Exchange of clathrin, AP2 and epsin on clathrin-coated pits in permeabilized tissue culture cells

The readily retrievable pool of synaptic vesicles

In the CNS communication between neurons occurs at synapses by secretion of neurotransmitter via exocytosis of synaptic vesicles (SVs) at the active zone. Given the limited number of SVs in presynaptic boutons a fast and efficient recycling of exocytosed membrane and proteins by triggered compensatory endocytosis is required to maintain neurotransmission. Thus, pre-synapses feature a unique tight coupling of exo- and endocytosis in time and space resulting in the reformation of SVs with uniform morphology and well-defined molecular composition. This rapid response requires early stages of endocytosis at the peri-active zone to be well choreographed to ensure reformation of SVs with high fidelity. The pre-synapse can address this challenge by a specialized membrane microcompartment, where a pre-sorted and pre-assembled readily retrievable pool (RRetP) of endocytic membrane patches is formed, consisting of the vesicle cargo, presumably bound within a nucleated Clathrin and adaptor complex. This review considers evidence for the RRetP microcompartment to be the primary organizer of presynaptic triggered compensatory endocytosis.

Cargo regulates clathrin-coated pit invagination via clathrin light chain phosphorylation

Phosphorylation of clathrin light chains (CLCs) regulates GPCR uptake but is dispensable for transferrin internalization. Maib et al. show that CLCb phosphorylation is required for efficient auxilin-mediated clathrin exchange to promote coated pit invagination in a cargo-specific manner.

Clathrin light chains (CLCs) control selective uptake of a range of G protein–coupled receptors (GPCRs), although the mechanism by which this occurs has remained elusive thus far. In particular, site-specific phosphorylation of CLCb controls the uptake of the purinergic GPCR P2Y₁₂, but it is dispensable for the constitutive uptake of the transferrin receptor (TfR). We demonstrate that phosphorylation of CLCb is required for the maturation of clathrin-coated pits (CCPs) through the transition of flat lattices into invaginated buds. This transition is dependent on efficient clathrin exchange regulated by CLCb phosphorylation and mediated through auxilin. Strikingly, this rearrangement is required for the uptake of P2Y₁₂ but not TfR. These findings link auxilin-mediated clathrin exchange to early stages of CCP invagination in a cargo-specific manner. This supports a model in which CCPs invaginate with variable modes of curvature depending on the cargo they incorporate.

Auxilin facilitates membrane traffic in the early secretory pathway

In this study, a proteomic approach links the J-domain chaperone auxilin, which uncoats clathrin-coated vesicles, to the other major coat complexes in the cell (COPII and COPI). Genetic and biochemical studies support the proposal that auxilin facilitates vesicle traffic in the early secretory pathway.

Coat protein complexes contain an inner shell that sorts cargo and an outer shell that helps deform the membrane to give the vesicle its shape. There are three major types of coated vesicles in the cell: COPII, COPI, and clathrin. The COPII coat complex facilitates vesicle budding from the endoplasmic reticulum (ER), while the COPI coat complex performs an analogous function in the Golgi. Clathrin-coated vesicles mediate traffic from the cell surface and between the trans-Golgi and endosome. While the assembly and structure of these coat complexes has been extensively studied, the disassembly of COPII and COPI coats from membranes is less well understood. We describe a proteomic and genetic approach that connects the J-domain chaperone auxilin, which uncoats clathrin-coated vesicles, to COPII and COPI coat complexes. Consistent with a functional role for auxilin in the early secretory pathway, auxilin binds to COPII and COPI coat subunits. Furthermore, ER–Golgi and intra-Golgi traffic is delayed at 15°C in swa2Δ mutant cells, which lack auxilin. In the case of COPII vesicles, we link this delay to a defect in vesicle fusion. We propose that auxilin acts as a chaperone and/or uncoating factor for transport vesicles that act in the early secretory pathway.

