Identification of a protein kinase as an intrinsic component of rat liver coated vesicles

Evidence on differential role for alpha 1 and alpha 2 subtypes of AP-2 adaptin in the Central Nervous System

Two subtypes of alpha (α)subunits , α1and α2, belonging to AP-2 complex have been described in the central nervous system (CNS). The specific role of each subtype is still unclear. In this study, we evaluated the expression and interaction with cell membranes of both subtypes in the postnatal developing cerebral cortex and cerebellum in two rat strains that display distinct developmental features. We observed that α2 displays higher variations than α1 during development, and at lesser extent in the rats with delayed rate of development. Additionally, by in vitro binding assays we evaluated the interaction of α subunits with bovine brain membranes. Both subtypes displayed clear differences in their performance, maximum binding of α1 was higher and α2 reached it faster than α1. In addition, both subtypes displayed different binding to membranes when bivalent cations or nucleotides were added. We conclude that both subtypes interact differently with membranes and that they may play different roles in clathrin-mediated endocytosis in the CNS.

Nanodissected elastically loaded clathrin lattices relax to increased curvature

Clathrin-mediated endocytosis (CME) is the major endocytosis pathway for the specific internalization of large compounds, growth factors, and receptors. Formation of internalized vesicles from the flat plasma membrane is accompanied by maturation of cytoplasmic clathrin coats. How clathrin coats mature and the mechanistic role of clathrin coats are still largely unknown. Maturation models proposed clathrin coats to mature at constant radius or constant area, driven by molecular actions or elastic energy. Here, combining high-speed atomic force microscopy (HS-AFM) imaging, HS-AFM nanodissection, and elasticity theory, we show that clathrin lattices deviating from the intrinsic curvature of clathrin form elastically loaded assemblies. Upon nanodissection of the clathrin network, the stored elastic energy in these lattices drives lattice relaxation to accommodate an ideal area-curvature ratio toward the formation of closed clathrin-coated vesicles. Our work supports that the release of elastic energy stored in curvature-frustrated clathrin lattices could play a major role in CME.

Dysregulation of the AP2M1 phosphorylation cycle by LRRK2 impairs endocytosis and leads to dopaminergic neurodegeneration

Mutations in the kinase LRRK2 and impaired endocytic trafficking are both implicated in the pathogenesis of Parkinson's disease (PD). Expression of the PD-associated LRRK2 mutant in mouse dopaminergic neurons was shown to disrupt clathrin-mediated endocytic trafficking. Here, we explored the molecular mechanism linking LRRK2 to endocytosis and found that LRRK2 bound to and phosphorylated the μ2 subunit of the adaptor protein AP2 (AP2M1), a core component of the clathrin-mediated endocytic machinery. Analysis of human SH-SY5Y cells and mouse neurons and tissues revealed that loss of LRRK2 abundance or kinase function resulted in decreased phosphorylation of AP2M1, which is required for the initial formation of clathrin-coated vesicles (CCVs). In contrast, overexpression of LRRK2 or expression of a Parkinson's disease-associated gain-of-function mutant LRRK2 (G2019S) inhibited the uncoating of AP2M1 from CCVs at later stages and prevented new cycles of CCV formation. Thus, the abundance and activity of LRRK2 must be calibrated to ensure proper endocytosis. Dysregulated phosphorylation of AP2M1 from the brain but not thyroid tissues of LRRK2 knockout and G2019S-knockin mice suggests a tissue-specific regulatory mechanism of endocytosis. Furthermore, we found that LRRK2-dependent phosphorylation of AP2M1 mediated dopaminergic neurodegeneration in a Drosophila model of PD. Together, our findings provide a mechanistic link between LRRK2, AP2, and endocytosis in the pathogenesis of PD.

Dynamic instability of clathrin assembly provides proofreading control for endocytosis

Clathrin-mediated endocytosis depends on the formation of functional clathrin-coated pits that recruit cargos and mediate the uptake of those cargos into the cell. However, it remains unclear whether the cargos in the growing clathrin-coated pits are actively monitored by the coat assembly machinery. Using a cell-free reconstitution system, we report that clathrin coat formation and cargo sorting can be uncoupled, indicating that a checkpoint is required for functional cargo incorporation. We demonstrate that the ATPase Hsc70 and a dynamic exchange of clathrin during assembly are required for this checkpoint. In the absence of Hsc70 function, clathrin assembles into pits but fails to enrich cargo. Using single-molecule imaging, we further show that uncoating takes place throughout the lifetime of the growing clathrin-coated pits. Our results suggest that the dynamic exchange of clathrin, at the cost of the reduced overall assembly rates, primarily serves as a proofreading mechanism for quality control of endocytosis.

SMrT Assay for Real-Time Visualization and Analysis of Clathrin Assembly Reactions

Clathrin-mediated endocytosis manages the vesicular transport of the bulk of membrane proteins from the plasma membrane and the trans-Golgi network. During this process, discrete sets of adaptor proteins recognize specific classes of membrane proteins, which recruit and assemble clathrin lattices on the membrane. An important determinant to the success of this vesicular transport reaction is the intrinsic ability of adaptors to polymerize clathrin on a membrane surface. Adaptor-induced clathrin assembly has traditionally been analyzed using static electron microscopy-based approaches. Here, we describe a methodology to follow adaptor-induced clathrin assembly in real-time using fluorescence microscopy on a facile model membrane assay system of supported membrane tubes (SMrT). Results from such assays can be conveniently run through routine image analysis procedures to extract kinetic parameters of the clathrin assembly reaction.

The progressive ankylosis protein ANK facilitates clathrin- and adaptor-mediated membrane traffic at the trans-Golgi network-to-endosome interface

Dominant or recessive mutations in the progressive ankylosis gene ANKH have been linked to familial chondrocalcinosis (CCAL2), craniometaphyseal dysplasia (CMD), mental retardation, deafness and ankylosis syndrome (MRDA). The function of the encoded membrane protein ANK in cellular compartments other than the plasma membrane is unknown. Here, we show that ANK localizes to the trans-Golgi network (TGN), clathrin-coated vesicles and the plasma membrane. ANK functionally interacts with clathrin and clathrin associated adaptor protein (AP) complexes as loss of either protein causes ANK dispersion from the TGN to cytoplasmic endosome-like puncta. Consistent with its subcellular localization, loss of ANK results in reduced formation of tubular membrane carriers from the TGN, perinuclear accumulation of early endosomes and impaired transferrin endocytosis. Our data indicate that clathrin/AP-mediated cycling of ANK between the TGN, endosomes, and the cell surface regulates membrane traffic at the TGN/endosomal interface. These findings suggest that dysfunction of Golgi-endosomal membrane traffic may contribute to ANKH-associated pathologies.

Spatial Control of Epsin-induced Clathrin Assembly by Membrane Curvature

Epsins belong to the family of highly conserved clathrin-associated sorting proteins that are indispensable for clathrin-mediated endocytosis, but their precise functions remain unclear. We have developed an assay system of budded supported membrane tubes displaying planar and highly curved membrane surfaces to analyze intrinsic membrane curvature preference shown by clathrin-associated sorting proteins. Using real-time fluorescence microscopy, we find that epsin preferentially partitions to and assembles clathrin on highly curved membrane surfaces. Sorting of epsin to regions of high curvature strictly depends on binding to phosphatidylinositol 4,5-bisphosphate. Fluorescently labeled clathrins rapidly assemble as foci, which in turn cluster epsin, while maintaining tube integrity. Clathrin foci grow in intensity with a typical time constant of ∼75 s, similar to the time scales for coated pit formation seen in cells. Epsin therefore effectively senses membrane curvature to spatially control clathrin assembly. Our results highlight the potential role of membrane curvature in orchestrating the myriad molecular interactions necessary for the success of clathrin-mediated membrane budding.

