A third specificity-determining site in mu 2 adaptin for sequences upstream of Yxx phi sorting motifs

P-selectin mobility undergoes a sol-gel transition as it diffuses from exocytosis sites into the cell membrane

In response to vascular damage, P-selectin molecules are secreted onto the surface of cells that line our blood vessels. They then serve as mechanical anchors to capture leucocytes from the blood stream. Here, we track individual P-selectin molecules released at the surface of live endothelial cells following stimulated secretion. We find P-selectin initially shows fast, unrestricted diffusion but within a few minutes, movement becomes increasingly restricted and ~50% of the molecules become completely immobile; a process similar to a sol-gel transition. We find removal of the extracellular C-type lectin domain (ΔCTLD) and/or intracellular cytoplasmic tail domain (ΔCT) has additive effects on diffusive motion while disruption of the adapter complex, AP2, or removal of cell-surface heparan sulphate restores mobility of full-length P-selectin close to that of ΔCT and ΔCTLD respectively. We have found P-selectin spreads rapidly from sites of exocytosis and evenly decorates the cell surface, but then becomes less mobile and better-suited to its mechanical anchoring function.

P-selectin recruits leucocytes to regions of blood vessel damage. Using single molecule imaging, we find newly secreted P-selectin spreads rapidly across the plasma membrane and then becomes immobilized and better-suited to leucocyte capture.

Weak Molecular Interactions in Clathrin-Mediated Endocytosis

Clathrin-mediated endocytosis is a process by which specific molecules are internalized from the cell periphery for delivery to early endosomes. The key stages in this step-wise process, from the starting point of cargo recognition, to the later stage of assembly of the clathrin coat, are dependent on weak interactions between a large network of proteins. This review discusses the structural and functional data that have improved our knowledge and understanding of the main weak molecular interactions implicated in clathrin-mediated endocytosis, with a particular focus on the two key proteins: AP2 and clathrin.

Intracellular Trafficking of Neuropeptide Y Receptors

The multireceptor multiligand system of neuropeptide Y receptors and their ligands is involved in the regulation of a multitude of physiological and pathophysiological processes. Specific expression patterns, ligand-binding modes, and signaling properties contribute to the complex network regulating distinct cellular responses. Intracellular trafficking processes are important key steps that are regulated in context with accessory proteins. These proteins exert their influence by interacting directly or indirectly with the receptors, causing modification of the receptors, or operating as scaffolds for the assembly of larger signaling complexes. On the intracellular receptor faces, sequence-specific motifs have been identified that play an important role in this process. Interestingly, it is also possible to influence the receptor internalization by modification of the peptide ligand.

σ1B adaptin regulates adipogenesis by mediating the sorting of sortilin in adipose tissue

Here, we describe altered sorting of sortilin in adipocytes deficient for the σ1B-containing AP-1 complex, leading to the inhibition of adipogenesis. The AP-1 complex mediates protein sorting between the trans-Golgi network and endosomes. Vertebrates express three AP1 σ1 subunit isoforms - σ1A, σ1B and σ1C (also known as AP1S1, AP1S2 and AP1S3, respectively). σ1B-deficient mice display impaired recycling of synaptic vesicles and lipodystrophy. Here, we show that sortilin is overexpressed in adipose tissue from σ1B(-/-) mice, and that its overexpression in wild-type cells is sufficient to suppress adipogenesis. σ1B-specific binding of sortilin requires the sortilin DxxD-x12-DSxxxL motif. σ1B deficiency does not lead to a block of sortilin transport out of a specific organelle, but the fraction that reaches lysosomes is reduced. Sortilin binds to the receptor DLK1, an inhibitor of adipocyte differentiation, and the overexpression of sortilin prevents DLK1 downregulation, leading to enhanced inhibition of adipogenesis. DLK1 and sortilin expression are not increased in the brain tissue of σ1B(-/-) mice, although this is the tissue with the highest expression of σ1B and sortilin. Thus, adipose-tissue-specific and σ1B-dependent routes for the transport of sortilin exist and are involved in the regulation of adipogenesis and adipose-tissue mass.

Cargo recognition in clathrin-mediated endocytosis

The endosomal system is expansive and complex, characterized by swift morphological transitions, dynamic remodeling of membrane constituents, and intracellular positioning changes. To properly navigate this ever-altering membrane labyrinth, transmembrane protein cargoes typically require specific sorting signals that are decoded by components of protein coats. The best-characterized sorting process within the endosomal system is the rapid internalization of select transmembrane proteins within clathrin-coated vesicles. Endocytic signals consist of linear motifs, conformational determinants, or covalent modifications in the cytosolic domains of transmembrane cargo. These signals are interpreted by a diverse set of clathrin-associated sorting proteins (CLASPs) that translocate from the cytosol to the inner face of the plasma membrane. Signal recognition by CLASPs is highly cooperative, involving additional interactions with phospholipids, Arf GTPases, other CLASPs, and clathrin, and is regulated by large conformational changes and covalent modifications. Related sorting events occur at other endosomal sorting stations.

