Deep-etch visualization of proteins involved in clathrin assembly

Disordered hinge regions of the AP-3 adaptor complex promote vesicle budding from the late Golgi in yeast

Vesicles bud from maturing Golgi cisternae in a programmed sequence. Budding is mediated by adaptors that recruit cargoes and facilitate vesicle biogenesis. In Saccharomyces cerevisiae, the AP-3 adaptor complex directs cargoes from the Golgi to the lysosomal vacuole. The AP-3 core consists of small and medium subunits complexed with two non-identical large subunits, β3 (Apl6) and δ (Apl5). The C-termini of β3 and δ were thought to be flexible hinges linking the core to ear domains that bind accessory proteins involved in vesicular transport. We found by computational modeling that the yeast β3 and δ hinges are intrinsically disordered and lack folded ear domains. When either hinge is truncated, AP-3 is recruited to the Golgi, but vesicle budding is impaired and cargoes normally sorted into the AP-3 pathway are mistargeted. This budding deficiency causes AP-3 to accumulate on ring-like Golgi structures adjacent to GGA adaptors that, in wild-type cells, bud vesicles downstream of AP-3 during Golgi maturation. Thus, each of the disordered hinges of yeast AP-3 has a crucial role in mediating transport vesicle formation at the Golgi.

A novel ROS-activable self-immolative prodrug for tumor-specific amplification of oxidative stress and enhancing chemotherapy of mitoxantrone

Reactive oxygen species (ROS) as well-known endogenous stimuli has been widely used to activate drug delivery systems (DDSs) for tumor-specific therapy. Unfortunately, endogenous ROS in the tumor microenvironment (TME) is not enough to achieve effective therapeutic efficacy and cancer cells have adapted to high oxidative stress by upregulating glutathione (GSH) level. Herein, we devised a novel ROS-activable self-immolative prodrug CASDB with both GSH-depletion ability and ROS self-supply competence. Then, an stimuli-responsive nanoplatform integrating CASDB with clinical chemotherapeutics mitoxantrone (MTO) and PLGA was fabricated (denoted as CMPs) through nanoprecipitation method. The CMPs could achieve desired accumulation at tumor tissues through enhanced permeability and retention (EPR) effects. Then the accumulated CMPs could induce tumor cell apoptosis efficiently. Especially, ROS in tumor sites could trigger the immolation of CASDB to generate CA and quinone methide (QM). Then CA and QM cooperatively promoted damage of mitochondria due to oxidative stress and led to cancer cells more sensitive to MTO. Accordingly, MTO could perturb cellular microenvironment of cancer cells then promote the degradation of CASDB. The experiment results demonstrated that CMPs were ideal for desirable synergetic tumor-specific anticancer therapy with negligible systemic toxicity. The half-maximal inhibitory concentrations (IC50) value of CMPs was 6.53 μM, while the IC50 values of MTO was 14.76 μM. And the CMPs group showed the strongest tumor suppressor effect with the tumor sizes increased to 1.2-fold (Control group: 20.6-fold, MTO only: 3.0-fold). This study should be inspirational for designing efficient prodrugs to overcome the handicaps of traditional chemotherapy.

Generation of nanoscopic membrane curvature for membrane trafficking

Curved membranes are key features of intracellular organelles, and their generation involves dynamic protein complexes. Here we describe the fundamental mechanisms such as the hydrophobic insertion, scaffolding and crowding mechanisms these proteins use to produce membrane curvatures and complex shapes required to form intracellular organelles and vesicular structures involved in endocytosis and secretion. For each mechanism, we discuss its cellular functions as well as the underlying physical principles and the specific membrane properties required for the mechanism to be feasible. We propose that the integration of individual mechanisms into a highly controlled, robust process of curvature generation often relies on the assembly of proteins into coats. How cells unify and organize the curvature-generating factors at the nanoscale is presented for three ubiquitous coats central for membrane trafficking in eukaryotes: clathrin-coated pits, caveolae, and COPI and COPII coats. The emerging theme is that these coats arrange and coordinate curvature-generating factors in time and space to dynamically shape membranes to accomplish membrane trafficking within cells.