Endocytic accessory factors and regulation of clathrin-mediated endocytosis

Up to 60 different proteins are recruited to the site of clathrin-mediated endocytosis in an ordered sequence. These accessory proteins have roles during all the different stages of clathrin-mediated endocytosis. First, they participate in the initiation of the endocytic event, thereby determining when and where endocytic vesicles are made; later they are involved in the maturation of the clathrin coat, recruitment of specific cargo molecules, bending of the membrane, and finally in scission and uncoating of the nascent vesicle. In addition, many of the accessory components are involved in regulating and coupling the actin cytoskeleton to the endocytic membrane. We will discuss the different accessory components and their various roles. Most of the data comes from studies performed with cultured mammalian cells or yeast cells. The process of endocytosis is well conserved between these different organisms, but there are also many interesting differences that may shed light on the mechanistic principles of endocytosis.

A role of OCRL in clathrin-coated pit dynamics and uncoating revealed by studies of Lowe syndrome cells

Mutations in the inositol 5-phosphatase OCRL cause Lowe syndrome and Dent's disease. Although OCRL, a direct clathrin interactor, is recruited to late-stage clathrin-coated pits, clinical manifestations have been primarily attributed to intracellular sorting defects. Here we show that OCRL loss in Lowe syndrome patient fibroblasts impacts clathrin-mediated endocytosis and results in an endocytic defect. These cells exhibit an accumulation of clathrin-coated vesicles and an increase in U-shaped clathrin-coated pits, which may result from sequestration of coat components on uncoated vesicles. Endocytic vesicles that fail to lose their coat nucleate the majority of the numerous actin comets present in patient cells. SNX9, an adaptor that couples late-stage endocytic coated pits to actin polymerization and which we found to bind OCRL directly, remains associated with such vesicles. These results indicate that OCRL acts as an uncoating factor and that defects in clathrin-mediated endocytosis likely contribute to pathology in patients with OCRL mutations.

DOI:http://dx.doi.org/10.7554/eLife.02975.001

Oculo-Cerebro-Renal syndrome of Lowe (Lowe syndrome) is a rare genetic disorder that can cause cataracts, mental disabilities and kidney dysfunction. It is caused by mutations in the gene encoding OCRL, a protein that modifies a membrane lipid and that is found on membranes transporting molecules (cargo) into cells by a process known as endocytosis.

During endocytosis, the cell outer membrane is deformed into a pit that engulfs the cargo to be taken up by the cell. The pit then pinches off from the outer membrane to form a vesicle—a bubble-like compartment—inside the cell that transports the cargo to its destination. In one type of endocytosis, this process is mediated by a basket-like coat primarily made up from the protein clathrin that assembles at the membrane patch to be internalized. After the vesicle is released from the cell membrane, the clathrin coat is broken apart and its components are shed and recycled for use by new budding endocytic vesicles.

The OCRL protein had previously been observed associated to newly forming clathrin-coated vesicles, but the significance of this was not known. Now, Nández et al. have used a range of imaging and analytical techniques to further investigate the properties of OCRL, taking advantage of cells from patients with Lowe syndrome. These cells lack OCRL, and so allow the effect of OCRL's absence on cell function to be deduced. OCRL destroys the membrane lipid that helps to connect the clathrin coat to the membrane, and Nández et al. show that without OCRL the newly formed vesicle moves into the cell but fails to efficiently shed its clathrin coat. Thus, a large fraction of clathrin coat components remain trapped on the vesicles, reducing the amount of such components available to help new pits develop into vesicles. As a consequence, the cell has difficulty internalizing molecules.

Collectively, the findings of Nández et al. outline that OCRL plays a role in the regulation of endocytosis in addition to its previously reported actions in the control of intracellular membrane traffic. The results also help to explain some of the symptoms seen in Lowe syndrome patients.