Recruitment of coat proteins to liposomes and peptidoliposomes

Intracellular transport within the cell is generally mediated by membrane vesicles. Their formation is typically initiated by activation of small GTPases that then recruit cytosolic proteins to the membrane surface to form a coat, interact with cargo and accessory proteins, and deform the lipid bilayer to produce a transport vesicle. Liposomes proved to be a useful tool to study the molecular mechanisms of these processes in vitro. Here we describe the use of liposomes and peptidoliposomes presenting lipid-coupled cytosolic tails of cargo proteins for the in vitro analysis of the membrane recruitment of AP-1 adaptors in the process of forming AP-1/clathrin coats. AP-1 recruitment is mediated by the GTPase Arf1 and requires specific lipids and cargo signals. Interaction with cargo induces AP-1 oligomerization already in the absence of clathrin. Without cargo peptides, accessory proteins, such as amphiphysin 2, can be identified that stabilize AP-1 binding to liposomal membranes.

Isolation of clathrin-coated vesicles from tissue culture cells

The study of clathrin-coated vesicles (CCVs) isolated from various organs has revealed the identities and important features of many of the factors involved in membrane trafficking. The development of isolation methods using cultured cell lines has made it possible to manipulate the source material before isolation to ask important questions about the roles of these factors and the pathways in which they are involved. We discuss here the advantages and limitations of the use of cultured cell lines for the isolation of CCVs.

Presynaptic clathrin levels are a limiting factor for synaptic transmission

To maintain communication, neurons must recycle their synaptic vesicles with high efficiency. This process places a huge burden on the clathrin-mediated endocytic machinery, but the consequences of this are poorly understood. We found that the amount of clathrin in a presynaptic terminal is not fixed. During stimulation, clathrin moves out of synapses as a function of stimulus strength and neurotransmitter release probability, which, together with membrane coat formation, transiently reduces the available pool of free clathrin triskelia. Correlative functional and morphological experiments in cholinergic autapses established by superior cervical ganglion neurons in culture show that presynaptic terminal function is compromised if clathrin levels fall by 20% after clathrin heavy chain knock down using RNAi. Synaptic transmission is depressed due to a reduction of cytoplasmic and readily releasable pools of vesicles. However, synaptic depression reverts after dialysis of exogenous clathrin, thus compensating RNAi-induced depletion. Lowering clathrin levels also reduces quantal size, which occurs concomitantly with a decrease in the size of synaptic vesicles. Large dense-core vesicles are unaffected by clathrin knock down. Together, our results show that clathrin levels are a dynamic property of presynaptic terminals that can influence short-term plasticity in a stimulus-dependent manner.

Coordination of adjacent domains mediates TACC3-ch-TOG-clathrin assembly and mitotic spindle binding

A complex of transforming acidic coiled-coil protein 3 (TACC3), colonic and hepatic tumor overexpressed gene (ch-TOG), and clathrin has been implicated in mitotic spindle assembly and in the stabilization of kinetochore fibers by cross-linking microtubules. It is unclear how this complex binds microtubules and how the proteins in the complex interact with one another. TACC3 and clathrin have each been proposed to be the spindle recruitment factor. We have mapped the interactions within the complex and show that TACC3 and clathrin were interdependent for spindle recruitment, having to interact in order for either to be recruited to the spindle. The N-terminal domain of clathrin and the TACC domain of TACC3 in tandem made a microtubule interaction surface, coordinated by TACC3-clathrin binding. A dileucine motif and Aurora A-phosphorylated serine 558 on TACC3 bound to the "ankle" of clathrin. The other interaction within the complex involved a stutter in the TACC3 coiled-coil and a proposed novel sixth TOG domain in ch-TOG, which was required for microtubule localization of ch-TOG but not TACC3-clathrin.

Spatiotemporal control of endocytosis by phosphatidylinositol-3,4-bisphosphate

Phosphoinositides serve crucial roles in cell physiology, ranging from cell signalling to membrane traffic. Among the seven eukaryotic phosphoinositides the best studied species is phosphatidylinositol-4,5-bisphosphate (PI(4,5)P2), which is concentrated at the plasma membrane where, among other functions, it is required for the nucleation of endocytic clathrin-coated pits. No phosphatidylinositol other than PI(4,5)P2 has been implicated in clathrin-mediated endocytosis, whereas the subsequent endosomal stages of the endocytic pathway are dominated by phosphatidylinositol-3-phosphates(PI(3)P). How phosphatidylinositol conversion from PI(4,5)P2-positive endocytic intermediates to PI(3)P-containing endosomes is achieved is unclear. Here we show that formation of phosphatidylinositol-3,4-bisphosphate (PI(3,4)P2) by class II phosphatidylinositol-3-kinase C2α (PI(3)K C2α) spatiotemporally controls clathrin-mediated endocytosis. Depletion of PI(3,4)P2 or PI(3)K C2α impairs the maturation of late-stage clathrin-coated pits before fission. Timed formation of PI(3,4)P2 by PI(3)K C2α is required for selective enrichment of the BAR domain protein SNX9 at late-stage endocytic intermediates. These findings provide a mechanistic framework for the role of PI(3,4)P2 in endocytosis and unravel a novel discrete function of PI(3,4)P2 in a central cell physiological process.

Hsc70-induced changes in clathrin-auxilin cage structure suggest a role for clathrin light chains in cage disassembly

The molecular chaperone, Hsc70, together with its co-factor, auxilin, facilitates the ATP-dependent removal of clathrin during clathrin-mediated endocytosis in cells. We have used cryo-electron microscopy to determine the 3D structure of a complex of clathrin, auxilin^401-910 and Hsc70 at pH 6 in the presence of ATP, frozen within 20 seconds of adding Hsc70 in order to visualize events that follow the binding of Hsc70 to clathrin and auxilin before clathrin disassembly. In this map, we observe density beneath the vertex of the cage that we attribute to bound Hsc70. This density emerges asymmetrically from the clathrin vertex, suggesting preferential binding by Hsc70 for one of the three possible sites at the vertex. Statistical comparison with a map of whole auxilin and clathrin previously published by us reveals the location of statistically significant differences which implicate involvement of clathrin light chains in structural rearrangements which occur after Hsc70 is recruited. Clathrin disassembly assays using light scattering suggest that loss of clathrin light chains reduces the efficiency with which auxilin facilitates this reaction. These data support a regulatory role for clathrin light chains in clathrin disassembly in addition to their established role in regulating clathrin assembly.

Phosphorylation by Dyrk1A of clathrin coated vesicle-associated proteins: identification of the substrate proteins and the effects of phosphorylation

Dyrk1A phosphorylated multiple proteins in the clathrin-coated vesicle (CCV) preparations obtained from rat brains. Mass spectrometric analysis identified MAP1A, MAP2, AP180, and α- and β-adaptins as the phosphorylated proteins in the CCVs. Each protein was subsequently confirmed by [(32)P]-labeling and immunological methods. The Dyrk1A-mediated phosphorylation released the majority of MAP1A and MAP2 and enhanced the release of AP180 and adaptin subunits from the CCVs. Furthermore, Dyrk1A displaced adaptor proteins physically from CCVs in a kinase-concentration dependent manner. The clathrin heavy chain release rate, in contrast, was not affected by Dyrk1A. Surprisingly, the Dyrk1A-mediated phosphorylation of α- and β-adaptins led to dissociation of the AP2 complex, and released only β-adaptin from the CCVs. AP180 was phosphorylated by Dyrk1A also in the membrane-free fractions, but α- and β-adaptins were not. Dyrk1A was detected in the isolated CCVs and was co-localized with clathrin in neurons from mouse brain sections and from primary cultured rat hippocampus. Previously, we proposed that Dyrk1A inhibits the onset of clathrin-mediated endocytosis in neurons by phosphorylating dynamin 1, amphiphysin 1, and synaptojanin 1. Current results suggest that besides the inhibition, Dyrk1A promotes the uncoating process of endocytosed CCVs.