Control of clathrin-mediated endocytosis by NIMA family kinases

Endocytosis, the process by which cells internalize plasma membrane and associated cargo, is regulated extensively by posttranslational modifications. Previous studies suggested the potential involvement of scores of protein kinases in endocytic control, of which only a few have been validated in vivo. Here we show that the conserved NIMA-related kinases NEKL-2/NEK8/9 and NEKL-3/NEK6/7 (the NEKLs) control clathrin-mediated endocytosis in C. elegans. Loss of NEKL-2 or NEKL-3 activities leads to penetrant larval molting defects and to the abnormal localization of trafficking markers in arrested larvae. Using an auxin-based degron system, we also find that depletion of NEKLs in adult-stage C. elegans leads to gross clathrin mislocalization and to a dramatic reduction in clathrin mobility at the apical membrane. Using a non-biased genetic screen to identify suppressors of nekl molting defects, we identified several components and regulators of AP2, the major clathrin adapter complex acting at the plasma membrane. Strikingly, reduced AP2 activity rescues both nekl mutant molting defects as well as associated trafficking phenotypes, whereas increased levels of active AP2 exacerbate nekl defects. Moreover, in a unique example of mutual suppression, NEKL inhibition alleviates defects associated with reduced AP2 activity, attesting to the tight link between NEKL and AP2 functions. We also show that NEKLs are required for the clustering and internalization of membrane cargo required for molting. Notably, we find that human NEKs can rescue molting and trafficking defects in nekl mutant worms, suggesting that the control of intracellular trafficking is an evolutionarily conserved function of NEK family kinases.

In order to function properly, cells must continually import materials from the outside. This process, termed endocytosis, is necessary for the uptake of nutrients and for interpreting signals coming from the external environment or from within the body. These signals are critical during animal development but also affect many types of cell behaviors throughout life. In our current work, we show that several highly conserved proteins in the nematode Caenorhabditis elegans, NEKL-2 and NEKL-3, regulate endocytosis. The human counterparts of NEKL-2 and NEKL-3 have been implicated in cardiovascular and renal diseases as well as many types of cancers. However, their specific functions within cells is incompletely understood and very little is known about their role in endocytosis or how this role might impact disease processes. Here we use several complementary approaches to characterize the specific functions of C. elegans NEKL-2 and NEKL-3 in endocytosis and show that their human counterparts likely have very similar functions. This work paves the way to a better understanding of fundamental biological processes and to determining the cellular functions of proteins connected to human diseases.

Conformational regulation of AP1 and AP2 clathrin adaptor complexes

Heterotetrameric clathrin adaptor protein complexes (APs) orchestrate the formation of coated vesicles for transport among organelles of the cell periphery. AP1 binds membranes enriched for phosphatidylinositol 4-phosphate, such as the trans Golgi network, while AP2 associates with phosphatidylinositol 4,5-bisphosphate of the plasma membrane. At their respective membranes, AP1 and AP2 bind the cytoplasmic tails of transmembrane protein cargo and clathrin triskelions, thereby coupling cargo recruitment to coat polymerization. Structural, biochemical and genetic studies have revealed that APs undergo conformational rearrangements and reversible phosphorylation to cycle between different activity states. While membrane, cargo and clathrin have been demonstrated to promote AP activation, growing evidence supports that membrane-associated proteins such as Arf1 and FCHo also stimulate this transition. APs may be returned to the inactive state via a regulated process involving phosphorylation and a protein called NECAP. Finally, because antiviral mechanisms often rely on appropriate trafficking of membrane proteins, viruses have evolved novel strategies to evade host defenses by influencing the conformation of APs. This review will cover recent advances in our understanding of the molecular inputs that stimulate AP1 and AP2 to adopt structurally and functionally distinct configurations.