DOI:http://dx.doi.org/10.7554/eLife.02975.002

Unraveling protein-protein interactions in clathrin assemblies via atomic force spectroscopy

Atomic force microscopy (AFM), single molecule force spectroscopy (SMFS), and single particle force spectroscopy (SPFS) are used to characterize intermolecular interactions and domain structures of clathrin triskelia and clathrin-coated vesicles (CCVs). The latter are involved in receptor-mediated endocytosis (RME) and other trafficking pathways. Here, we subject individual triskelia, bovine-brain CCVs, and reconstituted clathrin-AP180 coats to AFM-SMFS and AFM-SPFS pulling experiments and apply novel analytics to extract force-extension relations from very large data sets. The spectroscopic fingerprints of these samples differ markedly, providing important new information about the mechanism of CCV uncoating. For individual triskelia, SMFS reveals a series of events associated with heavy chain alpha-helix hairpin unfolding, as well as cooperative unraveling of several hairpin domains. SPFS of clathrin assemblies exposes weaker clathrin-clathrin interactions that are indicative of inter-leg association essential for RME and intracellular trafficking. Clathrin-AP180 coats are energetically easier to unravel than the coats of CCVs, with a non-trivial dependence on force-loading rate.

Polymeric nucleic acid vehicles exploit active interorganelle trafficking mechanisms

Materials that self-assemble with nucleic acids into nanocomplexes (e.g. polyplexes) are widely used in many fundamental biological and biomedical experiments. However, understanding the intracellular transport mechanisms of these vehicles remains a major hurdle in their effective usage. Here, we investigate two polycation models, Glycofect (which slowly degrades via hydrolysis) and linear polyethyleneimine (PEI) (which does not rapidly hydrolyze), to determine the impact of polymeric structure on intracellular trafficking. Cells transfected using Glycofect underwent increasing transgene expression over the course of 40 h and remained benign over the course of 7 days. Transgene expression in cells transfected with PEI peaked at 16 h post-transfection and resulted in less than 10% survival after 7 days. While saccharide-containing Glycofect has a higher buffering capacity than PEI, polyplexes created with Glycofect demonstrate more sustained endosomal release, possibly suggesting an additional or alternative delivery mechanism to the classical "proton sponge mechanism". PEI appeared to promote release of DNA from acidic organelles more than Glycofect. Immunofluorescence images indicate that both Glycofect and linear PEI traffic oligodeoxynucleotides to the Golgi and endoplasmic reticulum, which may be a route towards nuclear delivery. However, Glycofect polyplexes demonstrated higher co-localization with the ER than PEI polyplexes, and co-localization experiments indicate the retrograde transport of polyplexes via COP I vesicles from the Golgi to the ER. We conclude that slow release and unique trafficking behaviors of Glycofect polyplexes may be due to the presence of saccharide units and the degradable nature of the polymer, allowing more efficacious and benign delivery.

Cyclin G-associated kinase is necessary for osteosarcoma cell proliferation and receptor trafficking

Osteosarcoma is the most frequent primary malignant bone tumor among the children. The advent of neoadjuvant chemotherapy significantly improved the prognosis of patients with osteosarcoma in the 1980s, but it has since plateaued in the past decades. Recently, one of the most researched areas in sarcoma treatment is tyrosine kinases. Here, we describe research on a serine/threonine kinase, cyclin G-associated kinase (GAK), which has not been reported in osteosarcoma previously. In this study, a lentiviral based human shRNA library was utilized to screen for kinases in KHOS and U-2OS osteosarcoma cells. The expression of GAK was examined in osteosarcoma and the effect on cell proliferation was analyzed by GAK siRNA knockdown. The level of GAK expression and its correlation to prognosis was analyzed in osteosarcoma tissue microarray. The effect of GAK depletion on insulin-like growth factor and epidermal growth factor receptor-mediated signal transduction was analyzed by Western blot. We observed that GAK was overexpressed in both osteosarcoma cell lines and tissue samples when compared with human osteoblasts. GAK knockdown by siRNA decreased cell proliferation in both drug-sensitive and multidrug-resistant osteosarcoma cell lines. Immunohistochemistry of osteosarcoma tissue microarray revealed that overexpression of GAK was associated with poor prognosis. Finally, knockdown of GAK resulted in alterations of receptor trafficking and several downstream proteins. In conclusion, our results suggest that osteosarcoma cell proliferation and survival are dependent on GAK. These findings may lead to the development of new therapeutic options for osteosarcoma.

