From the Department of Biology, The Johns Hopkins University, Baltimore, Maryland 21218
Received for publication, November 14, 2002, and in revised form, January 6, 2003
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ABSTRACT |
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In addition to its well known role in targeting
proteins for proteasomal degradation, ubiquitin (Ub) is also involved
in promoting internalization of cell surface proteins into the
endocytic pathway. Moreover, putative Ub interaction motifs (UIMs) as
well as Ub-associated (UBA) domains have been identified in key yeast
endocytic proteins (the epsins Ent1 and Ent2, and the Eps15 homolog
Ede1). In this study, we characterized the interaction of Ub with the
Ede1 UBA domain and with the UIMs of Ent1. Our data suggest that the
UIMs and the UBA are involved in binding these proteins to biological membranes. We also show that the Ent1 ENTH domain binds to
phosphoinositides in vitro and that Ent1 NPF motifs
interact with the EH domain-containing proteins Ede1 and Pan1. Our
findings indicate that the ENTH domain interaction with membrane lipids
cooperates with the binding of membrane-associated Ub moieties. These
events may in turn favor the occurrence of other interactions, for
instance EH-NPF recognition, thus stabilizing networks of low affinity
binding partners at endocytic sites.
Endocytosis is a multistep process in which cells selectively
internalize plasma membrane proteins and lipids, as well as extracellular macromolecules such as nutrients and peptide hormones. It
is an effective way to control the composition of the plasma membrane,
and thus the physiological responses of the cell, so it is a tightly
regulated pathway. Many cytosolic proteins are required for the early
stages of clathrin-dependent endocytosis, including the
adaptor protein complex AP2, the coat protein clathrin, and more
recently, a class of proteins called accessory factors (1, 2). Because
accessory factors interact with AP2 and are required for endocytosis,
but are not enriched in purified clathrin-coated vesicles, they have
been suggested to play a regulatory rather than a structural role in
clathrin-coated vesicle formation (3). However, the precise functions,
interactions, and order of operation of most accessory factors have not
yet been determined.
One accessory factor of interest to many investigators is the protein
epsin, which is conserved from yeast to humans. Rat epsin 1 (Eps15 interactor 1) was first identified in a
yeast two-hybrid screen as a protein that binds to another endocytic
accessory factor, Eps15 (4). Epsin and eps15 are localized to endocytic sites at the plasma membrane and interact directly through the Asn-Pro-Phe (NPF) tripeptide motifs of epsin binding to the
Eps15 homology
(EH)1 domains of Eps15 (4).
The yeast epsins Ent1 and Ent2 were similarly identified as binding
partners of the EH domain-containing endocytic protein Pan1 (5, 6).
Several independent lines of evidence have indicated an important role
for epsin in the internalization step of endocytosis. First,
overexpressing fragments of epsin in cultured cells inhibits
internalization of ligands such as transferrin and epidermal growth
factor (4, 7, 8). Second, mutation of the yeast epsins results in
growth defects and reduced internalization of plasma membrane lipids
and proteins (6, 9, 10). Third, Drosophila melanogaster
epsin (liquid facets) exhibits genetic interactions with
genes encoding other endocytic machinery components (11). Fourth,
in vitro studies show that epsin can stimulate clathrin
polymerization into the spherical basket that encases endocytic
vesicles (12). Finally, recent work suggests that epsin binding to
phospholipids initiates the membrane curvature necessary to form an
endocytic vesicle (13).
Epsins from all species share several features (Fig.
1A), including a conserved
epsin amino-terminal homology (ENTH) domain that binds the phospholipid
phosphatidylinositol-4,5-bisphosphate (PI(4,5)P2), two or
three copies of the ubiquitin interaction motif (UIM, Fig.
1A) that binds ubiquitin (Ub), and a carboxyl-terminal region containing numerous short linear peptide sequences that serve as
ligands for binding to components of the endocytic machinery such as
AP2 (mammalian epsins), EH domains, and clathrin (Fig. 1A;
Refs. 4, 6, 7, 10, and 14). To understand the function of such
multivalent proteins, it is necessary to characterize the nature of
each interaction in isolation as well as in the context of the entire
protein, where cooperativity may influence some of its properties. In
this study, we have defined the binding partners for each of the
characterized domains of the yeast epsin Ent1, using in
vitro recombinant protein binding, yeast two hybrid, and
immobilized phospholipid binding experiments.
INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
REFERENCES
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Fig. 1.
The bacterially produced yeast epsin Ent1
interacts with ubiquitin through its UIMs in a cooperative manner.