A primer on resolving the nanoscale structure of the plasma membrane with light and electron microscopy

Taraska reviews the imaging methods that are being used to understand the structure of the plasma membrane at the molecular level.

The plasma membrane separates a cell from its external environment. All materials and signals that enter or leave the cell must cross this hydrophobic barrier. Understanding the architecture and dynamics of the plasma membrane has been a central focus of general cellular physiology. Both light and electron microscopy have been fundamental in this endeavor and have been used to reveal the dense, complex, and dynamic nanoscale landscape of the plasma membrane. Here, I review classic and recent developments in the methods used to image and study the structure of the plasma membrane, particularly light, electron, and correlative microscopies. I will discuss their history and use for mapping the plasma membrane and focus on how these tools have provided a structural framework for understanding the membrane at the scale of molecules. Finally, I will describe how these studies provide a roadmap for determining the nanoscale architecture of other organelles and entire cells in order to bridge the gap between cellular form and function.

Correlative Fluorescence Super-Resolution Localization Microscopy and Platinum Replica EM on Unroofed Cells

Platinum replicas of unroofed mammalian cells can be imaged with a transmission electron microscope (TEM) to produce high contrast, high resolution images of the structure of the cytoplasmic side of a plasma membrane. A complementary approach, super-resolution fluorescence localization microscopy, can be used to localize labeled molecules with better than 20 nm precision in cells. Here, we describe a correlative method that couples these two techniques and produces images where localization microscopy data can be used to highlight specific proteins of interest within the structural context of the platinum replica TEM image. This combined method is uniquely suited to investigate the nanometer-scale structural organization of the plasma membrane and its associated organelles and proteins.

Using selenomethionyl derivatives to assign sequence in low-resolution structures of the AP2 clathrin adaptor

Selenomethionine incorporation is a powerful technique for assigning sequence to regions of electron density at low resolution. Genetic introduction of methionine point mutations and the subsequent preparation and crystallization of selenomethionyl derivatives permits unambiguous sequence assignment by enabling the placement of the anomalous scatterers (Se atoms) thus introduced. Here, the use of this approach in the assignment of sequence in a part of the AP2 clathrin adaptor complex that is responsible for clathrin binding is described. AP2 plays a pivotal role in clathrin-mediated endocytosis, a tightly regulated process in which cell-surface transmembrane proteins are internalized from the plasma membrane by incorporation into lipid-enclosed transport vesicles. AP2 binds cargo destined for internalization and recruits clathrin, a large trimeric protein that helps to deform the membrane to produce the transport vesicle. By selenomethionine labelling of point mutants, it was shown that the clathrin-binding site is buried within a deep cleft of the AP2 complex. A membrane-stimulated conformational change in AP2 releases the clathrin-binding site from autoinhibition, thereby linking clathrin recruitment to membrane localization.

Molecular structure, function, and dynamics of clathrin-mediated membrane traffic

Clathrin is a molecular scaffold for vesicular uptake of cargo at the plasma membrane, where its assembly into cage-like lattices underlies the clathrin-coated pits of classical endocytosis. This review describes the structures of clathrin, major cargo adaptors, and other proteins that participate in forming a clathrin-coated pit, loading its contents, pinching off the membrane as a lattice-enclosed vesicle, and recycling the components. It integrates as much of the structural information as possible at the time of writing into a sketch of the principal steps in coated-pit and coated-vesicle formation.

The role of clathrin in mitotic spindle organisation

Clathrin, a protein best known for its role in membrane trafficking, has been recognised for many years as localising to the spindle apparatus during mitosis, but its function at the spindle remained unclear. Recent work has better defined the role of clathrin in the function of the mitotic spindle and proposed that clathrin crosslinks the microtubules (MTs) comprising the kinetochore fibres (K-fibres) in the mitotic spindle. This mitotic function is unrelated to the role of clathrin in membrane trafficking and occurs in partnership with two other spindle proteins: transforming acidic coiled-coil protein 3 (TACC3) and colonic hepatic tumour overexpressed gene (ch-TOG; also known as cytoskeleton-associated protein 5, CKAP5). This review summarises the role of clathrin in mitotic spindle organisation with an emphasis on the recent discovery of the TACC3-ch-TOG-clathrin complex.