Endocytosis and clathrin-uncoating defects at synapses of auxilin knockout mice

Neuronally expressed auxilin and ubiquitously expressed cyclin-G-dependent kinase (GAK) are homologous proteins that act as cochaperones to support the Hsc70-dependent clathrin uncoating of clathrin-coated vesicles. GAK was previously shown to be essential in mouse during embryonic development and in the adult. We have now engineered an auxilin knockout mouse. Mutant mice had a high rate of early postnatal mortality and surviving pups generally had a lower body weight than wild-type pups, although they had a normal life span. GAK was up-regulated as much as 3-fold in the brains of both surviving neonates and adult mutant mice. An increased number of clathrin-coated vesicles and empty cages were present at knockout synapses both in situ and in primary neuronal cultures. Additionally, clathrin-mediated endocytosis of synaptic vesicles in knockout hippocampal neurons was impaired, most likely due to sequestration of coat components in assembled coats and cages. Collectively, our results demonstrate the specialized role of auxilin in the recycling of synaptic vesicles at synapses, but also show that its function can be partially compensated for by up-regulation of GAK.

Dab2 regulates clathrin assembly and cell spreading

The recruitment of clathrin to the membrane and its assembly into coated pits results from its interaction with endocytic adaptors and other regulatory proteins in the context of a specific lipid microenvironment. Dab2 (disabled 2) is a mitotic phosphoprotein and a monomeric adaptor for clathrin-mediated endocytosis. In the present study, we employed GFP (green fluorescent protein) fusion constructs of different isoforms and mutants of rat Dab2 and characterized their effect on the size, distribution and dynamics of clathrin assemblies. Enhanced levels of expression of the p82 isoform of Dab2 in COS7 cells induced enlarged clathrin assemblies at the plasma membrane. p82-clathrin assemblies, which concentrate additional endocytic proteins, such as AP2 (adaptor protein 2) and epsin, are dynamic structures in which both p82 and clathrin exchange actively between the membrane-bound and cytosolic sub-populations. The ability of p82 to induce enlarged clathrin assemblies is dependent on the presence of a functional PTB domain (phosphotyrosine-binding domain), on binding to clathrin and phospholipids, and on a newly identified and evolutionarily conserved poly-lysine stretch which precedes the PTB domain. These same molecular features are required for Dab2 to enhance the spreading of COS7 cells on fibronectin. The ability of the p82 isoform of Dab2 to enhance cell spreading was confirmed in both HeLa cells and HBL cells (human breast epithelial cells). COS7 cells expressing GFP-p82 and plated on to fibronectin concentrate the beta1 integrin into clathrin-p82 assemblies. Furthermore, during cell spreading, p82-clathrin assemblies concentrate at the site of the initial cell-matrix contact and are absent from regions of intense membrane ruffling. We propose a role for Dab2 and clathrin in integrin-mediated cell spreading.

Architecture of clathrin fullerene cages reflects a geometric constraint--the head-to-tail exclusion rule--and a preference for asymmetry

Fullerene cages have n trivalent vertices, 12 pentagonal faces, and (n-20)/2 hexagonal faces. The smallest cage in which all of the pentagons are surrounded by hexagons and thus isolated from each other has 60 vertices and is shaped like a soccer ball. The protein clathrin self-assembles into fullerene cages of a variety of sizes and shapes, including smaller ones with adjacent pentagons as well as larger ones, but the variety is limited. To explain the range of clathrin architecture and how these fullerene cages self-assemble, we proposed a hypothesis, the "head-to-tail exclusion rule" (the "Rule"). Of the 5769 small clathrin cage isomers with n< or =60 vertices and adjacent pentagons, the Rule permits just 15, three identified in 1976 and 12 others. A "weak version" of the Rule permits another 99. Based on cryo-electron tomography, Cheng et al. reported six raw clathrin fullerene cages. One was among the three identified in 1976. Here, (1) we identify the remaining five. (2) Four are new and are among the 12 others permitted by the Rule. (3) One, also new, is among the 99 weak version cages. (4) Of particular note, none of the remaining 5565 excluded cages has been identified. These findings provide powerful experimental confirmation of the Rule and the principle on which it is based. (5) Surprisingly, the newly identified clathrin cages are among the least symmetric of those permitted. (6) By devising a method for counting assembly paths, (7) we show that asymmetric cages can be assembled by larger numbers of paths, thus providing a kinetic explanation for the prevalence of asymmetric cages. (8) Finally, we show that operation during cage growth of the Rule greatly increases the likelihood of producing a closed fullerene cage, specifically one of those permitted, but efficient assembly still appears to require internal remodeling.

