(Received for publication, May 10, 1995; and in revised form, August 11, 1995)
From the
Dynamin is a GTP-binding protein that is involved in the release
of coated endocytic vesicles from the plasma membrane. We have been
characterizing the enzymatic properties of purified rat brain dynamin
to better understand how GTP binding and hydrolysis relate to its
proposed function. Previously, we have demonstrated that activation of
dynamin GTPase results from positive cooperative associations between
dynamin molecules as they are bound to a polymeric surface. Our present
report has extended these studies and has examined the structural
features of dynamin self-association. After treatment with the
zero-length protein cross-linking reagent,
1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide, dynamin in
solution was found cross-linked into dimers. This homodimer likely
reflects the native soluble state of the molecule. After binding to
brain vesicles, dynamin was cross-linked into higher order oligomers of
greater than 800 kDa. Dynamin, copurified on brain membranous
organelles, also formed multimeric complexes when cross-linked
suggesting dynamin exists in polymeric form in vivo. No
cross-linked species other than homo-oligomers were observed, providing
no evidence for close interactions between dynamin and membrane
proteins. From experiments examining the effects of GTP, GDP, guanosine
5`-3-O-(thio)triphosphate, and
5`-guanylyl-,
-imidodiphosphate on cross-linking, we have
determined that both dynamin membrane binding and self-association
occur independently from the nucleotide-bound state of the enzyme. An
80-kDa dynamin fragment that is lacking its carboxyl-terminal domain is
not cross-linked into higher order oligomers, suggesting that this
domain is required for binding of dynamin to membranes and the
subsequent enhancement of oligomerization. However, the dynamin
fragment was found to form dimers indicating that this domain is not
required for dynamin dimerization. Cross-linked dynamin was able to
cooperatively bind microtubules, but did not exhibit GTPase activation.
We propose that intramolecular cross-links in the dynamin monomer
impart structural constraints that prevent the enhancement of GTP
hydrolysis. We describe a model of the dynamin activation process to be
considered in further investigations of the role for dynamin in
endocytic vesicle formation.
Dynamin is a GTP-binding protein that functions in the early stages of endocytosis. In particular, it is thought to be required for endocytic coated vesicle formation and budding. Recent reports have suggested that dynamin participates in this process by catalyzing a GTP-dependent fission reaction at the necks of invaginated coated pits that results in the release of coated vesicles from the plasma membrane(1, 2) . Mutational analyses of the dynamin GTP-binding site have indicated that GTPase activity is required for in vivo function(2, 3, 4) . However, it is not clear how dynamin's enzymatic properties are related to its proposed activity in endocytic vesicle formation. We have been investigating the enzymatic properties of purified rat brain dynamin in order to define parameters affecting GTP hydrolysis. Our goal is to identify and characterize the agents which function as physiological regulators of dynamin activity.
Dynamin GTP hydrolysis rates are accelerated when in the presence of a diverse set of agents including microtubules(5, 6) , acidic phospholipid liposomes(6) , rat brain vesicles(6) , and certain SH3-domain proteins(7, 8) . Dynamin associates with each of these agents via its 100-amino acid carboxyl-terminal domain(6, 8) . However, binding of rat brain dynamin to acidic phospholipids or microtubules does not stimulate GTPase activity directly. Rather, binding these agents promotes cooperative associations between dynamin molecules leading to activation of GTP hydrolysis(9, 10) . We have determined that at least two dynamin molecules, each of which is a homodimer, must self-associate for optimal GTPase activity to occur(9) . Each of the activating agents listed above provide a multivalent surface to which dynamin binds that promotes cooperative associations between dynamin molecules.
It is not clear with which agents (if any) dynamin associates in vivo. Although dynamin is routinely purified from brain preparations by exploiting its affinity for microtubules(6, 11) , dynamin has not been shown to colocalize with microtubules in the cell. Instead, biochemical and immunolocalization studies have indicated dynamin plasma membrane associations(2, 3, 4, 6, 12, 13) . Whether dynamin binds the membrane directly through interactions with acidic phospholipids or binding is mediated by other proteins (SH3-domain or clathrin coat proteins?) is not known.