A, Ent1 domain organization. The scheme depicts
the multimodular domain architecture of the yeast epsin Ent1 (from
amino to carboxyl terminus): ENTH domain, UIMs, NPF tripeptides, and a
CBD. B, Ent1 binds ubiquitin. Agarose beads were
incubated with purified His6-Ent1 for 1 h at 4 °C.
After washing, bound His6-tagged protein was visualized by
chemiluminescent Western blotting with an anti-His6
antibody. Starting material (lane 1) and fractions bound to
Ub-agarose beads in the presence or absence of 10 mM Ub
(lanes 3 and 2, respectively). C, Ent1
interaction with Ub is mediated by its UIMs in a cooperative fashion.
Ub-agarose beads were incubated with 35S-labeled Ent1 WT
and UIM-double and single S D mutants (see text for details) for
1 h at 4 °C and washed, and the bound radioactivity (in cpm)
was measured in a scintillation counter. Values are the means ± S.D. of triplicates from one of three independent experiments.
Statistical significance in comparison with binding showed by WT Ent1
is indicated by ** (p < 0.005).
The yeast epsins are almost exclusively localized in cortical patches
at the plasma membrane (6, 9), but the cis- and trans-acting components
necessary for their membrane association have not been determined. Here
we show that Ub covalently attached to transmembrane proteins (often a
requirement for internalization; reviewed in Ref. 15) or to peripheral
membrane proteins mediates the recruitment of Ent1 to membranes. By
examining the requirements for recruitment of Ent1 to biological
membranes, we have uncovered evidence for cooperativity between the
ENTH domain and the UIMs for efficient membrane binding. Other data
indicate that the NPF-containing regions of Ent1 also contribute to
additional layers of cooperative interactions. Together, our data are
consistent with a model in which epsin might function as an endocytic
adaptor protein. We suggest that epsin binds
PI(4,5)P2-enriched membrane domains where it recruits
components of the endocytic machinery and perhaps selects certain
ubiquitinated transmembrane proteins for inclusion into endocytic vesicles.
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EXPERIMENTAL PROCEDURES |
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Recombinant DNA Constructs-- The Gal4AD and BD fusion constructs were prepared by ligating the appropriate PCR fragments (digested with BamHI-SalI) into the BamHI-XhoI and BamHI-SalI sites of the pGADT7 (LEU2, Clontech) and pGBT9 (TRP1, Clontech) vectors, respectively. To prepare His6-tagged and GST fusion proteins, cDNAs were subcloned into the pET28a (Novagen) or pGEX-4t-1 (Amersham Biosciences) vectors, respectively. Amino acid substitutions were made using the QuikChange XL site-directed mutagenesis kit (Stratagene, La Jolla, CA) according to the manufacturer's instructions. Murine p53 in pAS2-1 and SV40 large T-antigen in pACT2 were obtained from Clontech.
Ubiquitin and the terminal domain of clathrin cloned in pGEX-2t-1 were kindly provided by Dr. Scott Emr's lab (UCSD) and Dr. Peter McPherson (McGill University), respectively.
Two-hybrid Assays--
The yeast strain AH 109 (MATa, trp1-901,
leu2-3, 112, ura3-52, his3-200, gal4, gal80
,
LYS2::GAL1UAS- GAL1TATA-HIS3,
GAL2UAS-GAL2TATA-ADE2, URA3::MEL1UAS- MEL1TATA-lacZ,
Clontech) was maintained on YPD agar plates.
Transformations were performed according to the
Clontech's Yeast Protocols Handbook.
Recombinant His6-tagged and GST Fusion Protein
Expression and Purification--
Protein expression was induced in
Rosetta cells (Novagen) transformed with pET28a- or
pGEX-derived constructs by the addition of
isopropyl--D-thiogalactopyranoside (1 mM
final concentration) followed by a 5-h incubation at 30 °C. The
cells were harvested, resuspended in TBST (10 mM Tris, 140 mM NaCl, 0.1% Tween 20, pH 8.0), and incubated with 1 mg/ml lysozyme (Sigma) for 30 min at 4 °C and sonicated. The lysates
were centrifuged for 30 min at 15,000 rpm at 4 °C in a Sorvall
centrifuge, and the supernatants were collected.