Focusing on clathrin-mediated endocytosis

Investigations into the mechanisms which regulate entry of integral membrane proteins, and associated ligands, into the cell through vesicular carriers (endocytosis) have greatly benefited from the application of live-cell imaging. Several excellent recent reviews have detailed specific aspects of endocytosis, such as entry of particular cargo, or the different routes of internalization. The aim of the present review is to highlight how advances in live-cell fluorescence microscopy have affected the study of clathrin-mediated endocytosis. The last decade has seen a tremendous increase in the development and dissemination of methods for imaging endocytosis in live cells, and this has been followed by a dramatic shift in the way this critical cellular pathway is studied and understood. The present review begins with a description of the technical advances which have permitted new types of experiment to be performed, as well as potential pitfalls of these new technologies. Subsequently, advances in the understanding of three key endocytic proteins will be addressed: clathrin, dynamin and AP-2 (adaptor protein 2). Although great strides have clearly been made in these areas in recent years, as is often the case, each answer has bred numerous questions. Furthermore, several examples are highlighted where, because of seemingly minor differences in experimental systems, what appear at first to be very similar studies have, at times, yielded vastly differing results and conclusions. Thus this is an exceedingly exciting time to study endocytosis, and this area serves as a clear demonstration of the power of applying live-cell imaging to answer fundamental biological questions.

The clathrin-binding motif and the J-domain of Drosophila Auxilin are essential for facilitating Notch ligand endocytosis

Background

Ligand endocytosis plays a critical role in regulating the activity of the Notch pathway. The Drosophila homolog of auxilin (dAux), a J-domain-containing protein best known for its role in the disassembly of clathrin coats from clathrin-coated vesicles, has recently been implicated in Notch signaling, although its exact mechanism remains poorly understood.

Results

To understand the role of auxilin in Notch ligand endocytosis, we have analyzed several point mutations affecting specific domains of dAux. In agreement with previous work, analysis using these stronger dAux alleles shows that dAux is required for several Notch-dependent processes, and its function during Notch signaling is required in the signaling cells. In support of the genetic evidences, the level of Delta appears elevated in dAux deficient cells, suggesting that the endocytosis of Notch ligand is disrupted. Deletion analysis shows that the clathrin-binding motif and the J-domain, when over-expressed, are sufficient for rescuing dAux phenotypes, implying that the recruitment of Hsc70 to clathrin is a critical role for dAux. However, surface labeling experiment shows that, in dAux mutant cells, Delta accumulates at the cell surface. In dAux mutant cells, clathrin appears to form large aggregates, although Delta is not enriched in these aberrant clathrin-positive structures.

Conclusion

Our data suggest that dAux mutations inhibit Notch ligand internalization at an early step during clathrin-mediated endocytosis, before the disassembly of clathrin-coated vesicles. Further, the inhibition of ligand endocytosis in dAux mutant cells possibly occurs due to depletion of cytosolic pools of clathrin via the formation of clathrin aggregates. Together, our observations argue that ligand endocytosis is critical for Notch signaling and auxilin participates in Notch signaling by facilitating ligand internalization.