To further complicate our understanding of the regulation of the dynamin GTP hydrolytic cycle, it has been demonstrated that activation of dynamin in the presence of acidic phospholipids, rat brain vesicles, microtubules, or Grb2 does not display Michaelis-Menten kinetics(6, 8) . Although positive cooperative interactions between dynamin molecules result in optimal activation, it is not known what features of dynamin's interactions with itself or with the binding substrates accounts for the non-classical behavior observed. It is also highly likely that additional regulatory aspects of dynamin function remain to be identified.
In this study we have further defined properties of dynamin GTP binding and hydrolysis. In particular, we have examined dynamin self-association in the presence and absence of activating agents and have determined that dynamin is self-associated when bound to membranous organelles. We have found that dynamin membrane binding and cooperative associations occur independently from nucleotide binding. We have also examined dynamin intra- and intermolecular interactions that are important in the stimulation of dynamin GTPase. We provide a structural model that describes the features of the activation process that must be strongly considered as we continue our investigation of the role dynamin GTP hydrolysis plays in endocytosis.
The extent of dynamin self-association in solution, in the absence of lipid vesicles, was limited (Fig. 1). Within 5 min of EDC treatment, dynamin dimers (200-kDa immunoreactive species) were formed and, after 60 min, were the prominent oligomers detected. Higher order cross-linked species were also observed, but only after 30-60 min of EDC treatment and in much lower amounts (Fig. 1). In the presence of brain vesicles, the extent of dynamin oligomerization was enhanced (Fig. 1). As was observed for dynamin in the absence of brain vesicles, dimers were the prominent cross-linked species detected after 5 min of EDC treatment. However, the formation of dynamin multimeric complexes was greatly accelerated. Within 15 min of EDC treatment, species of 300 and 400 kDa were detected, and by 60 min, nearly all dynamin was detected in complexes of more than 400 kDa. Examination of these complexes on high resolving phosphate gels revealed cross-linked dynamin complexes in multiples of 100 kDa, up to greater than 800 kDa (data not shown).
Figure 1: Dynamin self-association is enhanced in the presence of brain vesicles. Dynamin (60 nM) was treated with 10 mM EDC for 0, 5, 15, 30, or 60 min in the absence or presence of lipid vesicles and the samples were processed for Western blot analysis using anti-dynamin antibodies. The relative levels of dynamin monomer remaining, as determined by densitometric analysis, are indicated below each lane of the blot. The molecular masses of the dynamin immunoreactive species are indicated at the right of the figure in kilodaltons. The corresponding Coomassie Blue-stained gel revealed only minor cross-linking of other vesicle proteins (data not shown).
The extent of dynamin oligomerization was calculated and the relative levels of dynamin remaining as a monomer are indicated (Fig. 1). By 60 min, nearly twice the amount of dynamin in the absence of vesicles remained as a monomer as compared to dynamin cross-linked in the presence of vesicles (63 and 33%, respectively). Of the dynamin that was cross-linked, the relative amounts of dimers, trimers, and tetramers were determined for each time point (Fig. 2). In the absence of vesicles, dimers were the prominent oligomer observed and dimer formation was approximately linear with time (Fig. 2A). After 60 min of EDC treatment, 73% of the cross-linked dynamin was present as a dimer. For dynamin cross-linked in the presence of vesicles (Fig. 2B), dynamin dimers, trimers, and tetramers were present within 5 min of EDC treatment and dimers were observed to be the prominent species formed. Decreases in dimer and trimer formation were observed with time as increases in higher order oligomer formation occurred. After 60 min of treatment, only 13% of the cross-linked dynamin remained as a dimer.
Figure 2:
The
formation of dynamin higher order oligomers displays altered kinetics
in the presence of brain vesicles. The relative amounts of dimers
(), trimers (
), and tetramers (
) were determined for
dynamin (60 nM) treated with 10 mM EDC for 0, 5, 15,
30, or 60 min in the absence (A) or presence (B) of
brain vesicles. Arbitrary units refer to values obtained from
densitometric analysis of individual anti-dynamin immunoreactive bands
on the Western blot shown in Fig. 1.
These results provide evidence that dynamin forms higher order oligomers when bound to multivalent activating surfaces. These data also support the conclusions drawn from our earlier analysis of dynamin enzymatic behavior that indicated dynamin self-association is favored when bound to polymeric surfaces, and that this self-association is required for maximal GTPase activity levels(9) . Although dynamin has been shown to self-associate in vitro in the absence of polymeric substrates, supra-physiological concentrations were used (21) . We found some higher order oligomer formation of dynamin in solution, but we suggest that dynamin self-association either requires or is greatly enhanced upon interaction with its in vivo binding site.