The apoptotic engulfment protein Ced-6 participates in clathrin-mediated yolk uptake in Drosophila egg chambers

During oogenesis in Drosophila, the phagocytic engulfment protein Ced-6 recognizes the atypical endocytic sorting signal within the vitellogenin receptor Yolkless. Because Ced-6 displays all of the features of an authentic clathrin adaptor, an unrecognized clathrin dependence for Ced-6/Gulp operation during phagocytosis is possible.

Clathrin-mediated endocytosis and phagocytosis are both selective surface internalization processes but have little known mechanistic similarity or interdependence. Here we show that the phosphotyrosine-binding (PTB) domain protein Ced-6, a well-established phagocytosis component that operates as a transducer of so-called “eat-me” signals during engulfment of apoptotic cells and microorganisms, is expressed in the female Drosophila germline and that Ced-6 expression correlates with ovarian follicle development. Ced-6 exhibits all the known biochemical properties of a clathrin-associated sorting protein, yet ced-6–null flies are semifertile despite massive accumulation of soluble yolk precursors in the hemolymph. This is because redundant sorting signals within the cytosolic domain of the Drosophila vitellogenin receptor Yolkless, a low density lipoprotein receptor superfamily member, occur; a functional atypical dileucine signal binds to the endocytic AP-2 clathrin adaptor directly. Nonetheless, the Ced-6 PTB domain specifically recognizes the noncanonical Yolkless FXNPXA sorting sequence and in HeLa cells promotes the rapid, clathrin-dependent uptake of a Yolkless chimera lacking the distal dileucine signal. Ced-6 thus operates in vivo as a clathrin adaptor. Because the human Ced-6 orthologue GULP similarly binds to clathrin machinery, localizes to cell surface clathrin-coated structures, and is enriched in placental clathrin-coated vesicles, new possibilities for Ced-6/Gulp operation during phagocytosis must be considered.

Coupling between clathrin-dependent endocytic budding and F-BAR-dependent tubulation in a cell-free system

Cell-free reconstitution of membrane traffic reactions and the morphological characterization of membrane intermediates that accumulate under these conditions have helped to elucidate the physical and molecular mechanisms involved in membrane transport. To gain a better understanding of endocytosis, we have reconstituted vesicle budding and fission from isolated plasma membrane sheets and imaged these events. Electron and fluorescence microscopy, including subdiffraction-limit imaging by stochastic optical reconstruction microscopy (STORM), revealed F-BAR (FBP17) domain coated tubules nucleated by clathrin-coated buds when fission was blocked by GTPgammaS. Triggering fission by replacing GTPgammaS with GTP led not only to separation of clathrin-coated buds, but also to vesicle formation by fragmentation of the tubules. These results suggest a functional link between FBP17-dependent membrane tubulation and clathrin-dependent budding. They also show that clathrin spatially directs plasma membrane invaginations that lead to the generation of endocytic vesicles larger than those enclosed by the coat.

Dyrk1A binds to multiple endocytic proteins required for formation of clathrin-coated vesicles

In spite of a nuclear targeting sequence, a substantial amount of dual-specificity tyrosine phosphorylation-regulated kinase (Dyrk1A) is located within the cytoplasm of neurons. Analysis of fractionated rat brains revealed that the majority of Dyrk1A was in the postnuclear precipitate. The kinase in this fraction was resistant to high salt and Triton X-100 extraction at pH 6.5. Hypothesizing that Dyrk1A binds tightly with cell constituents, we searched for Dyrk1A binding proteins in the Triton X-100-insoluble fraction extracted with urea and fractionated by column chromatography. An overlay assay using the recombinant kinase revealed that multiple proteins are capable of binding to Dyrk1A. Among them, we identified clathrin heavy chain and dynamin 1 as potential candidates. An overlay assay using purified and partially purified proteins showed the binding of Dyrk1A with both proteins. Under native conditions, Dyrk1A precipitated with newly formed clathrin cages and with dynamin via the GST-amphiphysin SH3 domain. We also identified another endocytic protein, endophilin 1, as an additional Dyrk1A binding protein. We then tested whether the clathrin-coated vesicle (CCV)-associated proteins could be phosphorylated by Dyrk1A. Multiple proteins apparently distinctive from the known substrates were phosphorylated in the brain CCV. Our findings suggest a role for Dyrk1A in controlling synaptic vesicle recycling processes.

Coordinated regulation of AP2 uncoating from clathrin-coated vesicles by rab5 and hRME-6

Here we investigate the role of rab5 and its cognate exchange factors rabex-5 and hRME-6 in the regulation of AP2 uncoating from endocytic clathrin-coated vesicles (CCVs). In vitro, we show that the rate of AP2 uncoating from CCVs is dependent on the level of functional rab5. In vivo, overexpression of dominant-negative rab5(S34N), or small interfering RNA (siRNA)-mediated depletion of hRME-6, but not rabex-5, resulted in increased steady-state levels of AP2 associated with endocytic vesicles, which is consistent with reduced uncoating efficiency. hRME-6 guanine nucleotide exchange factor activity requires hRME-6 binding to alpha-adaptin ear, which displaces the ear-associated mu2 kinase AAK1. siRNA-mediated depletion of hRME-6 increases phospho-mu2 levels, and expression of a phosphomimetic mu2 mutant increases levels of endocytic vesicle-associated AP2. Depletion of hRME-6 or rab5(S35N) expression also increases the levels of phosphoinositide 4,5-bisphosphate (PtdIns(4,5)P(2)) associated with endocytic vesicles. These data are consistent with a model in which hRME-6 and rab5 regulate AP2 uncoating in vivo by coordinately regulating mu2 dephosphorylation and PtdIns(4,5)P(2) levels in CCVs.

Endocytosis assays in intact and permeabilized cells

Clathrin-coated pits and vesicles represent the major ports of entry into most eukaryotic cells. As well as performing housekeeping functions (e.g., allowing cells to take up essential nutrients), the endocytic pathway participates in a number of tissue-specific events such as synaptic-vesicle recycling, control of morphogen gradients during development, downregulation of receptor tyrosine kinases, and immune surveillance. To understand the role played by clathrin-mediated uptake, it is therefore essential to have robust endocytosis assays in intact cells. The clathrin-coated vesicle cycle requires a complicated interplay of proteins and lipids that is regulated in space and time. Reconstitution assays in permeabilized cells provide a powerful approach to understanding how this complex process is regulated.

Recruitment of coat proteins to peptidoliposomes

Intracellular transport between compartments within the cell is generally mediated by membrane vesicles. Their formation is initiated by activation of small GTPases that then recruit cytosolic proteins to the membrane surface to form a coat, interact with cargo proteins, and deform the lipid bilayer. Liposomes proved to be a useful tool to study the molecular mechanisms of these processes in vitro. To analyze the involvement of membrane proteins, the cytosolically exposed sequences may be coupled chemically to reactive lipids in the membrane. Here we describe the use of such peptidoliposomes presenting lipid-coupled cytosolic tails of cargo proteins for the in vitro analysis of the membrane recruitment of AP-1 adaptors in the process of forming AP-1/clathrin coats. AP-1 recruitment is mediated by the GTPase Arf1, requires specific lipids, and cargo signals. Interaction with cargo induces AP-1 oligomerization already in the absence of clathrin.