For His6-tagged protein purification, the lysates were incubated in batch with nickel-nitrilotriacetic acid Superflow resin (Qiagen, Valencia, CA) in the presence of 5 mM imidazole (Sigma) for 1 h at 4 °C. The unbound fraction was separated by centrifugation, and the resin containing the bound His6-tagged protein was packed into a disposable polypropylene column (Pierce). The column was washed with TBST/15 mM imidazole, and the protein was then eluted with TBST/150 mM imidazole, and fractions containing the His6-tagged protein were pooled. The imidazole from the His6-tagged protein and the glutathione from the GST fusion protein fractions were eliminated by using a PD-10 desalting column (Amersham Biosciences).
For GST fusion protein purification, the lysates were incubated in batch with agarose beads coupled to glutathione for 2 h at 4 °C and then washed 4 times with TBST. The beads were then incubated with 20 mM glutathione (Sigma) in TBST for 30 min at room temperature, and the supernatant containing the purified fusion protein was collected.
Yeast Membrane Preparation--
RL120 cells (MATa
lys2-801 leu2-3 112 ura3-52 his3-200 trp1-1
ubi1::TRP1
ubi2-
::ura3 ubi3-
ub2
ubi4-
2::LEU2) expressing copper
promoter-driven His-Ub (p6209) (described in Ref. 16) or
pep4
cells (MAT
leu2-3 ura3-52
his3-
200 trp1-
901 lys2-ho1 suc2
9 pep4
::LEU2)
expressing copper promoter-driven myc-tagged ubiquitin were grown
overnight in selective medium to mid-log phase and induced with 0.1 M CuSO4 for 3 h. Twenty to fifty
A600 (1 A600 nm
corresponds to ~1-2 × 107 cells) were harvested
(200 × g) and incubated for 10 min at room temperature
in softening solution (Tris, pH 9.4, 1 mM DTT). Cells were
harvested and spheroplasted in YPD medium containing 1 M sorbitol and 1µl of zymolyase per A600 for 30 min at 30 °C. Spheroplasts were washed once, and pellets were
Dounce-homogenized in HEPES/KOAc lysis buffer containing 50 mM N-ethylmaleimide (Sigma) and protease inhibitor mixture (Roche, Indianapolis, IN). Homogenates were spun at
300 × g for 5 min, and the resulting pellets were
stored at
80 °C before use in membrane-binding experiments.
GST and Ub-Agarose Pull-down Assays-- All binding assays were incubated for 1 h at 4 °C in the presence or absence of 10 mM free ubiquitin (Sigma). For GST pull-downs, glutathione-agarose beads (Sigma) loaded with either GST or GST fusion proteins were incubated with purified His6-tagged recombinant protein. For Ub binding, Ub coupled to agarose beads (Sigma) was incubated with either purified His6-tagged recombinant protein or 35S-labeled in vitro transcription/translation products. After washing with TBST, bound radiolabeled samples were quantified in a scintillation counter, whereas all other samples were boiled in Laemmli's protein sample buffer and resolved by SDS-PAGE followed by immunoblotting using either a rabbit polyclonal antibody raised against the Ent1 protein (previously described in Ref. 9) or an anti-His6 tag mouse monoclonal antibody (Clontech).
In Vitro Transcription/Translation and Binding of the in Vitro-transcribed/translated Products to Yeast Membranes-- 35S-Labeled proteins were obtained by in vitro transcription/translation of pET28a constructs using the TNT T7 Quick coupled transcription/translation system (Promega, Madison, WI) and Easytag expression protein labeling mixture (PerkinElmer Life Sciences) according to the manufacturer's instructions. The transcription/translation products were diluted 1:100 in TBS, centrifuged (136,000 × g, 1 h, 4 °C), and their specific activity was determined by a conventional trichloroacetic acid precipitation assay (only tracers showing at least 10% incorporation were used for further experiments). The predominance of a single radiolabeled species was verified by SDS-PAGE followed by autoradiography. The concentration of radioactive material for different mutants was normalized according to 35S incorporation and the number of methionines present in the corresponding Ent1 truncations.
3-10 × 106 cpm of 35S-labeled proteins were incubated with membrane suspensions prepared from 1 A600 of cells in a total volume of 300 µl, for 1 h at either room temperature or 4 °C, and in the presence or absence of 10 mM purified ubiquitin. After washing, membrane associated radioactivity was measured in a scintillation counter. Experiments were conducted in triplicate, and their S.D. was calculated. Slightly different washing procedures resulted in some variability in background between experiments. Another source of variability may be caused by differences in the initial status and in the preservation (from batch to batch and time of storage) of the two major reagents used in these assays: (i) the membrane suspensions (composition of phosphoinositides and ubiquitinated proteins); (ii) the 35S-labeled proteins (concentration of binding-competent species). Each experiment was performed two to four times (as indicated in the corresponding figure legend) and always showed similar trends.