The prevalence of dynamin dimer formation, both in the absence and presence of vesicles, indicates that this is the minimal dynamin multimer and represents the native form of dynamin. This result agrees with size estimations obtained from sucrose density gradient sedimentation values(22) , gel permeation chromatography(23) , and electron microscopic observations(11, 23) . However, recent reports have suggested that dynamin exists in its native form as a tetramer (21) . Data for these reports were obtained using significantly higher dynamin concentrations which may have promoted some self-association.
Figure 3: Dynamin is self-associated when isolated from rat brain. A high speed pellet preparation (HSP) enriched in clathrin-coated vesicles (1.9 mg of protein/ml) was treated with 10 mM EDC for 0, 15, 30, or 60 min and the samples processed for Western blot analysis with anti-dynamin antibodies. For comparison, purified dynamin (53 nM) that was cross-linked in the presence of a salt-stripped HSP preparation (vesicles) is also shown. The dashes to the right of the figure correspond to the molecular mass of the dynamin immunoreactive monomer (100 kDa), dimer (200 kDa), and other higher order cross-linked species (>200 kDa). The corresponding Coomassie Blue-stained gel revealed only minor cross-linking of other vesicle proteins (data not shown).
Figure 4:
GTP
does not affect dynamin oligomerization. Dynamin (53 nM) was
cross-linked with 10 mM EDC for 0, 5, 15, 30, or 60 min in the
absence (,
) or presence of lipid vesicles (
,
). The relative levels of dynamin oligomeric species (>200
kDa) were determined and are expressed as % dynamin oligomerized. The
addition of 1 mM GTP had no effect on the rates or extent of
dynamin oligomerization (
,
).
The addition of 1 mM GTP had no effect on dynamin self-association in the presence of lipid vesicles. As shown in Fig. 4, neither the rate of dynamin polymeric complex formation nor the extent of cross-linking after 60 min were altered by the addition of GTP. After 60 min of EDC treatment, 49% of dynamin in the absence of GTP was present in multimers of greater than 200 kDa while 54% of dynamin oligomerized in the presence of GTP. This indicates that GTP does not alter the properties of dynamin binding to vesicles (see ``Discussion'') or its subsequent self-association.
The
next stage of our analysis examined the effect of nucleotides on
dynamin associated with vesicles. For these experiments, a high speed
pellet preparation enriched in dynamin (6) was treated with 10
mM EDC after preincubation with 1 mM GTP, GDP,
GTPS, or GMPPNP. The extent of dynamin cross-linking was detected
on Western blots and compared to preparations cross-linked without
added nucleotide. None of these nucleotides affected the rate or the
extent of dynamin oligomerization (Table 2). The rates of dynamin
oligomerization ranged from 0.9 to 1.2% of total protein/min. The final
extents of higher order complex formation at 60 min were equivalent,
ranging from 62 to 67%. These results further indicate that both
dynamin membrane binding and self-association occur independently of
its nucleotide-bound state. It is interesting to note that the addition
of GDP did not disrupt dynamin self-association. This indicates that
dynamin binding to membranes is not regulated by its nucleotide-bound
state such that it does not cycle on and off as it hydrolyzes GTP.
Figure 5:
The stimulated rates of dynamin GTP
hydrolysis are inhibited by treatment with EDC. Dynamin (52
nM) that was prebound to 0.1 mg/ml 25% phosphatidylserine
liposomes () or 0.15 mg/ml microtubules (
) was treated with
increasing concentrations of EDC for 20 min and subsequently assayed
for GTPase activity (A). Results are given as the percent of
activity remaining, after subtracting basal GTPase levels. In B, dynamin (56 nM) was treated with EDC in the
absence of activating agent and basal GTPase activity levels were
determined. Maximal activation of dynamin in the presence of
microtubules or liposomes was 456 and 436 nmol/min/mg,
respectively.