Endocytosis at the nanoscale

Endocytosis is a fundamental process in which eukaryotic cells internalise molecules and macromolecules via deformation of the membrane and generation of membrane-bound carriers. Functional aspects are not only limited to uptake of nutrients, but also play a primary role in evolutionary conserved processes such as the regulation of plasma membrane protein activity (i.e. signal-transducing receptors, small-molecule transporters and ion channels), cell motility and mitosis. The macromolecular nature of the material transported by endocytosis makes this route one of the most important targets for nanomedicine. Indeed, many nanoparticle formulations have been customised to enter cells through endocytosis and deliver the cargo within the cell. In this critical review, we present an overview of the biology of endocytosis and discuss its implications in cell internalisation of nanoparticles. We discuss how nanoparticle size, shape and surface chemistry can control this process effectively. Finally, we discuss different drug delivery strategies on how to evade lysosomal degradation to promote effective release of the cargo (376 references).

Full-length structural model of RET3 and SEC21 in COPI: identification of binding sites on the appendage for accessory protein recruitment motifs

COPI, a 600 kD heptameric complex (consisting of subunits α, β, γ, δ, ε, ζ, and β′) “coatomer,” assembles non-clathrin-coated vesicles and is responsible for intra-Golgi and Golgi-to-ER protein trafficking. Here, we report the three-dimensional structures of the entire sequences of yeast Sec21 (γ-COPI mammalian ortholog), yeast Ret3 (ζ-COPI mammalian ortholog), and the results of successive molecular dynamics investigations of the subunits and assembly based on a protein–protein docking experiment. The three-dimensional structures of the subunits in their complexes indicate the residues of the two subunits that impact on assembly, the conformations of Ret3 and Sec21, and their binding orientations in the complexed state. The structure of the appendage domain of Sec21, with its two subdomains—the platform and the β-sandwich, was investigated to explore its capacity to bind to accessory protein recruitment motifs. Our study shows that a binding site on the platform is capable of binding the Eps15 DPF and epsin DPW2 peptides, whereas the second site on the platform and the site on the β-sandwich subdomain were found to selectively bind to the amphiphysin FXDXF and epsin DPW1 peptides, respectively. Identifying the regions of both the platform and sandwich subdomains involved in binding each peptide motif clarifies the mechanism through which the appendage domain of Sec21 engages with the accessory proteins during the trafficking process of non-clathrin-coated vesicles.

The Kaposi's sarcoma-associated herpesvirus ORF6 DNA binding protein forms long DNA-free helical protein filaments

The Kaposi's sarcoma-associated herpesvirus ORF6 has a 41% sequence identity with Balf2 protein of Epstein-Barr virus and 23% with ICP8 protein of Herpes Simplex type I. Balf2 and ICP8 are multi-functional DNA binding proteins with roles central to viral DNA replication and recombination. In this study, we cloned the KSHV ORF6 gene, expressed the full length ORF6 protein in insect cells and purified it to homogeneity. Gel filtration revealed the protein to be present in a broad spectrum of sizes ranging from monomers to high molecular weight oligomers. Transmission electron microscopy (TEM) using negative staining under conditions favoring monomers and small oligomers revealed fields of globular particles measuring 11nm in diameter consistent with the size of a protein monomer. Incubation of ORF6 protein at room temperature for extended periods of time resulted in the bulk of the protein forming very long helical filaments. Measurements from negative staining revealed that the filaments were up to 2600nm in length, with a width of 13.7nm and a long gentle helical periodicity of 42.9nm along the filament axis. Using rapid freezing and freeze-drying, it was possible to show that the filaments consist of two protein chains wrapped around each other. The possibility that these protein filaments generate a scaffold upon which viral DNA replication, recombination, and encapsidation occur in the infected cell nucleus is discussed.