Binding of Recombinant Proteins to Phosphoinositides Immobilized onto Nitrocellulose-- Nitrocellulose membranes containing immobilized phospholipids (PIP-Strips membranes, Echelon, Salt Lake City, UT), blocked with TBST with 3% fatty acid-free bovine serum albumin (Sigma), were incubated with purified His6-tagged recombinant protein for 1 h at room temperature. The bound proteins were detected by immunoblotting using an anti-His6 tag mouse monoclonal antibody (Clontech).
Rabbit Antisera--
Anti-Ent1/2 serum was described (9).
Anti-Ub serum was a gift of Cecile Pickart (The Johns Hopkins Bloomberg
School of Public Health). Anti-Ede1 serum was produced in our lab
against a peptide and preincubated with extract from ede1 cells.
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RESULTS |
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Ent1 Binds Ub via Its UIMs in a Cooperative Fashion-- It has been shown previously that Ent1 binds Ub via its UIMs (10). We confirmed and further characterized the specificity of this interaction. In an in vitro pull-down assay, we found that recombinant His6-Ent1 protein bound Ub-agarose beads as assessed by immunodetection of starting and bound material with an anti-His6 antibody (Fig. 1B, lanes 1 and 2). Importantly, this interaction was specifically inhibited by the presence of an excess (10 mM) of free Ub (Fig. 1B, lane 3).
We next investigated whether both UIMs are necessary for the
recognition of Ub. Both conserved serines within the Ent1 putative UIMs
(Ser177 and Ser201) were mutated to
aspartate, glutamate, lysine, arginine, or cysteine residues, which
were selected based on random permutation probabilities according to
Dayhoff's PAM250 matrix (i.e. the residue substitutions least likely to cause structural perturbation in the protein). 35S-labeled His6-Ent1 wild type, single, and
double mutants were produced by in vitro
transcription/translation, incubated with Ub-conjugated agarose beads,
and bound radioactivity was quantified. Compared with Ent1 WT, all five
of the Ent1 UIM double mutants exhibited significantly impaired
ubiquitin binding (data not shown), and as demonstrated by the 34%
reduction for the S D double mutant (Fig. 1C). Also,
mutation of a single conserved serine within either UIM decreased the
binding of the mutant proteins for Ub-agarose beads by at least 50%
(Fig. 1C). These results suggest that the two UIMs cooperate
for the interaction of Ent1 with Ub.
Ent1 Binds Yeast Membranes in a Ub- and UIM-dependent
and Cooperative Fashion--
It has been demonstrated that certain
cell-surface transmembrane proteins, such as Ste2 and Ste6, are
ubiquitinated as a prerequisite to their internalization (17, 18).
Thus, it is possible that these and/or other Ub-modified proteins such
as peripheral membrane proteins may act as docking sites for Ub-binding
proteins such as Ent1. To test this possibility, we prepared a crude
membrane fraction from yeast that contains plasma membrane, which is
the major localization site of many endocytic proteins in
vivo (Fig. 2A), including
the yeast epsins (6). We confirmed by Western blotting that these
membranes contained an abundance of Ub-conjugated proteins (Fig.
2A), and tested whether these membranes could recruit recombinant His6-Ent1 in a Ub-dependent
manner.
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Purified His6-Ent1 was incubated with a suspension of
membranes for 1 h at 4 °C in the presence or absence of 10 mM free Ub. After washing, the presence of the recombinant
protein associated with the membrane pellet was assessed by SDS-PAGE,
followed by Western blotting with an anti-His6 antibody.
Our results showed that His6-Ent1 interacts with membranes
prepared from either MATa (Fig. 2B,
lane 1) or MAT cells (Fig. 2B,
lane 3). 10 mM free Ub greatly reduced this
membrane recruitment (Fig. 2B, lanes 2 and
4). We also observed that even in cases where our
His6-Ent1 preparations contained some protein degradation
products (some as abundant as the intact protein), only full-length
Ent1 was recruited to the yeast membranes. Additionally, an excess of
yeast cytosolic proteins did not compete the interaction (data not shown).