Identification of a structural motif crucial for infectivity of hepatitis B viruses

Infectious entry of hepatitis B viruses (HBV) has nonconventional facets. Here we analyzed whether a cell-permeable peptide [translocation motif (TLM)] identified within the surface protein of human HBV is a general feature of all hepadnaviruses and plays a role in the viral life cycle. Surface proteins of all hepadnaviruses contain conserved functional TLMs. Genetic inactivation of the duck HBV TLMs does not interfere with viral morphogenesis; however, these mutants are noninfectious. TLM mutant viruses bind to cells and are taken up into the endosomal compartment, but they cannot escape from endosomes. Processing of surface protein by endosomal proteases induces their exposure on the virus surface. This unmasking of TLMs mediates translocation of viral particles across the endosomal membrane into the cytosol, a prerequisite for productive infection. The ability of unmasked TLMs to translocate processed HBV particles across cellular membranes was shown by confocal immunofluorescence microscopy and by infection of nonpermissive cell lines with HBV processed in vitro with endosomal lysate. Based on these data, we propose an infectious entry mechanism unique for hepadnaviruses that involves virus internalization by receptor-mediated endocytosis followed by processing of surface protein in endosomes. This processing activates the function of TLMs that are essential for viral particle translocation through the endosomal membrane into the cytosol and productive infection.

The small G-protein Arf6GTP recruits the AP-2 adaptor complex to membranes

The small GTP-binding protein ADP-ribosylation factor 6 (Arf6) is involved in plasma membrane/endosomes trafficking. However, precisely how the activation of Arf6 regulates vesicular transport is still unclear. Here, we show that, in vitro, recombinant Arf6GTP recruits purified clathrin-adaptor complex AP-2 (but not AP-1) onto phospholipid liposomes in the absence of phosphoinositides. We also show that phosphoinositides and Arf6 tightly cooperate to translocate AP-2 to the membrane. In vivo, Arf6GTP (but not Arf6GDP) was found associated to AP-2. The expression of the GTP-locked mutant of Arf6 leads to the plasma membrane redistribution of AP-2 in Arf6GTP-enriched areas. Finally, we demonstrated that the expression of the GTP-locked mutant of Arf6 inhibits transferrin receptor internalization without affecting its recycling. Altogether, our results demonstrated that Arf6GTP interacts specifically with AP-2 and promotes its membrane recruitment. These findings strongly suggest that Arf6 plays a major role in clathrin-mediated endocytosis by directly controlling the assembly of the AP-2/clathrin coat.

Reconstitution of Rab4-dependent vesicle formation in vitro

We have developed an in vitro assay to reconstitute the formation of endosomal recycling vesicles. To achieve specificity for endosomes as the donor organelle, cells are surface-biotinylated and allowed to endocytose for 10 min, after which the remaining surface-biotin is stripped off. The cells are then permeabilized and the cytosol washed away. Upon addition of exogenous cytosol and energy, sealed vesicles containing biotinylated recycling receptors are produced. Modification of the cytosol, for example, by immunodepletion or addition of purified proteins, allows the identification of proteins involved in vesicle formation. The results show that recycling is mediated by AP-1/clathrin-coated vesicles, requires Rab4, and is negatively regulated by rabaptin-5/rabex-5.

In vitro formation of recycling vesicles from endosomes requires adaptor protein-1/clathrin and is regulated by rab4 and the connector rabaptin-5

The involvement of clathrin and associated adaptor proteins in receptor recycling from endosomes back to the plasma membrane is controversial. We have used an in vitro assay to identify the molecular requirements for the formation of recycling vesicles. Cells expressing the asialoglycoprotein receptor H1, a typical recycling receptor, were surface biotinylated and then allowed to endocytose for 10 min. After stripping away surface-biotin, the cells were permeabilized and the cytosol washed away. In a temperature-, cytosol-, and nucleotide-dependent manner, the formation of sealed vesicles containing biotinylated H1 could be reconstituted. Vesicle formation was strongly inhibited upon immunodepletion of adaptor protein (AP)-1, but not of AP-2 or AP-3, from the cytosol, and was restored by readdition of purified AP-1. Vesicle formation was stimulated by supplemented clathrin, but inhibited by brefeldin A, consistent with the involvement of ARF1 and a brefeldin-sensitive guanine nucleotide exchange factor. The GTPase rab4, but not rab5, was required to generate endosome-derived vesicles. Depletion of rabaptin-5/rabex-5, a known interactor of both rab4 and gamma-adaptin, stimulated and addition of the purified protein strongly inhibited vesicle production. The results indicate that recycling is mediated by AP-1/clathrin-coated vesicles and regulated by rab4 and rabaptin-5/rabex-5.

Regulation of Tyrosinase Processing and Trafficking by Organellar pH and by Proteasome Activity

Pigmentation of the hair, skin, and eyes of mammals results from a number of melanocyte-specific proteins that are required for the biosynthesis of melanin. Those proteins comprise the structural and enzymatic components of melanosomes, the membrane-bound organelles in which melanin is synthesized and deposited. Tyrosinase (TYR) is absolutely required for melanogenesis, but other melanosomal proteins, such as TYRP1, DCT, and gp100, also play important roles in regulating mammalian pigmentation. However, pigmentation does not always correlate with the expression of TYR mRNA/protein, and thus its function is also regulated at the post-translational level. Thus, TYR does not necessarily exist in a catalytically active state, and its post-translational activation could be an important control point for regulating melanin synthesis. In this study, we used a multidisciplinary approach to examine the processing and sorting of TYR through the endoplasmic reticulum (ER), Golgi apparatus, coated vesicles, endosomes and early melanosomes because those organelles hold the key to understanding the trafficking of TYR to melanosomes and thus the regulation of melanogenesis. In pigmented cells, TYR is trafficked through those organelles rapidly, but in amelanotic cells, TYR is retained within the ER and is eventually degraded by proteasomes. We now show that TYR can be released from the ER in the presence of protonophore or proton pump inhibitors which increase the pH of intracellular organelles, after which TYR is transported correctly to the Golgi, and then to melanosomes via the endosomal sorting system. The expression of TYRP1, which facilitates TYR processing in the ER, is down-regulated in the amelanotic cells; this is analogous to a hypopigmentary disease known as oculocutaneous albinism type 3 and further impairs melanin production. The sum of these results shows that organellar pH, proteasome activity, and down-regulation of TYRP1 expression all contribute to the lack of pigmentation in TYR-positive amelanotic melanoma cells.

Clathrin promotes incorporation of cargo into coated pits by activation of the AP2 adaptor micro2 kinase

Endocytic cargo such as the transferrin receptor is incorporated into clathrin-coated pits by associating, via tyrosine-based motifs, with the AP2 complex. Cargo–AP2 interactions occur via the μ2 subunit of AP2, which needs to be phosphorylated for endocytosis to occur. The most likely role for μ2 phosphorylation is in cargo recruitment because μ2 phosphorylation enhances its binding to internalization motifs. Here, we investigate the control of μ2 phosphorylation. We identify clathrin as a specific activator of the μ2 kinase and, in permeabilized cells, we show that ligand sequestration, driven by exogenous clathrin, results in elevated levels of μ2 phosphorylation. Furthermore, we show that AP2 containing phospho-μ2 is mainly associated with assembled clathrin in vivo, and that the level of phospho-μ2 is strongly reduced in a chicken B cell line depleted of clathrin heavy chain. Our results imply a central role for clathrin in the regulation of cargo selection via the modulation of phospho-μ2 levels.