Structural basis for endosomal trafficking of diverse transmembrane cargos by PX-FERM proteins

Transit of proteins through the endosomal organelle following endocytosis is critical for regulating the homeostasis of cell-surface proteins and controlling signal transduction pathways. However, the mechanisms that control these membrane-transport processes are poorly understood. The Phox-homology (PX) domain-containing proteins sorting nexin (SNX) 17, SNX27, and SNX31 have emerged recently as key regulators of endosomal recycling and bind conserved Asn-Pro-Xaa-Tyr-sorting signals in transmembrane cargos via an atypical band, 4.1/ezrin/radixin/moesin (FERM) domain. Here we present the crystal structure of the SNX17 FERM domain bound to the sorting motif of the P-selectin adhesion protein, revealing both the architecture of the atypical FERM domain and the molecular basis for recognition of these essential sorting sequences. We further show that the PX-FERM proteins share a promiscuous ability to bind a wide array of putative cargo molecules, including receptor tyrosine kinases, and propose a model for their coordinated molecular interactions with membrane, cargo, and regulatory proteins.

TGN morphology and sorting regulated by prolyl-oligopeptidase–like protein PREPL and AP-1 μ1A

The AP-1 complex recycles between membranes and the cytoplasm and dissociates from membranes during clathrin-coated-vesicle uncoating, but also independent of vesicular transport. The μ1A N-terminal seventy amino acids are involved in regulating AP-1 recycling. In a yeast-2-hybrid library screen we identified the cytoplasmic prolyl-oligopeptidase-like protein PREPL as an interaction partner of this domain. PREPL overexpression leads to reduced AP-1 membrane binding, whereas reduced PREPL expression increases membrane binding and it impairs AP-1 recycling. Altered AP-1 membrane binding in PREPL-deficient cells mirrors the membrane binding of the mutant AP-1* complex, not able to bind PREPL. Colocalisation of PREPL with residual membrane bound AP-1 can be demonstrated. Patient cell lines deficient in PREPL have an expanded TGN, which could be rescued by PREPL expression. These data demonstrate PREPL as an AP-1 effector, which takes part in the regulation of AP-1 membrane binding. PREPL is highly expressed in brain, and at lower levels also in muscle and kidney, and its deficiency causes hypotonia and growth hormone hyposecretion supporting essential PREPL functions in AP-1-dependent secretory pathways

A noncanonical mu-1A-binding motif in the N terminus of HIV-1 Nef determines its ability to downregulate major histocompatibility complex class I in T lymphocytes

Downregulation of major histocompatibility complex class I (MHC-I) by HIV-1 Nef protein is indispensable for evasion of protective immunity by HIV-1. Though it has been suggested that the N-terminal region of Nef contributes to the function by associating with a mu-1A subunit of adaptor protein 1, the structural basis of the interaction between Nef and mu-1A remains elusive. We found that a tripartite hydrophobic motif (Trp13/Val16/Met20) in the N terminus of Nef was required for the MHC-I downregulation. Importantly, the motif functioned as a noncanonical mu-1A-binding motif for the interaction with the tyrosine motif-binding site of the mu-1A subunit. Our findings will help understanding of how HIV-1 evades the antiviral immune response by selectively redirecting the cellular protein trafficking system.

CD63 is an essential cofactor to leukocyte recruitment by endothelial P-selectin

The activation of endothelial cells is critical to initiating an inflammatory response. Activation induces the fusion of Weibel-Palade Bodies (WPB) with the plasma membrane, thus transferring P-selectin and VWF to the cell surface, where they act in the recruitment of leukocytes and platelets, respectively. CD63 has long been an established component of WPB, but the functional significance of its presence within an organelle that acts in inflammation and hemostasis was unknown. We find that ablating CD63 expression leads to a loss of P-selectin-dependent function: CD63-deficient HUVECs fail to recruit leukocytes, CD63-deficient mice exhibit a significant reduction in both leukocyte rolling and recruitment and we show a failure of leukocyte extravasation in a peritonitis model. Loss of CD63 has a similar phenotype to loss of P-selectin itself, thus CD63 is an essential cofactor to P-selectin.

Conservation and diversification of dileucine signal recognition by adaptor protein (AP) complex variants