To characterize further this membrane association process, we studied
the recruitment of Ent1 UIM mutants. For these experiments, 35S-labeled in vitro transcription/translation
products were incubated with membrane suspensions for 1 h at
4 °C, and after washing, the radioactivity associated with the
membrane pellet was quantified. As shown in Fig. 2C, and
consistent with their reduced binding to Ub-agarose, the S D single
and double mutants exhibited at least 50% reduced recruitment to
membranes as compared with WT Ent1. We observed reduced membrane
binding with all five UIM double mutants (S
D, S
E, S
K, S
R, and S
C), and the reduction was comparable with the binding
of WT Ent1 to membranes in the presence of free Ub (ranging from 40%
to 60% inhibition, depending on the experiment). The remaining bound
material may be nonspecific binding or, alternatively, could represent
a Ub-independent component of membrane recruitment (see below). The
finding that a single mutation introduced into either the first or
second UIM of Ent1 was sufficient to drastically decrease the membrane
binding capability of the protein is again consistent with cooperative
binding of the two UIMs to Ub.
We also carried out saturation experiments by incubating increasing concentrations of 35S-labeled Ent1 with a constant amount of membrane in the presence or absence of free Ub (data not shown). Although we were not able to confirm that binding equilibrium was achieved after 1 h at 4 °C, we roughly estimated an avidity dissociation constant of ~20 µM for each UIM.
The ENTH Domain Cooperates with the UIMs for Membrane
Recruitment--
The ENTH domains of mammalian epsins bind
phosphoinositides (7); therefore, we tested whether the Ent1 ENTH
domain also interacts with lipids, thus possibly contributing to Ent1
membrane association. Purified His6-tagged proteins plus
3% fatty acid-free bovine serum albumin were incubated with
nitrocellulose strips containing a variety of immobilized
phospholipids. After washing, the bound proteins were detected using an
anti-His6 antibody and visualized by chemiluminescence.
Both His6-Ent11-454 (full-length) and
His6-Ent11-149 (ENTH domain) bound
phosphoinositides immobilized onto nitrocellulose (Fig.
3A). As expected, an Ent1
amino-terminal truncation lacking the ENTH domain
(Ent1149-454) did not exhibit phospholipid binding
activity (Fig. 3A). It should be noted that, because of the
qualitative nature of this binding assay, slight differences observed
between full-length and the ENTH domain binding were considered
insignificant.
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Given that the Ent1 ENTH domain binds phosphoinositides and its UIMs bind ubiquitin, we next explored a possible interplay between the UIMs and the ENTH domain for membrane recruitment. To this end, we prepared two Ent1 truncations (see Fig. 1A), one lacking the first 148 amino acids that comprise the ENTH domain (Ent1149-454, containing both UIMs plus the carboxyl-terminal part of the protein) and another consisting of amino-acids 1-240 (Ent11-240, containing the ENTH domain plus both UIMs and a putative coiled coil region).
We assayed the binding of the Ent11-240 and Ent1149-454 truncations at room temperature rather than 4 °C to preserve physiological membrane characteristics such as membrane lipid fluidity. As before, 35S-labeled in vitro transcription/translation products were incubated with membranes, and the membrane-associated radioactivity was quantified. Each Ent1 truncation bound the membranes, consistent with Ub-dependent recruitment. However, we found that Ent11-240 bound membranes to a greater extent than Ent1149-454 (Fig. 3B). This suggested that, in addition to the UIMs, the ENTH domain also contributed to the membrane recruitment of Ent1. Moreover, we also found that the highly phosphorylated soluble inositol phosphate IP6 (phytic acid) that binds ENTH domains (13, 19) was able to reduce significantly (by 20-40%) some of the membrane recruitment of Ent11-240 (Fig. 3B). In contrast, efficient membrane recruitment of Ent11-240 was still observed on incubation with IS6 (myoinositol hexasulfate) (Fig. 3B). Membrane recruitment of the Ent1149-454 truncation was the same in the presence of either IP6 or IS6 (Fig. 3B). These results further support the idea that the interaction of the Ent1 ENTH domain with phosphoinositides plays a role in its membrane association. It should be noted that the precise phosphoinositide content of these membranes was not characterized.
When studied at 4 °C, both Ent11-240 and Ent1149-454 truncated proteins showed levels of membrane recruitment similar to full-length Ent1 (data not shown), which suggests a predominant role for the UIMs (common to both truncations, see Fig. 1A) in membrane recruitment at low temperature. It is possible that when the binding to membranes is instead conducted at 4 °C, the ENTH domain interaction with lipids may be disfavored because of membrane rigidity and weakened entropic contributions of the hydrophobic contacts. Taken together, these data suggest that, whereas the contribution of the UIM-Ub interaction to the overall membrane binding is favored at 4 °C, the ENTH domain provides a significant contribution to membrane binding at room temperature, which is more similar to physiological conditions, and may be more akin to what occurs in vivo.