Munc18 interacting proteins: ADP-ribosylation factor-dependent coat proteins that regulate the traffic of beta-Alzheimer's precursor protein

Coat proteins cycle between soluble and membrane-bound locations at the time of vesicle biogenesis and act to regulate the assembly of the vesicle coat that determines the specificity in cargo selection and the destination of the vesicle. A transmembrane cargo protein, an Arf GTPase, and a coat protein (e.g. COPs, APs, or GGAs) are minimal components required for budding of vesicles. Munc18 interacting proteins (MINTs) are a family of three proteins implicated in the localization of receptors to the plasma membrane. We show that MINTs bind Arfs directly, co-localize with Arf and the Alzheimer's precursor protein (beta-APP) to regions of the Golgi/trans-Golgi network, and can co-immunoprecipitate clathrin. We demonstrate that MINTs bind Arfs through a region of the PTB domain and the PDZ2 domain, and Arf-MINT interaction is necessary for the increased cellular levels of beta-APP produced by MINT overexpression. Knockdown (small interference RNA) experiments implicate beta-APP as a transmembrane cargo protein that works together with MINTs. We propose that MINTs are a family of Arf-dependent, vesicle-coat proteins that can regulate the traffic of beta-APP.

The autosomal recessive hypercholesterolemia (ARH) protein interfaces directly with the clathrin-coat machinery

The low density lipoprotein (LDL) receptor plays a pivotal role in cholesterol metabolism. Inherited mutations that disturb the activity of the receptor lead to elevations in plasma cholesterol levels and early-onset coronary atherosclerosis. Defects in either the LDL receptor or apolipoprotein B, the proteinaceous component of LDL particles that binds the LDL receptor, elevate circulating LDL-cholesterol levels in an autosomal-dominant fashion, with heterozygotes displaying values between homozygous and normal individuals. Rarely, similar clinical phenotypes occur with a recessive pattern of inheritance, and several genetic lesions in the autosomal recessive hypercholesterolemia (ARH) gene on chromosome 1 have been mapped in this class of patients. ARH has an N-terminal phosphotyrosine-binding (PTB) domain evolutionarily related to that found in Disabled-2 and numb, two endocytic proteins. PTB domains bind to the consensus sequence FXNPXY, corresponding to the internalization motif of the LDL receptor. We show here that in addition to the FXNPXY sequence, ARH binds directly to soluble clathrin trimers and to clathrin adaptors by a mode involving the independently folded appendage domain of the beta subunit. At steady state, ARH colocalizes with endocytic proteins in HeLa cells, and the LDL receptor fluxes through peripheral ARH-positive sites before delivery to early endosomes. Because ARH also binds directly to phosphoinositides, which regulate clathrin bud assembly at the cell surface, our data suggest that in ARH patients, defective sorting adaptor function in hepatocytes leads to faulty LDL receptor traffic and hypercholesterolemia.

ARF1.GTP, tyrosine-based signals, and phosphatidylinositol 4,5-bisphosphate constitute a minimal machinery to recruit the AP-1 clathrin adaptor to membranes

At the trans-Golgi network, clathrin coats containing AP-1 adaptor complexes are formed in an ARF1-dependent manner, generating vesicles transporting cargo proteins to endosomes. The mechanism of site-specific targeting of AP-1 and the role of cargo are poorly understood. We have developed an in vitro assay to study the recruitment of purified AP-1 adaptors to chemically defined liposomes presenting peptides corresponding to tyrosine-based sorting motifs. AP-1 recruitment was found to be dependent on myristoylated ARF1, GTP or nonhydrolyzable GTP-analogs, tyrosine signals, and small amounts of phosphoinositides, most prominently phosphatidylinositol 4,5-bisphosphate, in the absence of any additional cytosolic or membrane bound proteins. AP-1 from cytosol could be recruited to a tyrosine signal independently of the lipid composition, but the rate of recruitment was increased by phosphatidylinositol 4,5-bisphosphate. The results thus indicate that cargo proteins are involved in coat recruitment and that the local lipid composition contributes to specifying the site of vesicle formation.

Identification of a non-canonical tyrosine-based endocytic motif in an ionotropic receptor

Rapid modulation of the surface number of certain ionotropic receptors is achieved by altering the relative rates of insertion and internalization. These receptors are internalized by a clathrin-mediated pathway; however, a motif that is necessary for endocytosis of ionotropic receptors has not yet been identified. Here, we identified a motif that is required for constitutive and agonist-regulated internalization of the ionotropic P2X(4) receptor. Three amino acids in the C terminus of P2X(4) (Tyr(378), Gly(381), and Leu(382)) compose a non-canonical tyrosine-based sorting signal of the form YXXGL. We found that P2X(4) protein was present in clathrin-coated vesicles isolated from rat brain and that a glutathione S-transferase fusion of the P2X(4) C terminus pulled down the adaptor protein-2 complex from brain extract. Mutation of either the tyrosine-binding pocket of the mu2 subunit of adaptor protein-2 or the YXXGL motif in the receptor C terminus caused a decrease in receptor internalization and a dramatic increase in the surface expression of P2X(4) receptors. The YXXGL motif represents a non-canonical tyrosine-based sorting signal that is necessary for efficient endocytosis of the P2X(4) receptor. Similar motifs are present in other receptors and may be important for the control of their functional expression.

CK2 and GAK/auxilin2 are major protein kinases in clathrin-coated vesicles

Several peripheral membrane proteins associated with clathrin-coated vesicles (CCVs) are reversibly phosphorylated, but it is not clear precisely which protein kinases are involved. In order to address this question directly, we have isolated highly purified CCVs from porcine brain. The peripheral membrane proteins have been removed and assayed for kinase activity using the CCV peripheral membrane proteins as substrate. The major kinase activity identified has a molecular mass of 40 kDa, is inhibited by known specific inhibitors of the protein kinase CK2 and is recognised by an antibody specific to CK2. We show that CK2 is responsible for the phosphorylation of the majority of CCV-associated proteins that are subject to phosphorylation. Intriguingly, CK2 is inactive when associated with CCVs but becomes active once the clathrin coat has been removed. The medium subunit of the AP2 adaptor complex (mu2) is not a substrate for CK2, but is phosphorylated by a second kinase that we show to be cyclin G-associated kinase (GAK/auxilin2). Unlike the situation for the CK2 substrates, mu2 is a substrate for GAK/auxilin2, both in intact CCVs and in solution. In addition, we show that the 'stripped' CCV membranes that remain once the peripheral membrane proteins have been removed from CCVs inhibit CK2 but not GAK/auxilin2 activity.

Phosphorylation of the AP2 mu subunit by AAK1 mediates high affinity binding to membrane protein sorting signals

During receptor-mediated endocytosis, AP2 complexes act as a bridge between the cargo membrane proteins and the clathrin coat by binding to sorting signals via the mu 2 subunit and to clathrin via the beta subunit. Here we show that binding of AP2 to sorting signals in vitro is regulated by phosphorylation of the mu 2 subunit of AP2. Phosphorylation of mu 2 enhances the binding affinity of AP2 for sorting motifs as much as 25-fold compared with dephosphorylated AP2. The recognition of sorting signals was not affected by the phosphorylation status of the alpha or beta 2 subunit, suggesting that phosphorylation of mu 2 is critical for regulation of AP2 binding to sorting signals. Phosphorylation of mu 2 occurs at a single threonine residue (Thr-156) and is mediated by the newly discovered adaptor-associated kinase, AAK1, which copurifies with AP2. We propose that phosphorylation of the AP2 mu 2 subunit by AAK1 ensures high affinity binding of AP2 to sorting signals of cargo membrane proteins during the initial steps of receptor-mediated endocytosis.