Essential role of cyclin-G-associated kinase (Auxilin-2) in developing and mature mice

Hsc70 with its cochaperone, either auxilin or GAK, not only uncoats clathrin-coated vesicles but also acts as a chaperone during clathrin-mediated endocytosis. However, because synaptojanin is also involved in uncoating, it is not clear whether GAK is an essential gene. To answer this question, GAK conditional knockout mice were generated and then mated to mice expressing Cre recombinase under the control of the nestin, albumin, or keratin-14 promoters, all of which turn on during embryonic development. Deletion of GAK from brain, liver, or skin dramatically altered the histology of these tissues, causing the mice to die shortly after birth. Furthermore, by expressing a tamoxifen-inducible promoter to express Cre recombinase we showed that deletion of GAK caused lethality in adult mice. Mouse embryonic fibroblasts in which the GAK was disrupted showed a lack of clathrin-coated pits and a complete block in clathrin-mediated endocytosis. We conclude that GAK deletion blocks development and causes lethality in adult animals by disrupting clathrin-mediated endocytosis.

A geometric principle may guide self-assembly of fullerene cages from clathrin triskelia and from carbon atoms

Clathrin triskelia and carbon atoms alike self-assemble into a limited selection of fullerene cages (with n three connected vertices, 3n/2 edges, 12 pentagonal faces, and (n-20)/2 hexagonal faces). We show that a geometric constraint-exclusion of head-to-tail dihedral angle discrepancies (DADs)-explains this limited selection as well as successful assembly into such closed cages in the first place. An edge running from a pentagon to a hexagon has a DAD, since the dihedral angles about the edge broaden from its pentagon (tail) end to its hexagon (head) end. Of the 21 configurations of a central face and surrounding faces, six have such DAD vectors arranged head-to-tail. Of the 5770 mathematically possible fullerene cages for n <or= 60, excluding those with any of the six configurations leaves just 15 cages plus buckminsterfullerene (n = 60), among them the known clathrin cages. Of the 216,739 mathematically possible cages for 60 < n <or= 84, just the 50 that obey the isolated-pentagon rule, among them known carbon cages, pass. The absence of likely fullerenes for some n (30,34,46,48,52-58,62-68) explains the abundance of certain cages, including buckminsterfullerene. These principles also suggest a "probable roads" path to self-assembly in place of pentagon-road and fullerene-road hypotheses.

The physical basis for the head-to-tail rule that excludes most fullerene cages from self-assembly

In the companion article, we proposed that fullerene cages with head-to-tail dihedral angle discrepancies do not self-assemble. Here we show why. If an edge abuts a pentagon at one end and a hexagon at the other, the dihedral angle about the edge increases, producing a dihedral angle discrepancy (DAD) vector. The DADs about all five/six edges of a central pentagonal/hexagonal face are determined by the identities-pentagon or hexagon-of its five/six surrounding faces. Each "Ring"-central face plus specific surrounding faces-may have zero, two, or four edges with DAD. In most Rings, the nonplanarity induced by DADs is shared among surrounding faces. However, in a Ring that has DADs arranged head of one to tail of another, the nonplanarity cannot be shared, so some surrounding faces would be especially nonplanar. Because the head-to-tail exclusion rule is an implicit geometric constraint, the rule may operate either by imposing a kinetic barrier that prevents assembly of certain Rings or by imposing an energy cost that makes those Rings unlikely to last in an equilibrium circumstance. Since Rings with head-to-tail DADs would be unlikely to self-assemble or last, fullerene cages with those Rings would be unlikely to self-assemble.

Cellular prion protein (PrPC) protects neuronal cells from the effect of huntingtin aggregation