The inhibition of dynamin GTPase observed in Fig. 5A occurred at low EDC concentrations that do not induce intermolecular cross-links as assayed by monomer content on Coomassie Blue-stained gels (data not shown). Rather, intramolecular cross-links or modifications were likely being formed that disrupted the activation of dynamin GTP hydrolysis through self-association. To examine the effects of this modification, dynamin was treated with EDC under conditions that did not favor intermolecular cross-link formation (Table 3). The treated dynamin was incubated with liposomes or microtubules for 10 min to allow binding, and GTPase activity was measured. As shown in Table 3, treatment with EDC drastically inhibited the stimulated rates of dynamin GTP hydrolysis. Upon treatment of dynamin with 15 mM EDC, GTPase activity levels were inhibited by 83% (in the presence of liposomes) and 88% (in the presence of microtubules). Even at this high EDC concentration, intermolecular cross-links were not being formed; only a small proportion (4 or 18%, in the presence of liposomes or microtubules, respectively) of dynamin was cross-linked into dimers.
These results indicate that EDC treatment was inducing either intramolecular cross-links or modifications that were disrupting the dynamin GTPase activation process. One possible explanation is that dynamin was being modified such that activator-dynamin interactions were being prevented. To test this possibility, we assayed for binding of EDC-cross-linked dynamin to microtubules. Dynamin was cross-linked for 20 min with 15 mM EDC. The reaction was stopped with 2-mercaptoethanol and the mixture was incubated in the presence of 0.3 mg/ml microtubules for 10 min. The microtubules and associated dynamin were recovered by centrifugation. In Fig. 6, the results of this experiment are shown. In the absence of microtubules, the EDC-cross-linked dynamin remained in the supernatant, indicating that these complexes are not forming large aggregates that could sediment under these conditions. However, in the presence of microtubules, virtually all of the dynamin (both monomer and cross-linked species) was recovered in the pellet fraction indicating that treatment with EDC was not disrupting dynamin-microtubule interactions.
Figure 6: EDC cross-linking of dynamin does not inhibit microtubule binding. EDC-cross-linked dynamin (100 nM) was incubated in the presence or absence of 0.3 mg/ml microtubules (MT) and centrifuged under conditions that pellet microtubules. Binding was assessed by assaying the resultant supernatant (S) and pellet (P) fractions on Western blots for immunoreactivity with anti-dynamin antibodies. The numbers to the right of the figure reflect the molecular mass of the dynamin immunoreactive species in kilodaltons.
Another possible explanation for the inhibition of GTPase activity observed is that EDC was modifying dynamin such that dynamin-dynamin interactions were being prevented. To test this possibility, we examined dynamin cooperative binding to microtubules. After treatment with 10 mM EDC for 5 min (a combination sufficient to inhibit stimulated GTP hydrolysis rates), dynamin was incubated with microtubules to allow binding and the samples were processed for negative stain electron microscopy. As shown in Fig. 7A, dynamin bound microtubules with uniform spacing (13 nm) and appeared in clusters. This pattern of dynamin binding to microtubules has been previously observed and represents cooperative associations between dynamin molecules(11) . Even after 30 min of cross-linking, dynamin was still able to bind microtubules in a cooperative manner (Fig. 7B). This indicates that neither dynamin-microtubule nor dynamin-dynamin interactions were perturbed by EDC treatment.
Figure 7: EDC cross-linking of dynamin does not inhibit self-association. Dynamin (45 nM) was treated with 10 mM EDC for 5 (A) or 30 min (B) and then added to 0.05 mg/ml microtubules for 10 min to allow binding. Samples were processed for negative stain electron microscopy. Arrows indicate clusters of dynamin molecules.
To better understand how EDC treatment was inhibiting dynamin-activated GTP hydrolysis, it was necessary to determine whether intramolecular cross-links were being formed and identify the regions of the dynamin molecule that were involved. Dynamin was treated with 10 mM EDC for the indicated times and subsequently digested with 0.25 µg/ml chymotrypsin at room temperature for 60 min (Fig. 8). In Fig. 8A, a silver-stained gel of the peptide maps is shown with no obvious changes in the profiles between conditions. However, when Western blots of the peptide maps were prepared and processed for reactivity with anti-dynamin polyclonal antibodies specific for the carboxyl-terminal domain, striking differences in the peptide maps were noticed (Fig. 8B). At time 0 (no EDC cross-linking), a characteristic chymotrypsin peptide map of dynamin carboxyl-terminal derived fragments was generated with immunoreactive peptides at approximately 50, 44, 42, 36, 21, 20.5, and 18.5 kDa. The fragments ranging from 18.5 to 21 kDa have been previously described as the regions deleted from the 80-kDa dynamin (6) . Within 5 min of EDC treatment, the levels of 18.5-21-kDa fragments detected were significantly reduced and by 15 min, were barely detectable. The 36-44-kDa fragments were also present in reduced amounts as the extent of dynamin cross-linking was increased. Similar alterations in peptide maps were obtained from digests of dynamin when cross-linked in the presence of lipid vesicles (data not shown).