The clathrin-associated, heterotetrameric adaptor protein (AP) complexes, AP-1, AP-2, and AP-3, recognize signals in the cytosolic domains of transmembrane proteins, leading to their sorting to endosomes, lysosomes, lysosome-related organelles, and/or the basolateral membrane of polarized epithelial cells. One type of signal, referred to as "dileucine-based," fits the consensus motif (D/E)XXXL(L/I). Previous biochemical analyses showed that (D/E)XXXL(L/I) signals bind to a combination of two subunits of each AP complex, namely the AP-1 γ-σ1, AP-2 α-σ2, and AP-3 δ-σ3 hemicomplexes, and structural studies revealed that an imperfect variant of this motif lacking the (D/E) residue binds to a site straddling the interface of α and σ2. Herein, we report mutational and binding analyses showing that canonical (D/E)XXXL(L/I) signals bind to this same site on AP-2, and to similar sites on AP-1 and AP-3. The strength and amino acid requirements of different interactions depend on the specific signals and AP complexes involved. We also demonstrate the occurrence of diverse AP-1 heterotetramers by combinatorial assembly of various γ and σ1 subunit isoforms encoded by different genes. These AP-1 variants bind (D/E)XXXL(L/I) signals with marked preferences for certain sequences, implying that they are not functionally equivalent. Our results thus demonstrate that different AP complexes share a conserved binding site for (D/E)XXXL(L/I) signals. However, the characteristics of the binding site on each complex vary, providing for the specific recognition of a diverse repertoire of (D/E)XXXL(L/I) signals.

Tickets to ride: selecting cargo for clathrin-regulated internalization

Clathrin-mediated endocytosis oversees the constitutive packaging of selected membrane cargoes into transport vesicles that fuse with early endosomes. The process is responsive to activation of signalling receptors and ion channels, promptly clearing post-translationally tagged forms of cargo off the plasma membrane. To accommodate the diverse array of transmembrane proteins that are variably gathered into forming vesicles, a dedicated sorting machinery cooperates to ensure that non-competitive uptake from the cell surface occurs within minutes. Recent structural and functional data reveal remarkable plasticity in how disparate sorting signals are recognized by cargo-selective clathrin adaptors, such as AP-2. Cargo loading also seems to govern whether coats ultimately bud or dismantle abortively at the cell surface.

AP180-mediated trafficking of Vamp7B limits homotypic fusion of Dictyostelium contractile vacuoles

Clathrin-coated vesicles play an established role in endocytosis from the plasma membrane, but they are also found on internal organelles. We examined the composition of clathrin-coated vesicles on an internal organelle responsible for osmoregulation, the Dictyostelium discoideum contractile vacuole. Clathrin puncta on contractile vacuoles contained multiple accessory proteins typical of plasma membrane-coated pits, including AP2, AP180, and epsin, but not Hip1r. To examine how these clathrin accessory proteins influenced the contractile vacuole, we generated cell lines that carried single and double gene knockouts in the same genetic background. Single or double mutants that lacked AP180 or AP2 exhibited abnormally large contractile vacuoles. The enlarged contractile vacuoles in AP180-null mutants formed because of excessive homotypic fusion among contractile vacuoles. The SNARE protein Vamp7B was mislocalized and enriched on the contractile vacuoles of AP180-null mutants. In vitro assays revealed that AP180 interacted with the cytoplasmic domain of Vamp7B. We propose that AP180 directs Vamp7B into clathrin-coated vesicles on contractile vacuoles, creating an efficient mechanism for regulating the internal distribution of fusion-competent SNARE proteins and limiting homotypic fusions among contractile vacuoles. Dictyostelium contractile vacuoles offer a valuable system to study clathrin-coated vesicles on internal organelles within eukaryotic cells.

Characterization of the neuron-specific L1-CAM cytoplasmic tail: naturally disordered in solution it exercises different binding modes for different adaptor proteins

L1, a highly conserved transmembrane glycoprotein member of the immunoglobulin superfamily of cell adhesion molecules, mediates many developmental processes in the nervous system. Here we present the biophysical characterization and the binding properties of the least structurally defined part of this receptor: its cytoplasmic tail (CT). We have shown by analytical ultracentrifugation and dynamic light scattering experiments that it is mostly monomeric and unstructured in aqueous solution. We have defined by nuclear magnetic resonance the molecular details of L1-CT binding to two major targets: a membrane-cytoskeletal linker (MCL), ezrin, and an endocytosis mediator, AP2. Surprisingly, in addition to the two previously identified ezrin binding motifs, the juxtamembrane and the (1176)YRSLE regions, we have discovered a third one, a part of which has been previously associated with binding to another MCL, ankyrin. For the L1 interaction with AP2 we have determined the precise interaction region surrounding the (1176)YRSLE binding site and that this overlaps with the second ezrin binding site. In addition, we have shown that the juxtamembrane region of L1-CT has some binding affinity to AP2-mu2, although the specificity of this interaction needs further investigation. These data indicate that L1-CT belongs to the class of intrinsically disordered proteins. Endogenous flexibility of L1-CT might play an important role in dynamic regulation of intracellular signaling: the ability of cytoplasmic tails to accommodate different targets has the potential to fine-tune signal transduction via cell surface receptors.