The effect of normal cellular prion protein (PrP(C)) on abnormal protein aggregation was examined by transfecting huntingtin fragments (Htt) into SN56 neuronal-derived cells depleted of PrP(C) by RNA interference. PrP(C) depletion caused an increase in both the number of cells containing granules and the number of apoptotic cells. Consistent with the increase in Htt aggregation, PrP(C) depletion caused an decrease in proteasome activity and a decrease in the activities of cellular defense enzymes compared with control cells whereas reactive oxygen species (ROS) increased more than threefold. Therefore, PrP(C) may protect against Htt toxicity in neuronal cells by increasing cellular defense proteins, decreasing ROS and increasing proteasome activity thereby increasing Htt degradation. Depletion of endogenous PrP(C) in non-neuronal Caco-2 and HT-29 cells did not affect ROS levels or proteasome activity suggesting that only in neuronal cells does PrP(C) confer protection against Htt toxicity. The protective effect of PrP(C) was further evident in that overexpression of mouse PrP(C) in SN56 cells transfected with Htt caused a decrease in both the number of cells with Htt granules and the number of apoptotic cells, whereas there was no effect of PrP(C) expression in non-neuronal NIH3T3 or CHO cells. Finally, in chronically scrapie (PrP(Sc))-infected cells, ROS increased more than twofold while proteasome activity was decreased compared to control cells. Although this could be a direct effect of PrP(Sc), it is also possible that, since PrP(C) specifically prevents pathological protein aggregation in neuronal cells, partial loss of PrP(C) itself increases PrP(Sc) aggregation.

Endocytosis: clathrin-mediated membrane budding

Clathrin-dependent endocytosis is the major pathway for the uptake of nutrients and signaling molecules in higher eukaryotic cells. The long-held tenet that clathrin-coated vesicles are created from flat coated plasma membrane patches by a sequential process of invagination, bud formation and fission recently received strong support from the results of advanced live cell fluorescence microscopy. The data on the critical components that deform the plasma membrane locally into a coated bud suggest that membrane bending is a team effort requiring membrane-curving protein domains, actin dynamics and, last but not least, clathrin. The scission step requires the mechano-enzymatic function of dynamin, actin dynamics and possibly myosin motor proteins. Finally, a burst of auxilin/GAK initiates the uncoating of the vesicle.

Multiple roles of auxilin and hsc70 in clathrin-mediated endocytosis

The ATP-dependent dissociation of clathrin from clathrin-coated vesicles (CCVs) by the molecular chaperone Hsc70 requires J-domain cofactor proteins, either auxilin or cyclin-G-associated kinase (GAK). Both the nerve-specific auxilin and the ubiquitous GAK induce CCVs to bind to Hsc70. The removal of auxilin or GAK from various organisms and cells has provided definitive evidence that Hsc70 uncoats CCVs in vivo. In addition, evidence from various studies has suggested that Hsc70 and auxilin are involved in several other key processes that occur during clathrin-mediated endocytosis. First, Hsc70 and auxilin are required for the clathrin exchange that occurs during coated-pit invagination and constriction; this clathrin exchange may catalyze any rearrangement of the clathrin-coated pit (CCP) structure that is required during invagination and constriction. Second, Hsc70 and auxilin may chaperone clathrin after it dissociates from CCPs so that it does not aggregate in the cytosol. Third, auxilin and Hsc70 may be involved in the rebinding of clathrin to the plasma membrane to form new CCPs and independently appear to chaperone adaptor proteins so that they can also rebind to membranes to nucleate the formation of new CCPs. Finally, if formation of the curved clathrin coat induces membrane curvature, then Hsc70 and auxilin provide the energy for this curvature by inducing ATP-dependent clathrin exchange and rearrangement during endocytosis and ATP-dependent dissociation of clathrin at the end of the cycle so that it is energetically primed to rebind to the plasma membrane.

Endocytic adaptors: recruiters, coordinators and regulators

Clathrin-dependent endocytosis allows cells to bring plasma membrane and extracellular molecules into the cell. Forming a clathrin-coated vesicle requires the sequential action of numerous factors, beginning with endocytic adaptors. Adaptors are thought to initiate the process in two ways: by selecting cargo for packaging into the vesicle and assembling the clathrin coat and other components necessary to shape the vesicle. Here, we review recent work focusing on the sequential and cooperative interactions of adaptors with their binding partners, and how adaptors contribute to initial stages of endocytic internalization. The regulation of adaptors might be a key step for controlling endocytosis, and thus aid in homeostasis and cell physiology.