Figure 8: EDC treatment of dynamin induces intramolecular cross-link formation. Dynamin (40 nM), pretreated with 10 mM EDC for 0, 5, 15, 30, or 60 min, was digested with chymotrypsin for 60 min. The silver-stained gel of the peptide maps obtained is shown (A). The corresponding Western blot was probed with anti-dynamin antibodies specific for the carboxyl-terminal domain (B). The position of the dynamin monomer and a cluster of dynamin carboxyl-terminal fragments are indicated in kilodaltons.
The altered sensitivity of the treated dynamin to digestion by chymotrypsin indicates that EDC was inducing intramolecular cross-links that involved the dynamin carboxyl-terminal domain. This rigid domain likely extends away from the rest of the molecule(26) , and upon treatment with EDC, may be covalently attached back to the intact molecule or adjacent molecule thus altering the availability of these sequences to digestion by chymotrypsin. The cross-links not only decrease the conformational freedom of this domain, but also likely confer structural rigidity to the whole dynamin molecule. This, in turn, may prevent conformational changes required for activated GTP hydrolysis. Recent reports have postulated that dynamin conformational flexibility is important for pinching off coated vesicles from the plasma membrane(1) . These experiments provide evidence that such conformational changes are directly related to dynamin GTP binding and hydrolysis.
Our previous analysis of dynamin enzymatic properties revealed that cooperative associations between dynamin molecules, as they are bound to a polymeric surface, are responsible for activation of GTP hydrolysis. In this study, we extended our analysis and have provided structural evidence that dynamin self-associates in vitro in the presence of brain vesicles. We have also determined that membrane-bound dynamin, copurified with a clathrin-coated vesicle preparation, is organized into high order polymers. Both dynamin-membrane and dynamin-dynamin interactions occur independently of its nucleotide-bound state. We found that structural constraints, imposed by the formation of intramolecular cross-links involving the dynamin carboxyl-terminal domain, inhibit dynamin activated GTP hydrolysis.
From these results and our biochemical analysis of
dynamin, we have developed a structural model that describes the
dynamin activation process (Fig. 9). In step 1, a dynamin dimer
(its native state) binds a multivalent surface via its
carboxyl-terminal domain that extends from the intact molecule. Upon
binding, dynamin adopts an orientation that promotes cooperative
interactions between dynamin molecules resulting in polymer formation
(step 2). GTP binding, hydrolysis, and release of products (GDP and
P) are associated with a concerted conformational change in
the polymer that likely promotes the release of endocytic vesicles from
the plasma membrane in vivo (step 3). Upon completion of the
hydrolytic cycle, the dynamin molecules either reset to a starting
conformation or are released from the binding surface (step 4). We and
others have shown that both membrane binding and self-association are
not dependent on dynamin nucleotide binding (steps 1, 2, and 4). Other
signaling events are likely required for regulation of membrane
binding, initiation of the GTP hydrolytic cycle and the resetting
process (see below). Treatment of dynamin with EDC inhibits step 3 of
the activation process by conferring structural rigidity to the
molecule, preventing the necessary conformational changes associated
with stimulated rates of GTP hydrolysis.
Figure 9: Model of the dynamin activation process. Dynamin binds a multivalent surface (step 1) and adopts an orientation that promotes cooperative associations (step 2). Upon binding and hydrolysis of GTP, the dynamin molecules undergo a concerted conformational change (step 3). The molecules either return to a starting conformation (step 3) or are released from the membrane (step 4). Dynamin monomers are represented by ovals with the carboxyl-terminal domains extending away from the intact molecule.