Regulation of synaptic inhibition by phospho-dependent binding of the AP2 complex to a YECL motif in the GABAA receptor gamma2 subunit

The regulation of the number of gamma2-subunit-containing GABA(A) receptors (GABA(A)Rs) present at synapses is critical for correct synaptic inhibition and animal behavior. This regulation occurs, in part, by the controlled removal of receptors from the membrane in clathrin-coated vesicles, but it remains unclear how clathrin recruitment to surface gamma2-subunit-containing GABA(A)Rs is regulated. Here, we identify a gamma2-subunit-specific Yxxvarphi-type-binding motif for the clathrin adaptor protein, AP2, which is located within a site for gamma2-subunit tyrosine phosphorylation. Blocking GABA(A)R-AP2 interactions via this motif increases synaptic responses within minutes. Crystallographic and biochemical studies reveal that phosphorylation of the Yxxvarphi motif inhibits AP2 binding, leading to increased surface receptor number. In addition, the crystal structure provides an explanation for the high affinity of this motif for AP2 and suggests that gamma2-subunit-containing heteromeric GABA(A)Rs may be internalized as dimers or multimers. These data define a mechanism for tyrosine kinase regulation of GABA(A)R surface levels and synaptic inhibition.

Clathrin/AP-2-dependent endocytosis: a novel playground for the pharmacological toolbox?

Endocytosis is a vital process for mammalian cells by which they communicate with their environment, internalize nutrients, hormones, or growth factors, or take up extracellular fluids and particles. The best studied among the various pathways to ingest material from the extracellular side is clathrin/AP-2-mediated endocytosis. The past several years have allowed us to gain unprecedented molecular insights into the role of the heterotetrameric AP-2 adaptor complex as a central protein-protein and protein-lipid interaction hub at the plasmalemma. During the initial stages of clathrin-coated pit formation, AP-2 interacts with phosphoinositides and cargo membrane proteins as well as with a variety of accessory proteins and clathrin to coordinate clathrin coat polymerization with membrane deformation and cargo recruitment. In addition, a growing list of alternative adaptors provides opportunity for clathrin-dependent cargo selective pathways of internalization and endosomal sorting. Many of these interactions are now understood in structural detail and are thus amenable to pharmacological interference. In this review we will summarize our present state of knowledge about AP-2 and its partners in endocytosis and delineate potential strategies for pharmacological manipulations.

Clathrin-mediated endocytosis of a lipid-raft-associated protein is mediated through a dual tyrosine motif

We have previously shown that the integral membrane protein CD317 has both a conventional transmembrane domain near its N-terminus and a C-terminal glycosyl-phosphatidylinositol (GPI) anchor. With the possible exception of a minor topological variant of the prion protein, there remain no other convincing examples of a mammalian protein with such a topology. CD317 is localised to cholesterol-rich lipid microdomains (`lipid rafts') in the plasma membrane and is internalised from the cell surface for delivery to a juxta-nuclear compartment (most probably the TGN). We have now investigated the mechanism by which CD317 is internalised and find that this raft-associated integral membrane protein is internalised through a clathrin-dependent pathway, internalisation is dependent upon a novel dual-tyrosine-based motif in the cytosolic domain of CD317, the cytosolic domain of CD317 can interact with the μ subunits of the AP2 and AP1 adaptor complexes, interaction with AP1 is required for delivery of CD317 back to the TGN, and removal of the GPI anchor from CD317 reduces the efficiency of CD317 internalisation. Collectively, these data indicate that CD317 is internalised and delivered back to the TGN by the sequential action of AP2 and AP1 adaptor complexes and that, surprisingly, the clathrin-mediated internalisation of CD317 occurs more efficiently if CD317 is localised to lipid rafts.

Molecular Switches Involving the AP-2 β2 Appendage Regulate Endocytic Cargo Selection and Clathrin Coat Assembly

Clathrin-associated sorting proteins (CLASPs) expand the repertoire of endocytic cargo sorted into clathrin-coated vesicles beyond the transmembrane proteins that bind physically to the AP-2 adaptor. LDL and GPCRs are internalized by ARH and beta-arrestin, respectively. We show that these two CLASPs bind selectively to the AP-2 beta2 appendage platform via an alpha-helical [DE](n)X(1-2)FXX[FL]XXXR motif, and that this motif also occurs and is functional in the epsins. In beta-arrestin, this motif maintains the endocytosis-incompetent state by binding back on the folded core of the protein in a beta strand conformation. Triggered via a beta-arrestin/GPCR interaction, the motif must be displaced and must undergo a strand to helix transition to enable the beta2 appendage binding that drives GPCR-beta-arrestin complexes into clathrin coats. Another interaction surface on the beta2 appendage sandwich is identified for proteins such as eps15 and clathrin, suggesting a mechanism by which clathrin displaces eps15 to lattice edges during assembly.

Non-canonical YXXGΦ endocytic motifs: recognition by AP2 and preferential utilization in P2X4 receptors

During clathrin-mediated endocytosis, proteins on the cell surface are selected for inclusion in clathrin-coated vesicles by clathrin adaptors, mainly the adaptor complex AP2. The P2X4 subtype of ATP-gated ion channel has in its C-terminus two putative endocytic motifs: a canonical YXXΦ motif and a non-canonical YXXGΦ motif (YEQGL). We demonstrate that endocytosis of P2X4 receptors is mediated preferentially by the YXXGΦ motif because the YXXΦ motif is inaccessible to AP2 owing to the structure of the channel. The crystal structure of a complex between residues 160-435 of the μ2 subunit of AP2 and a P2X4 C-terminal peptide showed that the YEQGL motif binds to μ2 at the same site as YXXΦ motifs. Y and Φ residues are accommodated in the same hydrophobic pockets in μ2 with the extra residue between them being accommodated by changes in the peptide's backbone configuration, when compared to YXXΦ motifs. These data demonstrate that the family of potential tyrosine-based endocytic signals must be expanded to include motifs with an additional glycine at Y+3 (YXXGΦ).