A large-scale conformational change couples membrane recruitment to cargo binding in the AP2 clathrin adaptor complex

The AP2 adaptor complex (alpha, beta2, sigma2, and mu2 subunits) crosslinks the endocytic clathrin scaffold to PtdIns4,5P(2)-containing membranes and transmembrane protein cargo. In the "locked" cytosolic form, AP2's binding sites for the two endocytic motifs, YxxPhi on the C-terminal domain of mu2 (C-mu2) and [ED]xxxL[LI] on sigma2, are blocked by parts of beta2. Using protein crystallography, we show that AP2 undergoes a large conformational change in which C-mu2 relocates to an orthogonal face of the complex, simultaneously unblocking both cargo-binding sites; the previously unstructured mu2 linker becomes helical and binds back onto the complex. This structural rearrangement results in AP2's four PtdIns4,5P(2)- and two endocytic motif-binding sites becoming coplanar, facilitating their simultaneous interaction with PtdIns4,5P(2)/cargo-containing membranes. Using a range of biophysical techniques, we show that the endocytic cargo binding of AP2 is driven by its interaction with PtdIns4,5P(2)-containing membranes.

Focusing on clathrin-mediated endocytosis

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

Mutational analysis of betaCOP (Sec26p) identifies an appendage domain critical for function

BACKGROUND

The appendage domain of the gammaCOP subunit of the COPI vesicle coat bears a striking structural resemblance to adaptin-family appendages despite limited primary sequence homology. Both the gammaCOP appendage domain and an equivalent region on betaCOP contain the FxxxW motif; the conservation of this motif suggested the existence of a functional appendage domain in betaCOP.

RESULTS

Sequence comparisons in combination with structural prediction tools show that the fold of the COOH-terminus of Sec26p is strongly predicted to closely mimic that of adaptin-family appendages. Deletion of the appendage domain of Sec26p results in inviability in yeast, over-expression of the deletion construct is dominant negative and mutagenesis of this region identifies residues critical for function. The ArfGAP Glo3p was identified via suppression screening as a potential downstream modulator of Sec26p in a manner that is independent of the GAP activity of Glo3p but requires the presence of the COOH-terminal ISS motifs.

CONCLUSION

Together, these results indicate an essential function for the predicted betaCOP appendage and suggest that both COPI appendages perform a biologically active regulatory role with a structure related to adaptin-family appendage domains.

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.

Role of activation of PIP5Kgamma661 by AP-2 complex in synaptic vesicle endocytosis

Synaptic vesicles (SVs) are retrieved by clathrin-mediated endocytosis at the nerve terminals. Phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] drives this event by recruiting the components of the endocytic machinery. However, the molecular mechanisms that result in local generation of PI(4,5)P2 remain unclear. We demonstrate here that AP-2 complex directly interacts with phosphatidylinositol 4-phosphate 5-kinase gamma661 (PIP5Kgamma661), the major PI(4,5)P2-producing enzyme in the brain. The beta2 subunit of AP-2 was found to bind to the C-terminal tail of PIP5Kgamma661 and cause PIP5Kgamma661 activation. The interaction is regulated by PIP5Kgamma661 dephosphorylation, which is triggered by depolarization in mouse hippocampal neurons. Finally, overexpression of the PIP5Kgamma661 C-terminal region in hippocampal neurons suppresses depolarization-dependent SV endocytosis. These findings provide evidence for the molecular mechanism through which PIP5Kgamma661 locally generates PI(4,5)P2 in hippocampal neurons and suggest a model in which the interaction trigger SV endocytosis.