The UBA Domain of Ede1 Binds Yeast Membranes in a
Ub-dependent Manner--
Ede1 is an endocytic protein (the
yeast homolog of eps15) with a multimodular domain architecture,
including 3 EH domains and a UBA domain (14, 20). Because the Ede1 UBA
domain binds Ub (10),2 and
given that this protein is also found almost exclusively associated with membranes (20),2 we tested whether the Ede1
UBA domain interacts with our membrane preparations similarly to the
Ent1 UIMs. As shown in Fig.
4B, the UBA domain of Ede1
(residues 1217-1382) was able to bind biological membranes, and in two
independent experiments this recruitment was impaired (30%
inhibition in the experiments shown) by the presence of free Ub. These
experiments required the use of high concentrations of
35S-labeled UBA domain and membranes to detect significant
binding, which could suggest a weaker interaction than that of the
UIM/Ub complex. However, by analogy to the cooperative membrane binding observed among different domains of Ent1, it is possible that full-length Ede1 may bind membranes more efficiently than its UBA
domain alone.
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Ent1 Is Involved in a Network of Interactions with Other Endocytic
Proteins That Localize to Membranes--
Ede1 exhibits a cortical
patch membrane localization pattern similar to that of yeast epsins (6,
9, 20, 21).2 It is possible that Ede1 and Ent1 colocalize
through recognition of ubiquitinated membrane proteins. Alternatively,
they could colocalize through an EH/NPF interaction, because the yeast
epsins were originally identified in a yeast two-hybrid screen with the EH domains of Pan1 (5, 6). We found that Ent1 is indeed able to
interact in a two-hybrid system with Ede1 EH domains, specifically with
the third EH domain (EH3), and not the first or second EH domains (Fig.
5A). The carboxyl-terminal
Ent1149-454 protein was sufficient to bind the Ede1 EH3
domain, consistent with the NPF motifs being the binding determinants
involved in this interaction (Fig. 5A). In fact, we
established that this interaction is indeed mediated by Ent1 NPF
motifs, as mutation of both NPF motifs to NPM abolished the binding
(Fig. 5B). Mutation of a single NPF motif decreased but did
not abrogate the EH domain-dependent recognition,
suggesting that each Ent1 NPF motif participated in the interaction
with Ede1 (Fig. 5B). Interestingly, we also found that the
Ent1 interaction with Pan1 is much weaker than with Ede1 (Fig.
5A), suggesting a preference for Ede1 over Pan1.
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To confirm the Ent1/Ede1 interaction, we performed GST pull-down
experiments using a GST-EH3 fusion protein. As shown in Fig. 5C, His6-Ent1 bound Ede1 EH3, whereas mutation
of the first NPF motif to NPM significantly decreased the interaction.
We previously showed that Ent1 binds clathrin through a
carboxyl-terminal clathrin-binding domain (CBD) (6), and this
experiment also confirmed that the Ent1 CBD binds the amino-terminal
domain of the clathrin heavy chain (22) (Fig. 5C). Deleting
the last four residues of Ent1 that constitute the CBD resulted in no
binding of the clathrin amino-terminal domain, suggesting that the
carboxyl-terminal CBD is the unique CBD present in Ent1. GST and GST-Ub
fusion proteins were used as negative and positive controls,
respectively (Fig. 5C).
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DISCUSSION |
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The mechanisms mediating the recruitment of the endocytic
molecular machinery at sites where internalization takes place are not
completely understood. In this study, we present evidence suggesting
that the yeast epsin Ent1, an integral part of the endocytic machinery,
is recruited to biological membranes via multiple, cooperative
interactions with lipids and proteins through its numerous binding
domains. Importantly, we show that biological membranes containing
plasma membrane proteins, endocytic machinery, and ubiquitinated
proteins can recruit epsins and Ede1. It has not yet been clearly
determined which specific ubiquitinated proteins are the targets in
these membranes; however, an attractive possibility would be membrane
receptors and/or ubiquitinated endocytic machinery. In fact,
preliminary evidence from cross-linking His6-Ent1 to MAT membranes indicated that ubiquitinated Ste3 is one of
the interacting partners for Ent1.2
Many studies have uncovered roles for ubiquitination in the endocytosis
and lysosomal/vacuolar degradation of membrane receptors (reviewed in
Ref. 15), which may be facilitated by Ub binding to some endocytic
proteins (10, 14, 23). We have demonstrated that Ent1 binds Ub in a
UIM-dependent manner, and that, similarly to Vps27 (10),
the two Ent1 UIMs cooperate in this interaction. Whether each UIM binds
to different Ub moieties to generate a stable multivalent-complex or
instead both UIMs together bind a single Ub unit requires further
investigation. A UIM is predicted to form an -helix (14), and other
Ub-binding domains such as UBA and CUE domains are composed of three
-helices. In fact, the CUE domain binds Ub via an interface of two
-helices,3 so it is
possible that two UIMs might similarly join to form a binding site for
a single Ub.