Selective Golgi export of Kir2.1 controls the stoichiometry of functional Kir2.x channel heteromers

Surface expression of ion channels and receptors often depends on intrinsic sequence motifs that control their intracellular transport along the secretory pathway. Although members of the Kir2.x subfamily share two such motifs – a diacidic ER export motif and a positively charged Golgi export motif – they strongly differ in their surface expression. Whereas Kir2.1 shows prominent plasma membrane localization, Kir2.4 channels accumulate within the Golgi complex. By constructing chimeras between Kir2.1 and Kir2.4 subunits, a stretch of 20 amino acids was identified in the Kir2.1 C-terminus that is both necessary and sufficient to promote anterograde transport of Kir channel subunits at the level of trafficking from the Golgi to the plasma membrane. The core element of the identified sequence bears a tyrosine-dependent YXXΦ consensus motif for adaptin binding, with the flanking residues determining its functional efficiency. As the signal is dominant in promoting surface transport of Kir2.1/Kir2.4 channel heteromers and is recognized by both the epithelial and neuronal intracellular sorting machinery, the preferential Golgi export of Kir2.1 will control the stoichiometry of Kir2.x heteromers expressed on the cell surface.

Sorting of H,K-ATPase beta-subunit in MDCK and LLC-PK cells is independent of mu 1B adaptin expression

The cytoplasmic tail of the H,K-ATPase beta-subunit contains a putative tyrosine-based motif that directs the beta-subunit's basolateral sorting when it is expressed in Madin-Darby Canine Kidney (MDCK) cells. When expressed in LLC-PK(1) cells, however, the beta-subunit is localized to the apical membrane. Several proteins that contain tyrosine-based motifs, including the low-density lipoprotein and transferrin receptors, show a similar sorting 'defect' when expressed in LLC-PK(1) cells. For low-density lipoprotein and transferrin receptors, this behavior is due to the differential expression of the mu 1B subunit of the AP-1B clathrin adaptor complex. mu 1B is expressed by MDCK cells, but not LLC-PK(1) cells, and transfection of mu 1B into LLC-PK(1) cells restores basolateral localization of low-density lipoprotein and transferrin receptors. For the beta-subunit, however, mu B expression in LLC-PK(1) cells does not induce its basolateral expression. We found that the beta-subunit interacts with both mu 1B and mu 1A in vitro and in vivo. The capacity to participate in a mu 1B interaction therefore is not sufficient to program the beta-subunit's basolateral localization in MDCK cells. Our data suggest that the H,K-ATPase beta-subunit's basolateral sorting signal is either masked in certain epithelial cells, or requires an interaction with sorting machinery other than AP-1B for delivery to the basolateral plasma membrane.

Arabidopsis mu A-adaptin interacts with the tyrosine motif of the vacuolar sorting receptor VSR-PS1

In receptor-mediated transport pathways in mammalian cells, clathrin-coated vesicle (CCV) mu-adaptins are the main binding partners for the tyrosine sorting/internalization motif (YXXØ). We have analyzed the function of the mu A-adaptin, one of the five mu-adaptins from Arabidopsis thaliana, by pull-down assays and plasmon resonance measurements using its receptor-binding domain (RBD) fused to a histidine tag. We show that this adaptin is able to bind the consensus tyrosine motif YXXØ from the pea vacuolar sorting receptor (VSR)-PS1, as well as from the mammalian trans-Golgi network (TGN)38 protein. Moreover, the tyrosine residue was revealed to be crucial for binding of the complete cytoplasmic tail of VSR-PS1 to the plant mu A-adaptin. The trans-Golgi localization of the mu A-adaptin strongly suggests its involvement in Golgi- to vacuole-trafficking events.

Clathrin-dependent endocytosis

The process by which clathrin-coated vesicles are produced involves interactions of multifunctional adaptor proteins with the plasma membrane, as well as with clathrin and several accessory proteins and phosphoinositides. Here we review recent findings highlighting new insights into mechanisms underlying clathrin-dependent endocytosis.