Internalization of metabotropic glutamate receptor in C6 cells through clathrin-coated vesicles

C6 glioma cells were treated for hours with 100 microM L-glutamate, quisqualate or trans-ACPD. In all cases, phospholipase C-coupled metabotropic glutamate receptors (mGluRs) present in these cells are down-regulated after this agonist treatment. Cell surface metabotropic glutamate receptor density was minimum at 6 h of agonist treatment and reached near control values after 30 h of treatment. This recovery was associated with a progressive increase in mGluR1 and mGluR1a mRNA level between 6 and 24 h and was not due to agonist removal. Specific L-[3H]glutamate or [3H](+/-)trans-ACPD binding decrease detected in C6 cells after 6 h of 100 microM L-glutamate treatment was associated with a remarkable increase of specific L-[3H]glutamate binding detected in clathrin-coated vesicles isolated from these treated cells. Moreover, this decrease was blocked in the presence of 0.5 M sucrose or 1 microM phenylarsine oxide, suggesting that desensitization and down-regulation of mGluR can be due to an endocytosis process through clathrin-coated pits and vesicles.

Mu 2 binding directs the cystic fibrosis transmembrane conductance regulator to the clathrin-mediated endocytic pathway

The cystic fibrosis transmembrane conductance regulator (CFTR) contains a conserved tyrosine-based internalization motif, (1424)YDSI, which interacts with the endocytic clathrin adaptor complex, AP-2, and is required for its efficient endocytosis. Although direct interactions between several endocytic sequences and the medium chain and endocytic clathrin adaptor complexes have been shown by protein-protein interaction assays, whether all these interactions occur in vivo or are physiologically important has not always been addressed. Here we show, using both in vitro and in vivo assays, a physiologically relevant interaction between CFTR and the mu subunit of AP-2. Cross-linking experiments were performed using photoreactive peptides containing the YDSI motif and purified adaptor complexes. CFTR peptides cross-linked a 50-kDa subunit of purified AP-2 complexes, the apparent molecular mass of mu 2. Furthermore, isolated mu 2 bound to the sorting motif, YDSI, both in cross-linking experiments and glutathione S-transferase pull-down experiments, confirming that mu 2 mediates the interaction between CFTR and AP-2 complexes. Inducible overexpression of dominant-negative mu 2 in HeLa cells results in AP-2 complexes that fail to interact with CFTR. Moreover, internalization of CFTR in mutant cells is greatly reduced compared with wild type HeLa cells. These results indicate that the AP-2 endocytic complex selectively interacts with the conserved tyrosine-based internalization signal in the carboxyl terminus of CFTR, YDSI. Furthermore, this interaction is mediated by the mu 2 subunit of AP-2 and mutations in mu 2 that block its interaction with YDSI inhibit the incorporation of CFTR into the clathrin-mediated endocytic pathway.

The assembly of AP-3 adaptor complex-containing clathrin-coated vesicles on synthetic liposomes

The heterotetrameric adaptor protein complex AP-3 has been shown to function in the sorting of proteins to the endosomal/lysosomal system. However, the mechanism of AP-3 recruitment onto membranes is poorly understood, and it is still uncertain whether AP-3 nucleates clathrin-coated vesicles. Using purified components, we show that AP-3 and clathrin are recruited onto protein-free liposomes and Golgi-enriched membranes by a process that requires ADP-ribosylation factor (ARF) and GTP but no other proteins or nucleotides. The efficiency of recruitment onto the two sources of membranes is comparable and independent of the composition of the liposomes. Clathrin binding occurred in a cooperative manner as a function of the membrane concentration of AP-3. Thin-section electron microscopy of liposomes and Golgi-enriched membranes that had been incubated with AP-3, clathrin, and ARF.GTP showed the presence of clathrin-coated buds and vesicles. These results establish that AP-3-containing clathrin-coated vesicles form in vitro and are consistent with AP-3-dependent protein transport being mediated by clathrin-coated vesicles.

Direct and GTP-dependent interaction of ADP-ribosylation factor 1 with clathrin adaptor protein AP-1 on immature secretory granules

ADP-ribosylation factor 1 (ARF1) mediates clathrin coat formation on PC12 immature secretory granules (ISGs). We have used two approaches to investigate whether ARF1 interacts directly with the clathrin adaptor protein, AP-1. Using an in vitro recruitment assay and co-immunoprecipitation, we could isolate an AP-1.ARF1 complex. Then we used a site-directed photocross-linking approach to determine the components that act downstream of ARF1 in clathrin coat formation on ISGs. Myristoylated ARF1, with a photolabile phenylalanine analogue incorporated into its putative effector domain (switch 1), showed a specific, GTP-dependent interaction with both the gamma- and beta-adaptin subunits of AP-1 on ISGs. These experiments provide evidence for a direct interaction of ARF1 with AP-1. On mature secretory granules myristoylated ARF1 does not bind, and hence clathrin coat formation cannot be initiated, supporting the hypothesis that molecules involved in coat recruitment are removed during ISG maturation.

Simian and human immunodeficiency virus Nef proteins use different surfaces to downregulate class I major histocompatibility complex antigen expression

Simian immunodeficiency virus (SIV) and human immunodeficiency virus type 1 (HIV-1) Nef proteins are related regulatory proteins that share several functions, including the ability to downregulate class I major histocompatibility complex (MHC) and CD4 expression on the cell surface and to alter T-cell-receptor-initiated signal transduction in T cells. We compared the mechanisms used by SIV mac239 Nef and HIV-1 Nef to downregulate class I MHC and found that the ability of SIV Nef to downregulate class I MHC requires a unique C-terminal region of the SIV mac239 Nef molecule which is not found in HIV-1 Nef. Interestingly, mutation of the PxxP motif in SIV Nef, unlike in HIV-1 Nef, does not affect class I MHC downregulation. We also found that downregulation of class I MHC by SIV Nef requires a conserved tyrosine in the cytoplasmic domain of the class I MHC heavy chain and involves accelerated endocytosis of class I complexes, as previously found with HIV-1 Nef. Thus, while SIV and HIV-1 Nef proteins use a similar mechanism to downregulate class I MHC expression, they have evolved different surfaces for molecular interactions with cell factors that regulate class I MHC traffic. Mutations in the C-terminal domain of SIV mac239 Nef selectively disrupt class I MHC downregulation, having no detectable effect on other functions of Nef, such as the downregulation of CD4 and CD3 surface expression, the stimulation of SIV virion infectivity, and the induction of SIV replication from T cells infected in the absence of stimulation. The resulting mutants will be useful reagents for studying the importance of class I MHC downregulation for SIV replication and AIDS pathogenesis in infected rhesus macaques.