Crystalline Protein Domains and Lipid Bilayer Vesicle Shape Transformations

Cellular membranes can take on a variety of shapes to assist biological processes including endocytosis. Membrane-associated protein domains provide a possible mechanism for determining membrane curvature. We study the effect of tethered streptavidin protein crystals on the curvature of giant unilamellar vesicles (GUVs) using confocal, fluorescence, and differential interference contrast microscopy. Above a critical protein concentration, streptavidin domains align and percolate as they form, deforming GUVs into prolate spheroidal shapes in a size-dependent fashion. We propose a mechanism for this shape transformation based on domain growth and jamming. Osmotic deflation of streptavidin-coated GUVs reveals that the relatively rigid streptavidin protein domains resist membrane bending. Moreover, in contrast to highly curved protein domains that facilitate membrane budding, the relatively flat streptavidin domains prevent membrane budding under high osmotic stress. Thus, crystalline streptavidin domains are shown to have a stabilizing effect on lipid membranes. Our study gives insight into the mechanism for protein-mediated stabilization of cellular membranes.

Beta-adaptin: key molecule for microglial scavenger receptor function under oxidative stress

Scavenger receptors internalize chemically modified low density lipoprotein particles (ac-LDL) and other ligands through the process of receptor-mediated endocytosis. During this investigation using amyloid-beta as a natural ligand for the SR, we studied under a ligand-induced oxidative stress condition, changes in protein expression of several adaptor proteins important in the organization of the endocytic machinery in microglia and macrophages. Differential expression experiments of beta-adaptin, alpha-adaptin, SR-AI, and SR-BI in RAW (macrophages) and EOC (microglia) cells were performed according to dosage and exposure time to amyloid-beta. Our results show that according to dosage, amyloid-beta produces an oxidative stress state that importantly affects the availability of beta-adaptin. Under these conditions, RT-PCR assays show that beta-adaptin mRNA is normally synthesized, reason why protein translation or protein structure of beta-adaptin might be altered. These observations might have impact in the understanding of the mechanisms microglia employ to process amyloid-beta in the brain.

Bending a membrane: how clathrin affects budding

Receptor-mediated endocytosis of ligands, such as transferrin and LDL, is suppressed when clathrin synthesis is blocked by RNA interference in HeLa cells. We have found that domains containing the adapter complex 2 (AP2)-coated vesicle adapter and the endocytic accessory proteins CALM (clathrin assembly lymphoid myeloid leukemia protein), epsin, and eps15/eps15R (EGF receptor pathway substrate 15-related) nevertheless persist at the plasma membrane. They are similar in size and number to those seen in clathrin-expressing cells. Here we characterize these membrane domains by fluorescence and electron microscopy in detail. Fluorescence recovery after photobleaching measurements suggest that the exchange between membrane-bound and free cytosolic AP2 molecules is not significantly influenced by the depletion of clathrin. The AP2 membrane domains are dispersed upon interfering with protein-protein interactions that involve the alpha appendage domain of AP2. Electron microscopy of cellular cortices revealed that the AP2 membrane domains lack any curvature, suggesting that clathrin is essential for driving coated pit invagination. A model for coated vesicle formation, incorporating a mechanism commonly referred to as a "Brownian ratchet," is consistent with our observations.

Ultrastructure of the membrane systems in the unicellular cyanobacterium Synechocystis sp. strain PCC 6803

Among prokaryotes, cyanobacteria are unique in having highly differentiated internal membrane systems. Like other Gram-negative bacteria, cyanobacteria such as Synechocystis sp. strain PCC 6803 have a cell envelope consisting of a plasma membrane, peptidoglycan layer, and outer membrane. In addition, these organisms have an internal system of thylakoid membranes where the electron transfer reactions of photosynthesis and respiration occur. A long-standing controversy concerning the cellular ultrastructures of these organisms has been whether the thylakoid membranes exist inside the cell as separate compartments, or if they have physical continuity with the plasma membrane. Advances in cellular preservation protocols as well as in image acquisition and manipulation techniques have facilitated a new examination of this topic. We have used a combination of electron microscopy techniques, including freeze-etched as well as freeze-substituted preparations, in conjunction with computer-aided image processing to generate highly detailed images of the membrane systems in Synechocystis cells. We show that the thylakoid membranes are in fact physically discontinuous from the plasma membrane in this cyanobacterium. Thylakoid membranes in Synechocystis sp. strain PCC 6803 thus represent bona fide intracellular organelles, the first example of such compartments in prokaryotic cells.