Two motifs target Batten disease protein CLN3 to lysosomes in transfected nonneuronal and neuronal cells

Batten disease is a neurodegenerative disorder resulting from mutations in CLN3, a polytopic membrane protein, whose predominant intracellular destination in nonneuronal cells is the lysosome. The topology of CLN3 protein, its lysosomal targeting mechanism, and the development of Batten disease are poorly understood. We provide experimental evidence that both the N and C termini and one large loop domain of CLN3 face the cytoplasm. We have identified two lysosomal targeting motifs that mediate the sorting of CLN3 in transfected nonneuronal and neuronal cells: an unconventional motif in the long C-terminal cytosolic tail consisting of a methionine and a glycine separated by nine amino acids [M(X)9G], and a more conventional dileucine motif, located in the large cytosolic loop domain and preceded by an acidic patch. Each motif on its own was sufficient to mediate lysosomal targeting, but optimal efficiency required both. Interestingly, in primary neurons, CLN3 was prominently seen both in lysosomes in the cell body and in endosomes, containing early endosomal antigen-1 along neuronal processes. Because there are few lysosomes in axons and peripheral parts of dendrites, the presence of CLN3 in endosomes of neurons may be functionally important. Endosomal association of the protein was independent of the two lysosomal targeting motifs.

Signals for sorting of transmembrane proteins to endosomes and lysosomes

Sorting of transmembrane proteins to endosomes and lysosomes is mediated by signals present within the cytosolic domains of the proteins. Most signals consist of short, linear sequences of amino acid residues. Some signals are referred to as tyrosine-based sorting signals and conform to the NPXY or YXXO consensus motifs. Other signals known as dileucine-based signals fit [DE]XXXL[LI] or DXXLL consensus motifs. All of these signals are recognized by components of protein coats peripherally associated with the cytosolic face of membranes. YXXO and [DE]XXXL[LI] signals are recognized with characteristic fine specificity by the adaptor protein (AP) complexes AP-1, AP-2, AP-3, and AP-4, whereas DXXLL signals are recognized by another family of adaptors known as GGAs. Several proteins, including clathrin, AP-2, and Dab2, have been proposed to function as recognition proteins for NPXY signals. YXXO and DXXLL signals bind in an extended conformation to the mu2 subunit of AP-2 and the VHS domain of the GGAs, respectively. Phosphorylation events regulate signal recognition. In addition to peptide motifs, ubiquitination of cytosolic lysine residues also serves as a signal for sorting at various stages of the endosomal-lysosomal system. Conjugated ubiquitin is recognized by UIM, UBA, or UBC domains present within many components of the internalization and lysosomal targeting machinery. This complex array of signals and recognition proteins ensures the dynamic but accurate distribution of transmembrane proteins to different compartments of the endosomal-lysosomal system.

Definition of the consensus motif recognized by gamma-adaptin ear domains

The heterotetrameric adaptor complex 1 (AP-1) and the monomeric Golgi-localized, gamma ear-containing, Arf-binding (GGA) proteins are components of clathrin coats associated with the trans-Golgi network and endosomes. The carboxyl-terminal ear domains (or gamma-adaptin ear (GAE) domains) of two gamma-adaptin subunit isoforms of AP-1 and of the GGAs are structurally similar and bind to a common set of accessory proteins. In this study, we have systematically defined a core tetrapeptide motif PsiG(P/D/E)(Psi/L/M) (where Psi is an aromatic residue), which is responsible for the interactions of accessory proteins with GAE domains. The definition of this motif has allowed us to identify novel GAE-binding partners named NECAP and aftiphilin, which also contain clathrin-binding motifs. These findings shed light on the mechanism of accessory protein recruitment to trans-Golgi network and endosomal clathrin coats.

Multiple cargo binding sites on the COPII subunit Sec24p ensure capture of diverse membrane proteins into transport vesicles

We have characterized the mechanisms of cargo selection into ER-derived vesicles by the COPII subunit Sec24p. We identified a site on Sec24p that recognizes the v-SNARE Bet1p and show that packaging of a number of cargo molecules is disrupted when mutations are introduced at this site. Surprisingly, cargo proteins affected by these mutations did not share a single common sorting signal, nor were proteins sharing a putative class of signal affected to the same degree. We show that the same site is conserved as a cargo-interaction domain on the Sec24p homolog Lst1p, which only packages a subset of the cargoes recognized by Sec24p. Finally, we identified an additional mutation that defines another cargo binding domain on Sec24p, which specifically interacts with the SNARE Sec22p. Together, our data support a model whereby Sec24p proteins contain multiple independent cargo binding domains that allow for recognition of a diverse set of sorting signals.

Endocytosis of synaptotagmin 1 is mediated by a novel, tryptophan-containing motif

The rate at which a membrane protein is internalized from the plasma membrane can be regulated by revealing a latent internalization signal in response to an appropriate stimulus. Internalization of the synaptic vesicle membrane protein, synaptotagmin 1, is controlled by two distinct regions of its intracytoplasmic C2B domain, an internalization signal present in the 29 carboxyterminal (CT) amino acids and a separate regulatory region. We have now characterized the internalization motif by mutagenesis and found that it involves an essential tryptophan in the last beta strand of the C2B domain, a region that is distinct from the AP2-binding site previously described. Internalization through the tryptophan-based motif is sensitive to eps15 and dynamin mutants and is therefore likely to be clathrin mediated. A tryptophan-to-phenylalanine mutation had no effect on internalization of the CT domain alone, but completely inhibited endocytosis of the folded C2B domain. This result suggests that recognition of sorting motifs can be influenced by their structural context. We conclude that endocytosis of synaptotagmin 1 requires a novel type of internalization signal that is subject to regulation by the rest of the C2B domain.