We propose that at 4 °C, Ent1 recruitment to membranes is primarily
UIM-dependent, as binding is decreased (from 40 to 60%) either by addition of free ubiquitin or by mutation of the UIMs. Similarly to the cooperative Ub/UIM interaction we observed with recombinant protein, our studies showed that the individual UIMs cooperate for membrane association. Analysis of the membrane binding data, considering the individual contributions of each UIM bound to Ub ([UIM1·Ub], [UIM2·Ub],
and [UIM1·Ub·UIM2] complexes),
gave an estimated apparent dissociation constant of ~20
µM, which is similar to the binding affinity of Ub to the
single UIM present in the mammalian protein Hrs (24). However, a
remarkable consequence of the UIM-bivalency of Ent1 is that the
effective overall avidity of the protein for membrane-linked Ub falls
in the 108 M range (data not shown).
Additionally, the apparently high affinity of Ent1 for membranes could
represent Ent1 cooperativity, as a Hill coefficient significantly
greater than 1 (nH = 1.78 ± 0.04) was
calculated from membrane binding experiments and Ent1/Ent1 interactions
were observed in a yeast two-hybrid study.2
Because a wild-type ENTH domain of yeast epsin is required for endocytosis and viability in yeast (6), ENTH domain-binding activity may be most central to the endocytic function of epsin. Previous studies have shown that ENTH domains bind phosphoinositides (7), and we have confirmed this for the Ent1 ENTH domain. Thus, it was surprising to find that a major contribution to membrane binding is provided by the recognition of Ub by the Ent1 UIMs (at 4 °C). These results suggest that alteration of membrane biophysical properties due to lipid phase transitions (likely to occur at temperatures below 20 °C) and/or the weakening temperature-dependent entropic interactions (the primary driving force for hydrophobic contacts) would result in unfavorable conditions for ENTH domain binding to membranes at 4 °C. Supporting this interpretation, our experiments conducted at room temperature indicated that the ENTH domain not only binds lipids, but also cooperates with the UIMs for binding to membranes. The order of events for ENTH domain- and UIM-mediated recruitment of Ent1 to sites of endocytosis remains to be determined. However, one possibility is that the ENTH domain localizes the epsins to phosphoinositide-rich plasma membrane domains, whereas the UIMs provide specific targeting to ubiquitinated transmembrane and/or peripherally associated proteins.
It should be noted that our recruitment to membranes assay using radiolabeled protein resulted in a degree of background variability for each experiment, although all experiments showed similar statistically significant trends. We are currently working on optimizing reconstitution assays using liposomes and/or noninvasive in vivo techniques. However, these membrane binding experiments do have the advantage of using biological materials that provide an indication of in vivo interactions.
We have shown Ub-dependent membrane recruitment of the Ede1 UBA domain, suggesting that Ede1 may also bind ubiquitinated membrane-associated proteins. We also found that Ede1 and Ent1 bind one another through a synergistic EH/NPF interaction, which illustrates a recurring theme of cooperativity for Ent1 interactions. Because an Ent1 amino-terminal truncation (Ent1149-454) still bound membranes (Fig. 3B), it is possible that EH domain-containing proteins like Ede1 may also recruit Ent1 to membranes via EH/NPF interactions, independently of Ent1 binding to lipids.
Additionally, and as expected, the Ent1 carboxyl-terminal CBD was found
to be sufficient for binding to the terminal domain of clathrin.
Because a yeast strain lacking the five protein complexes previously
recognized as homologues of clathrin assembly factors and/or adaptors
(deleted for genes encoding yAP180A, yAP180B, and the -subunits of
AP1, AP2, and AP3; Ref. 25) is viable, can perform endocytosis, and
form clathrin-coated vesicles, it is clear that other factors remain to
support these activities. Epsin is an obvious candidate, particularly
in light of the recent demonstration that rat epsin can stimulate
clathrin cage assembly (12).