Recruitment dynamics of GAK and auxilin to clathrin-coated pits during endocytosis

Cyclin G-associated kinase (GAK), the ubiquitous form of the neuronal-specific protein auxilin 1, is an essential cofactor for Hsc70-dependent uncoating of clathrin-coated vesicles. Total internal reflectance microscopy was used to determine the timing of GAK binding relative to dynamin and clathrin binding during invagination of clathrin-coated pits. Following transient recruitment of dynamin to the clathrin puncta, large amounts of GAK are transiently recruited. GAK and clathrin then disappear from the evanescent field as the pit invaginates from the plasma membrane and finally these proteins disappear from the epifluorescence field, probably as the clathrin is uncoated from the budded vesicles by Hsc70. The recruitment of GAK is dependent on its PTEN-like domain, which we found binds to phospholipids. This suggests that interaction with phospholipids is essential for recruitment of GAK and, in turn, Hsc70, but Hsc70 recruitment alone might not be sufficient to induce irreversible clathrin uncoating. When budding of clathrin-coated pits is inhibited by actin depolymerization, there is repeated flashing of GAK on the clathrin-coated pit but neither scission nor irreversible uncoating occur. Therefore, budding as well as synchronous recruitment of GAK might be required for irreversible clathrin uncoating.

A single common portal for clathrin-mediated endocytosis of distinct cargo governed by cargo-selective adaptors

Sorting of transmembrane cargo into clathrin-coated vesicles requires endocytic adaptors, yet RNA interference (RNAi)-mediated gene silencing of the AP-2 adaptor complex only disrupts internalization of a subset of clathrin-dependent cargo. This suggests alternate clathrin-associated sorting proteins participate in cargo capture at the cell surface, and a provocative recent proposal is that discrete endocytic cargo are sorted into compositionally and functionally distinct clathrin coats. We show here that the FXNPXY-type internalization signal within cytosolic domain of the LDL receptor is recognized redundantly by two phosphotyrosine-binding domain proteins, Dab2 and ARH; diminishing both proteins by RNAi leads to conspicuous LDL receptor accumulation at the cell surface. AP-2-dependent uptake of transferrin ensues relatively normally in the absence of Dab2 and ARH, clearly revealing delegation of sorting operations at the bud site. AP-2, Dab2, ARH, transferrin, and LDL receptors are all present within the vast majority of clathrin structures at the surface, challenging the general existence of specialized clathrin coats for segregated internalization of constitutively internalized cargo. However, Dab2 expression is exceptionally low in hepatocytes, likely accounting for the pathological hypercholesterolemia that accompanies ARH loss.

Depletion of GAK/auxilin 2 inhibits receptor-mediated endocytosis and recruitment of both clathrin and clathrin adaptors

Cyclin G-associated kinase (GAK/auxilin 2), the ubiquitous form of the neuronal-specific protein auxilin 1, is an essential cofactor for the Hsc70-dependent uncoating of clathrin-coated vesicles. We have now investigated the effect of knocking down GAK in HeLa cells by vector-based small hairpin RNA. Functionally, depletion of GAK caused a marked decrease in internalization of both transferrin and epidermal growth factor and altered mannose 6-phosphate receptor trafficking, but had little effect on the recycling of transferrin receptor back to the plasma membrane. Structurally, depletion of GAK caused a marked reduction in perinuclear clathrin associated with the trans-Golgi network and in the number of clathrin-coated pits on the plasma membrane, and reduced clathrin exchange on the few clathrin-coated pits that remained. Surprisingly, while clathrin depletion does not prevent adaptors from assembling on the membrane, depletion of GAK caused a dramatic reduction in AP2 and epsin on the plasma membrane and AP1 and GGA at the trans-Golgi network. A similar effect was caused by expression of a dominant negative Hsp70 mutant. These results suggest that GAK, in conjunction with Hsc70, not only uncoats clathrin-coated vesicles and induces clathrin exchange on clathrin-coated pits, but also mediates binding of clathrin and adaptors to the plasma membrane and the trans-Golgi network.