Epsin binds to clathrin by associating directly with the clathrin-terminal domain. Evidence for cooperative binding through two discrete sites

Epsin is a recently identified protein that appears to play an important role in clathrin-mediated endocytosis. The central region of epsin 1, the so-called DPW domain, binds to the heterotetrameric AP-2 adaptor complex by associating directly with the globular appendage of the alpha subunit. We have found that this central portion of epsin 1 also associates with clathrin. The interaction with clathrin is direct and not mediated by epsin-bound AP-2. Alanine scanning mutagenesis shows that clathrin binding depends on the sequence (257)LMDLADV located within the epsin 1 DPW domain. This sequence, related to the known clathrin-binding sequences in the adaptor beta subunits, amphiphysin, and beta-arrestin, facilitates the association of epsin 1 with the terminal domain of the clathrin heavy chain. Unexpectedly, inhibiting the binding of AP-2 to the GST-epsin DPW fusion protein by progressively deleting DPW triplets but leaving the LMDLADV sequence intact, diminishes the association of clathrin in parallel with AP-2. Because the beta subunit of the AP-2 complex also contains a clathrin-binding site, optimal association with soluble clathrin appears to depend on the presence of at least two distinct clathrin-binding sites, and we show that a second clathrin-binding sequence (480)LVDLD, located within the carboxyl-terminal segment of epsin 1, also interacts with clathrin directly. The LMDLADV and LVDLD sequences act cooperatively in clathrin recruitment assays, suggesting that they bind to different sites on the clathrin-terminal domain. The evolutionary conservation of similar clathrin-binding sequences in several metazoan epsin-like molecules suggests that the ability to establish multiple protein-protein contacts within a developing clathrin-coated bud is an important aspect of epsin function.

The epithelial Na(+)/H(+) exchanger, NHE3, is internalized through a clathrin-mediated pathway

Trafficking of the Na(+)/H(+) exchanger isoform 3 (NHE3) between sub-apical vesicles and apical membrane of epithelial cells is a suggested mechanism of regulation of NHE3 activity. When epitope-tagged NHE3 was stably expressed in NHE-deficient Chinese hamster ovary cells, a sizable fraction was found in recycling endosomes. This system was used to analyze the mechanism of endocytosis of NHE3. Immunofluorescence and radiolabeling experiments showed that inhibition of clathrin-mediated endocytosis using hypertonicity, acid treatment, or K(+) depletion inhibited internalization of NHE3. Moreover, transient transfection of an inhibitory mutant of dynamin (DynS45N) blocked the clathrin-mediated uptake of transferrin, as well as the endocytosis of NHE3. In ileal villus cells, endogenous NHE3 was also found to co-purify with isolated clathrin-coated vesicles, thereby confirming their association in native tissues. The role of COP-I subunits in the intracellular traffic of NHE3 was evaluated using ldlF cells, which bear a temperature-sensitive mutation in the epsilon-COP subunit. At the permissive temperature, NHE3 distributed normally, whereas at the restrictive temperature, which induces rapid degradation of epsilon-COP, NHE3 was still internalized, but its subcellular distribution was altered. These results indicate that endocytosis of NHE3 occurs primarily via clathrin-coated pits and vesicles and that normal intracellular trafficking of NHE3 involves an epsilon-COP-dependent step.

Clathrin-coated vesicles bearing GAIP possess GTPase-activating protein activity in vitro

Galpha-interacting protein (GAIP) is a member of the RGS (regulators of G protein signaling) family, which serve as GAPs (GTPase-activating proteins) for Galpha subunits. Previously, we demonstrated that GAIP is localized on clathrin-coated vesicles (CCVs). Here, we tested whether GAIP-enriched vesicles could accelerate the GTPase activity of Galphai proteins. A rat liver fraction containing vesicular carriers (CV2) was enriched (4.5x) for GAIP by quantitative immunoblotting, and GAIP was detected on some of the vesicles in the CV2 fraction by immunoelectron microscopy. When liver fractions were added to recombinant Galphai3 and tested for GAP activity, only the CV2 fraction contained GAP activity. Increasing amounts of CV2 increased the activity, whereas immunodepletion of the CV2 fraction with an antibody against the C terminus of GAIP decreased GAP activity. CCV fractions were prepared from rat liver by using a protocol that maintains the clathrin coats. GAIP was enriched in these fractions and was detected on CCVs by immunogold labeling. Addition of increasing amounts of CCV to recombinant Galphai3 protein increased the GTPase activity. We conclude that CCVs possess GAP activity for Galphai3 and that membrane-associated GAIP is capable of interacting with Galphai3. The reconstitution of the interaction between a heterotrimeric G protein and GAIP on CCVs provides biochemical evidence for a model whereby the G protein and its GAP are compartmentalized on different membranes and come into contact at the time of vesicle fusion. Alternatively, they may be located on the same membrane and segregate at the time of vesicle budding.

Two elements target SIV Nef to the AP-2 clathrin adaptor complex, but only one is required for the induction of CD4 endocytosis

The simian immunodeficiency virus (SIV) and human immunodeficiency virus type 1 (HIV-1) Nef proteins induce the endocytosis of CD4 and class I MHC molecules. Here we show that SIV Nef interacts with the AP-2 adaptor complex via two elements located in the N-terminal region of the Nef molecule, but only the N-distal element is required to induce CD4 endocytosis. This N-distal AP-2 targeting element contains no canonical endocytic signals and probably contacts the AP-2 complex via a novel interaction surface. The data support a model where SIV Nef induces CD4 endocytosis by promoting the normal interactions between the di-leucine sorting signal in the CD4 cytoplasmic domain and AP-2, but does not substitute for the CD4-AP-2 adaptor interaction. Neither element is important for the induction of class I MHC endocytosis, thus indicating that different mechanisms underlie the induction of class I MHC and CD4 endocytosis by Nef. In contrast to SIV Nef, HIV-1 Nef interacts with AP-2 via a surface containing a di-leucine endocytosis signal in the C-terminal disordered loop of Nef. The fact that genetic selection maintains similar molecular interactions via different surfaces in SIV and HIV-1 Nef proteins indicates that these interactions have critical roles for the viral life cycle in vivo.

Functional partnership between amphiphysin and dynamin in clathrin-mediated endocytosis

Amphiphysin, a protein that is highly concentrated in nerve terminals, has been proposed to function as a linker between the clathrin coat and dynamin in the endocytosis of synaptic vesicles. Here, using a cell-free system, we provide direct morphological evidence in support of this hypothesis. Unexpectedly, we also find that amphiphysin-1, like dynamin-1, can transform spherical liposomes into narrow tubules. Moreover, amphiphysin-1 assembles with dynamin-1 into ring-like structures around the tubules and enhances the liposome-fragmenting activity of dynamin-1 in the presence of GTP. These results show that amphiphysin binds lipid bilayers, indicate a potential function for amphiphysin in the changes in bilayer curvature that accompany vesicle budding, and imply a close functional partnership between amphiphysin and dynamin in endocytosis.

ADP-ribosylation factor 1 dependent clathrin-coat assembly on synthetic liposomes

The assembly of clathrin-coated vesicles on Golgi membranes is initiated by the GTP-binding protein ADP ribosylation factor (ARF), which generates high-affinity membrane-binding sites for the heterotetrameric AP-1 adaptor complex. Once bound, the AP-1 recruits clathrin triskelia, which polymerize to form the coat. We have found that ARF.GTP also recruits AP-1 and clathrin onto protein-free liposomes. The efficiency of this process is modulated by the composition of the liposomes, with phosphatidylserine being the most stimulatory phospholipid. There is also a requirement for cytosolic factor(s) other than ARF. Thin-section electron microscopy shows the presence of clathrin-coated buds and vesicles that resemble those formed in vivo. These results indicate that AP-1-containing clathrin-coated vesicles can form in the absence of integral membrane proteins. Thus, ARF.GTP, appropriate lipids, and cytosolic factor(s) are the minimal components necessary for AP-1 clathrin-coat assembly.