Binding partners for the COOH-terminal appendage domains of the GGAs and gamma-adaptin

The adaptor appendage domains are believed to act as binding platforms for coated vesicle accessory proteins. Using glutathione S-transferase pulldowns from pig brain cytosol, we find three proteins that can bind to the appendage domains of both the AP-1 gamma subunit and the GGAs: gamma-synergin and two novel proteins, p56 and p200. p56 elicited better antibodies than p200 and was generally more tractable. Although p56 and gamma-synergin bind to both GGA and gamma appendages in vitro, immunofluorescence labeling of nocodazole-treated cells shows that p56 colocalizes with GGAs on TGN46-positive membranes, whereas gamma-synergin colocalizes with AP-1 primarily on a different membrane compartment. Furthermore, in AP-1-deficient cells, p56 remains membrane-associated whereas gamma-synergin becomes cytosolic. Thus, p56 and gamma-synergin show very strong preferences for GGAs and AP-1, respectively, in vivo. However, the GGA and gamma appendages share the same fold as determined by x-ray crystallography, and mutagenesis reveals that the same amino acids contribute to their binding sites. By overexpressing wild-type GGA and gamma appendage domains in cells, we can drive p56 and gamma-synergin, respectively, into the cytosol, suggesting a possible mechanism for selectively disrupting the two pathways.

A tyrosine-based sorting signal is involved in connexin43 stability and gap junction turnover

The gap junction protein connexin43 is known to have a rapid turnover, involving degradation by both the proteasomal and lysosomal systems, but the structural features of connexin43 that govern these actions are not known. The connexin43 C-terminal sequence contains a proline-rich region corresponding to the consensus of a protein-protein interaction PY-motif (xPPxY), and an overlapping putative tyrosine-based sorting signal (Yxxphi; =hydrophobic), known to play a role in the intracellular trafficking of many membrane proteins. As both motifs may control turnover of connexin43, we used a combination of metabolic radiolabelling, immuno-precipitation and functional assays to determine the possible role of these motifs in controlling degradation of human connexin43 expressed in SKHep1 cells. Mutation V289D in the tyrosine-based sorting motif increased the steady-state pool of connexin43 by approximately 3.5-fold, while mutation P283L in the PY-motif produced a comparatively modest augmentation (1.7-fold). No additive effect was observed when the overlapping tyrosine was mutated. In pulse-chase experiments, the Y286A substitution increased the half-life of connexin43 from 2 to 6 hours, indicating that the increased steady-state levels reflected reduced protein degradation. Moreover, expression at the junctional membrane, as well as gap junction-mediated intercellular communication (GJC), were nearly abolished by lysosomal inhibitors and Brefeldin A in cells expressing wild-type connexin43, but were unaffected in the tyrosine mutant. These results provide strong evidence that the tyrosine-based motif of human connexin43 is a prime determinant controlling connexin43 stability, and consequently GJC, by targeting connexin43 for degradation in the endocytic/lysosomal compartment.

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.

Biogenesis of Weibel–Palade bodies

Weibel-Palade bodies (WPBs) are the lysosome-related secretory organelles of endothelial cells. Their content protein von Willebrand factor, plays a key role in haemostasis, whilst P-selectin in the membranes is critical in the initiation of inflammation. Biogenesis of these rod-shaped structures is driven by von Willebrand factor, since its heterologous expression leads to formation of organelles morphologically indistinguishable from bona fide WPBs. The two main membrane proteins of WPBs, CD63 and P-selectin, have complex itineraries controlled largely by cytoplasmic targeting signals. We are only just beginning to understand the way in which these three proteins come together to form mature WPBs.

Differential recognition of tyrosine-based basolateral signals by AP-1B subunit mu1B in polarized epithelial cells

To investigate the importance of tyrosine recognition by the AP-1B clathrin adaptor subunit mu1B for basolateral sorting of integral membrane proteins in polarized epithelial cells, we have produced and characterized a mutant form of mu1B. The mutant (M-mu1B) contains alanine substitutions of each of the four conserved residues, which in the AP-2 adaptor subunit micro2 are critical for interacting with tyrosine-based endocytosis signals. We show M-mu1B is defective for tyrosine binding in vitro, but is nevertheless incorporated into AP-1 complexes in transfected cells. Using LLC-PK1 cells expressing either wild type or M-mu1B, we find that there is inefficient basolateral expression of membrane proteins whose basolateral targeting signals share critical tyrosines with signals for endocytosis. In contrast, membrane proteins whose basolateral targeting signals are distinct from their endocytosis signals (transferrin and low-density lipoprotein receptors) accumulate at the basolateral domain normally, although in a manner that is strictly dependent on mu1B or M-mu1B expression. Our results suggest that mu1B interacts with different classes of basolateral targeting signals in distinct ways.

