may recycle to the cell surface or be transported to a later
compartment en route to lysosomes. Rab5 would be expected to increase the flux of ligand through the early
compartment. Rab7 stimulates transport from the early endosome to a later compartment where inactivation and/or
degradation occurs. If Rab7 were rate limiting in the pathway
and the magnitude of the Rab7 effect was dependent on the
concentration of solute or ligand present in the early endosomal compartment, Rab7 would be expected to increase endocytosis and to act additively with Rab5. The relationship
between sequentially acting Rabs, the levels of expression,
the use of common factors (e.g., GDP dissociation inhibitor),
and the regulation by respective guanine nucleotide exchange
factors poses interesting and important questions that concern the overall regulation of intracellular transport.
Received for publication 27 November 1996 and in revised form 14 December 1996.
P.D. Stahl is supported by grants from the National Institutes of Health. A. Mukhopadhyay is supported by an associateship from DBT, Government of India.We thank Rita Boshans for technical help and Mike Mueckler for introducing us to oocytes. We also thank Angela Wandinger-Ness and Marisa Colombo for critically reading the manuscript.
GST, glutathione-S-transferase; MBS, modified Barth's saline; NSF, N-ethylmaleimide-sensitive factor; WT, wild type.