Sorting of furin at the trans-Golgi network. Interaction of the cytoplasmic tail sorting signals with AP-1 Golgi-specific assembly proteins

The eukaryotic subtilisin-like endoprotease furin is found predominantly in the trans-Golgi network (TGN) and cycles between this compartment, the cell surface, and the endosomes. There is experimental evidence for endocytosis from the plasma membrane and transport from endosomes to the TGN, but direct exit from the TGN to endosomes via clathrin-coated vesicles has only been discussed but not directly shown so far. Here we present data showing that expression of furin promotes the first step of clathrin-coat assembly at the TGN, the recruitment of the Golgi-specific assembly protein AP-1 on Golgi membranes. Further, we report that furin indeed is present in isolated clathrin-coated vesicles. Packaging into clathrin-coated vesicles requires signal components in the furin cytoplasmic domain which can be recognized by AP-1 assembly proteins. We found that besides depending on the phosphorylation state of a casein kinase II site, interaction of the furin tail with AP-1 and its mu1subunit is mediated by a tyrosine motif and to less extent by a leucine-isoleucine signal, whereas a monophenylalanine motif is only involved in binding to the intact AP-1 complex. This study implies that high affinity interaction of AP-1 or mu1 with the cytoplasmic tail of furin needs a complex interplay of signal components rather than one distinct signal.

In vitro binding study of adaptor protein complex (AP-1) to lysosomal targeting motif (LI-motif)

Lysosomal membrane glycoproteins carry targeting information in cytoplasmic regions. Two distinct targeting motifs in these regions, GY (glycine-tyrosine) and LI (leucine-isoleucine), have been identified and characterized. Accumulating evidence suggests that the adaptor complexes (AP-1, AP-2, and AP-3) recognize this information in cytoplasmic tails of transmembrane proteins. Here we report two different in vitro analyses (affinity beads and surface plasmon resonance) which revealed specific interaction between the cytoplasmic tail of LGP85 and AP-1 but not so with AP-2. We also noted requirement of the LI motif of the LGP85 tail in binding to the AP-1 complex. Our data and others which indicated the binding of AP-3 to the LIMP II (synonym of LGP85) tail suggest that the cytoplasmic tail of LGP85 interacts with AP-1 at the trans-Golgi network (TGN) and AP-3 at late endosomes, respectively. We propose that this sequential interaction between the lysosomal targeting signal and distinct APs along its transport pathway is responsible for the critical sorting of lysosomal membrane proteins and/or the potential proofreading system of mistargeted molecules.

Clathrin coats at 21 A resolution: a cellular assembly designed to recycle multiple membrane receptors

We present a map at 21 A resolution of clathrin assembled into cages with the endocytic adaptor complex, AP-2. The map was obtained by cryo-electron microscopy and single-particle reconstruction. It reveals details of the packing of entire clathrin molecules as they interact to form a cage with two nested polyhedral layers. The proximal domains of each triskelion leg depart from a cage vertex in a skewed orientation, forming a slightly twisted bundle with three other leg domains. Thus, each triskelion contributes to two connecting edges of the polyhedral cage. The clathrin heavy chains continue inwards under the vertices with local 3-fold symmetry, the terminal domains contributing to 'hook-like' features which form an intermediate network making possible contacts with the surface presented by the inner adaptor shell. A node of density projecting inwards from the vertex may correspond to the C-termini of clathrin heavy chains which form a protrusion on free triskelions at the vertex. The inter-subunit interactions visible in this map provide a structural basis for considering the assembly of clathrin coats on a membrane and show the contacts which will need to be disrupted during disassembly.

The LDL receptor clustering motif interacts with the clathrin terminal domain in a reverse turn conformation

Previously the hexapeptide motif FXNPXY807 in the cytoplasmic tail of the LDL receptor was shown to be essential for clustering in clathrin-coated pits. We used nuclear magnetic resonance line-broadening and transferred nuclear Overhauser effect measurements to identify the molecule in the clathrin lattice that interacts with this hexapeptide, and determined the structure of the bound motif. The wild-type peptide bound in a single conformation with a reverse turn at residues NPVY. Tyr807Ser, a peptide that harbors a mutation that disrupts receptor clustering, displayed markedly reduced interactions. Clustering motif peptides interacted with clathrin cages assembled in the presence or absence of AP2, with recombinant clathrin terminal domains, but not with clathrin hubs. The identification of terminal domains as the primary site of interaction for FXNPXY807 suggests that adaptor molecules are not required for receptor-mediated endocytosis of LDL, and that at least two different tyrosine-based internalization motifs exist for clustering receptors in coated pits.

Generation of coated intermediates of clathrin-mediated endocytosis on protein-free liposomes

Clathrin-coated buds and dynamin-coated tubules morphologically similar to corresponding structures observed in synaptic membranes can be generated on protein-free liposomes by incubation with cytosol, or with clathrin coat proteins and purified dynamin, respectively. Dynamin- and clathrin-coated intermediates may form independently of each other, despite the coupling between the two processes typically observed in synaptic membranes. Formation of both structures on liposomes can occur in the absence of nucleotides. These findings indicate that interfaces between lipids and cytosolic proteins are fully sufficient to deform lipids bilayers into buds and tubules. They suggest that a main function of membrane proteins is to act as positive and negative regulators of coat assembly, therefore controlling these processes in time and space.

ADP-ribosylation factor 1 transiently activates high-affinity adaptor protein complex AP-1 binding sites on Golgi membranes

Association of the Golgi-specific adaptor protein complex 1 (AP-1) with the membrane is a prerequisite for clathrin coat assembly on the trans-Golgi network (TGN). The AP-1 adaptor is efficiently recruited from cytosol onto the TGN by myristoylated ADP-ribosylation factor 1 (ARF1) in the presence of the poorly hydrolyzable GTP analog guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS). Substituting GTP for GTPgammaS, however, results in only poor AP-1 binding. Here we show that both AP-1 and clathrin can be recruited efficiently onto the TGN in the presence of GTP when cytosol is supplemented with ARF1. Optimal recruitment occurs at 4 microM ARF1 and with 1 mM GTP. The AP-1 recruited by ARF1.GTP is released from the Golgi membrane by treatment with 1 M Tris-HCl (pH 7) or upon reincubation at 37 degreesC, whereas AP-1 recruited with GTPgammaS or by a constitutively active point mutant, ARF1(Q71L), remains membrane bound after either treatment. An incubation performed with added ARF1, GTP, and AlFn, used to block ARF GTPase-activating protein activity, results in membrane-associated AP-1, which is largely insensitive to Tris extraction. Thus, ARF1. GTP hydrolysis results in lower-affinity binding of AP-1 to the TGN. Using two-stage assays in which ARF1.GTP first primes the Golgi membrane at 37 degreesC, followed by AP-1 binding on ice, we find that the high-affinity nucleating sites generated in the priming stage are rapidly lost. In addition, the AP-1 bound to primed Golgi membranes during a second-stage incubation on ice is fully sensitive to Tris extraction, indicating that the priming stage has passed the ARF1.GTP hydrolysis point. Thus, hydrolysis of ARF1.GTP at the priming sites can occur even before AP-1 binding. Our finding that purified clathrin-coated vesicles contain little ARF1 supports the concept that ARF1 functions in the coat assembly process rather than during the vesicle-uncoating step. We conclude that ARF1 is a limiting factor in the GTP-stimulated recruitment of AP-1 in vitro and that it appears to function in a stoichiometric manner to generate high-affinity AP-1 binding sites that have a relatively short half-life.