Molecular architecture and functional model of the endocytic AP2 complex

AP2 is the best-characterized member of the family of heterotetrameric clathrin adaptor complexes that play pivotal roles in many vesicle trafficking pathways within the cell. AP2 functions in clathrin-mediated endocytosis, the process whereby cargo enters the endosomal system from the plasma membrane. We describe the structure of the 200 kDa AP2 "core" (alpha trunk, beta2 trunk, mu2, and sigma2) complexed with the polyphosphatidylinositol headgroup mimic inositolhexakisphosphate at 2.6 A resolution. Two potential polyphosphatidylinositide binding sites are observed, one on alpha and one on mu2. The binding site for Yxxphi endocytic motifs is buried, indicating that a conformational change, probably triggered by phosphorylation in the disordered mu2 linker, is necessary to allow Yxxphi motif binding. A model for AP2 recruitment and activation is proposed.

Biological basket weaving: formation and function of clathrin-coated vesicles

There has recently been considerable progress in understanding the regulation of clathrin-coated vesicle (CCV) formation and function. These advances are due to the determination of the structure of a number of CCV coat components at molecular resolution and the identification of novel regulatory proteins that control CCV formation in the cell. In addition, pathways of (a) phosphorylation, (b) receptor signaling, and (c) lipid modification that influence CCV formation, as well as the interaction between the cytoskeleton and CCV transport pathways are becoming better defined. It is evident that although clathrin coat assembly drives CCV formation, this fundamental reaction is modified by different regulatory proteins, depending on where CCVs are forming in the cell. This regulatory difference likely reflects the distinct biological roles of CCVs at the plasma membrane and trans-Golgi network, as well as the distinct properties of these membranes themselves. Tissue-specific functions of CCVs require even more-specialized regulation and defects in these pathways can now be correlated with human diseases.

AP-3 adaptor functions in targeting P-selectin to secretory granules in endothelial cells

P-selectin, a cell adhesion protein participating in the early stages of inflammation, contains multiple sorting signals that regulate its cell surface expression. Targeting to secretory granules regulates delivery of P-selectin to the cell surface. Internalization followed by sorting from early to late endosomes mediates rapid removal of P-selectin from the surface. We show here that the P-selectin cytoplasmic domain bound AP-2 and AP-3 adaptor complexes in vitro. The amino acid substitution L768A, which abolishes endosomal sorting and impairs granule targeting of P-selectin, reduced binding of AP-3 adaptors but not AP-2 adaptors. Turnover of P-selectin was 2.4-fold faster than turnover of transferrin receptor in AP-3-deficient mocha fibroblasts, similar to turnover of these two proteins in AP-3-competent cells, demonstrating that AP-3 function is not required for endosomal sorting. However, sorting P-selectin to secretory granules was defective in endothelial cells from AP-3-deficient pearl mice, demonstrating a role for AP-3 adaptors in granule assembly in endothelial cells. P-selectin sorting to platelet alpha-granules was normal in pearl mice, consistent with earlier evidence that granule targeting of P-selectin is mechanistically distinct in endothelial cells and platelets. These observations establish that AP-3 adaptor functions in assembly of conventional secretory granules, in addition to lysosomes and the 'lysosome-like' secretory granules of platelets and melanocytes.

Internalization-competent influenza hemagglutinin mutants form complexes with clathrin-deficient multivalent AP-2 oligomers in live cells

Most membrane proteins are endocytosed through clathrin-coated pits via AP-2 adaptor complexes. However, little is known about the interaction of internalization signals with AP-2 in live cells in the absence of clathrin lattices. To investigate this issue, we employed cells cotransfected with pairs of antigenically distinct influenza hemagglutinin (HA) mutants containing different internalization signals of the YXXZ family. To enable studies on the possible association of the naturally trimeric HAs into higher order complexes via binding to AP-2, we exploited the inability of HAs from different influenza strains to form mutual trimers. Thus, we coexpressed HA pairs from different strains (Japan and X:31) bearing similar cytoplasmic tails mutated to include internalization signals. Using antibody-mediated immunofluorescence co-patching on live cells, we demonstrate that internalization-competent HA mutants form higher order complexes and that this clustering depends on the strength of the internalization signal. The clustering persisted in cells treated with hypertonic medium to disperse the clathrin lattices, as validated by co-immunoprecipitation experiments. The clustering of HAs bearing strong internalization signals appears to be mediated via binding to AP-2, as indicated by (i) the coprecipitation of alpha-adaptin with these HAs, even in hypertonically treated cells; (ii) the co-localization (after hypertonic treatment) of AP-2 with antibody-mediated patches of these mutants; and (iii) the dispersal of the higher order HA complexes following chlorpromazine treatment, which removes AP-2 from the plasma membrane. These results suggest that even in the absence of clathrin lattices, AP-2 exists in multivalent complexes capable of simultaneously binding several internalization signals from the same family.