Biochemical subfractionation of rat brain has shown dynamin peripheral association with membranous organelles(6, 12, 13) . Immunolocalization studies at both the light and electron microscopic levels have shown that dynamin is found at the plasma membrane(2, 3, 4) . However, it is not known whether dynamin binds membranes directly or through other membrane-associated proteins. It is compelling to speculate that dynamin is binding acidic phospholipid domains of the membrane. Acidic phospholipid liposomes and salt-stripped brain vesicles have both been shown to support dynamin-activated GTP hydrolysis in vitro(6) . Furthermore, cross-linking experiments of isolated brain vesicles with which dynamin associates provided no evidence for dynamin-receptor interactions (Fig. 3). However, it is possible that dynamin-receptor binding is substoichiometric; as one dynamin molecule binds a receptor, it adopts an orientation that favors cooperative associations such that binding proceeds without additional receptormediated interactions.
Whether dynamin binds membrane lipids or proteins, the interactions are likely GTP-insensitive (Fig. 4B). This conclusion is supported by the observation that mutant dynamin molecules that bind GTP with greatly reduced affinity maintain their plasma membrane localization when overexpressed in HeLa cells(2) . Although dynamin binding to microtubules has been regarded as GTP-sensitive, the ionic strength of the added nucleotide is sufficient to disrupt ionic interactions between dynamin and microtubules that results in an apparent GTP-dependent elution(9) . Furthermore, we found that dynamin was not eluted from membranes by the addition of GDP (Table 2). Although biochemical subfractionation studies have shown that dynamin partitions approximately equally between soluble and particulate fractions(6, 12, 13) , our results suggest that dynamin membrane binding is not regulated by GTP hydrolysis and occurs independently from the nucleotide-bound state of the enzyme. Since self-association stimulates dynamin GTPase, there must be other signals to regulate membrane binding, initiate the GTP hydrolytic cycle, and/or reset the process. The dynamin carboxyl-terminal 20-kDa fragment is required for vesicle binding and subsequent enhancement of oligomerization in vitro (this report) and targeting to membranes in vivo(4) . The pleckstrin homology domain of the dynamin molecules used in these studies remained intact and recently it has been shown that this domain can bind membrane lipids(27) . It is possible that lipid binding by this domain may be involved in the regulation of these processes. Also, dynamin carboxyl-terminal phosphorylation, as regulated by synaptic terminal hyperpolarization, remains a possible mechanism for the regulation of dynamin membrane binding(13, 28) .
Once bound to
membranes in vivo, dynamin likely adopts an orientation that
favors positive cooperative associations, resulting in dynamin polymer
formation. In the absence of brain vesicles, the extent of dynamin
oligomerization is limited primarily to dimer formation. However, upon
the addition of ves-icles to the mixture, dynamin rapidly forms
multimeric complexes. That dynamin polymer formation occurs is further
supported by studies performed in isolated synaptosomes that were
treated with GTPS. These synaptosomes were characterized by large
tubular plasma membrane invaginations which were positive for high
density dynamin immunoreactivity along their lengths (1) .
Although high concentrations of dynamin have been shown to
self-associate in the absence of polymeric binding sites in a
GTP-independent manner(21) , we suggest that in vivo,
dynamin-dynamin interactions occur following binding to membranes.
The finding that a dynamin-related protein, Mx1, also self-associates in vitro suggests that this property may be conserved among the dynamin family members. In fact, a ``self-assembly domain'' has been identified in Mx1 that is shared among all the related proteins (25) . However, our present analysis suggests that dynamin self-association is regulated differently. Soluble Mx1 self-assembles in vitro and this is enhanced in the presence of GTP(25) . This protein is found in rat liver nuclei in what appears to be Mx1 aggregates(25) . Our analysis has provided direct evidence that both dynamin membrane binding and self-association occur independently from the GTP hydrolytic cycle. Mutational analyses of dynamin molecules have shown that GTPase activity is required for its role in endocytosis (2, 3, 4) indicating that other properties of dynamin must be regulated by GTP binding and hydrolysis. Our experiments indicate that dynamin structural rigidity, resulting from EDC-induced intramolecular cross-links, inhibit association-stimulated GTPase activity. This suggests that conformational flexibility, in combination with membrane binding and self-association, is required for dynamin function. We propose that dynamin polymers, as they are bound to the membrane, are able to bind GTP and upon hydrolysis, undergo a concerted conformational change of the polymer relative to the surface. We suggest that this GTP hydrolysis-dependent conformational change brings the opposing membranes of the neck of the invaginated coated pit into close proximity, allowing membrane fusion and subsequent release of the endocytic vesicle.