Based on our results, we propose a model (Fig.
6) in which Ent1 is recruited to nascent
endocytic sites as a combined result of its lipid-membrane binding
property (through its ENTH domain) and its UIM-mediated specificity for
ubiquitinated proteins, depicted here as cargo. This could be
followed by the recruitment of Ede1 that may also interact with cargo
and/or covalently linked Ub moieties via its UBA domain and
cooperatively bind Ent1 NPFs through its EH domain. In turn, other
accessory proteins could bind to both Ent1 and Ede1 and the endocytic
coat would be assembled by interaction with the Ent1 CBD, among other
clathrin-binding determinants. Therefore, we suggest that the epsins
may play a key role as adaptors linking ubiquitinated cargoes to
the endocytic machinery, recruiting proteins with specific activities
and/or exerting undiscovered functions by themselves that make
endocytosis possible.
|
One interesting prediction of our model is that multiple mutations would be necessary to affect significantly the function/localization of the epsins. In agreement with this hypothesis, neither mutation nor deletion of the Ent1 UIMs is sufficient to render a phenotype in yeast cells expressing the mutant protein as the exclusive source of epsin (10).2 Our model predicts that such UIM mutants would still be able to properly localize at endocytic sites via their ENTH and NPF interactions, consistent with the finding by Shih et al. (10) that both deletion of Ede1 and Ent1 UIM inactivation are required for a significant endocytic defect in vivo. In fact, mutating the ENTH domain is the only known case in which single-domain mutations produce a phenotype (6). This is consistent with the findings of Ford et al. (13) and supports our model in which epsin binds to membranes via its ENTH domain as an early and required step for membrane localization.
We also expect Ent1 recruitment to membranes to be highly regulated by reversible post-translational modifications such as phosphorylation (9, 23, 26) that may influence the stability of the endocytic network. For example, phosphorylation of Ent1 Thr394 and Thr416, flanking the second NPF motif, has been shown to affect the endocytic function of Ent1 (9). Consistent with studies of phosphoregulation of endocytic complex function in nerve terminals (25), our preliminary data indicate that this modification impairs the Ent1/Ede1 interaction.2 On the other hand, it was recently demonstrated that UIM-containing proteins can be ubiquitinated in an UIM-dependent manner (23). Although the functional consequences of covalent Ub modification of epsins are unknown, it is possible that an intramolecular Ub/UIMs interaction may compete for the recognition of Ub linked to cargo by the UIMs (27).
In summary, we provide evidence suggesting that the epsins, as well as
other proteins such as Ede1, are recruited to membranes through a
network stabilized by cooperative interactions throughout their
multiple modular domain architecture. Future in vitro
reconstitution studies will be necessary to determine the order of
action of these and other proteins in initiating and assembling the
endocytic machinery.
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ACKNOWLEDGEMENTS |
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We thank Catharine Sciambi for excellent technical assistance. We also thank Pietro De Camilli, David Katzmann, Douglas Fambrough, and Jon Shaw for critical reading of the manuscript and members of the Wendland Laboratory for helpful comments and suggestions throughout the course of the work.
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FOOTNOTES |
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* This work was partially supported by grants from the National Institutes of Health (R01 GM60979) and the Human Frontiers Scientific Program.The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Supported by a Ford Foundation Dissertation Fellowship.
§ Burroughs Wellcome New Investigator in the Pharmacological Sciences. To whom correspondence should be addressed: Dept. of Biology, The Johns Hopkins University, Mudd Hall, Rm. 35, 3400 N. Charles St., Baltimore, MD 21218. Tel.: 410-516-0460; Fax: 410-516-5213; E-mail: bwendland@jhu.edu.
Published, JBC Papers in Press, January 14, 2003, DOI 10.1074/jbc.M211622200
2 R. C. Aguilar, H. A. Watson, and B. Wendland, unpublished results.
3 J. Hurley, personal communication.
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ABBREVIATIONS |
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The abbreviations used are: EH, eps15 homology; Ub, ubiquitin; WT, wild type; GAL4bd, GAL4 DNA-binding domain; GAL4ad, GAL4 transcription activation domain; ENTH, epsin amino-terminal homology domain; UIM, ubiquitin interaction motif; UBA, ubiquitin-associated domain; NPF, asparagine-proline-phenylalanine tripeptide; PI(4, 5)P2, phosphatidylinositol (4,5) bisphosphate; GST, glutathione S-transferase; CBD, clathrin-binding domain.
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