Hermansky-Pudlak syndrome: a disease of protein trafficking and organelle function

The Hermansky-Pudlak syndrome (HPS) is a collection of related autosomal recessive disorders which are genetically heterogeneous. There are eight human HPS subtypes, characterized by oculocutaneous albinism and platelet storage disease; prolonged bleeding, congenital neutropenia, pulmonary fibrosis, and granulomatous colitis can also occur. HPS is caused primarily by defects in intracellular protein trafficking that result in the dysfunction of intracellular organelles known as lysosome-related organelles. HPS gene products are all ubiquitously expressed and all associate in various multi-protein complexes, yet HPS has cell type-specific disease expression. Impairment of specialized secretory cells such as melanocytes, platelets, lung alveolar type II epithelial cells and cytotoxic T cells are observed in HPS. This review summarizes recent molecular, biochemical and cell biological analyses together with clinical studies that have led to the correlation of molecular pathology with clinical manifestations and led to insights into such diverse disease processes such as albinism, fibrosis, hemorrhage, and congenital neutropenia.

Life of a clathrin coat: insights from clathrin and AP structures

Membrane sorting between secretory and endocytic organelles is predominantly controlled by small carrier vesicles or tubules that have specific protein coats on their cytoplasmic surfaces. Clathrin-clathrin-adaptor coats function in many steps of intracellular transport and are the most extensively studied of all transport-vesicle coats. In recent years, the determination of structures of clathrin assemblies by electron microscopy, of domains of clathrin and of its adaptors has improved our understanding of the molecular mechanisms of clathrin-coated-vesicle assembly and disassembly.

Deep-etch EM reveals that the early poxvirus envelope is a single membrane bilayer stabilized by a geodetic "honeycomb" surface coat

Three-dimensional "deep-etch" electron microscopy (DEEM) resolves a longstanding controversy concerning poxvirus morphogenesis. By avoiding fixative-induced membrane distortions that confounded earlier studies, DEEM shows that the primary poxvirus envelope is a single membrane bilayer coated on its external surface by a continuous honeycomb lattice. Freeze fracture of quick-frozen poxvirus-infected cells further shows that there is only one fracture plane through this primary envelope, confirming that it consists of a single lipid bilayer. DEEM also illustrates that the honeycomb coating on this envelope is completely replaced by a different paracrystalline coat as the poxvirus matures. Correlative thin section images of infected cells freeze substituted after quick-freezing, plus DEEM imaging of Tokuyasu-type cryo-thin sections of infected cells (a new application introduced here) all indicate that the honeycomb network on immature poxvirus virions is sufficiently continuous and organized, and tightly associated with the envelope throughout development, to explain how its single lipid bilayer could remain stable in the cytoplasm even before it closes into a complete sphere.

Natively unfolded domains in endocytosis: hooks, lines and linkers

It is commonly assumed that a protein must adopt a tertiary structure to achieve its active native state and that regions of a protein that are devoid of alpha-helix or beta-sheet structures are functionally inert. Although extended proline-rich regions are recognized as presenting binding motifs to, for example, Src homology 2 (SH2) and SH3 domains, the idea persists that natively unfolded regions in functional proteins are simply 'spacers' between the folded domains. Such a view has been challenged in recent years and the importance of natively unfolded proteins in biology is now being recognized. In this review, we highlight the role of natively unfolded domains in the field of endocytosis, and show that some important endocytic proteins lack a traditionally folded structure and harbour important binding motifs in their unstructured linker regions.

Evolving nature of the AP2 alpha-appendage hub during clathrin-coated vesicle endocytosis

Clathrin-mediated endocytosis involves the assembly of a network of proteins that select cargo, modify membrane shape and drive invagination, vesicle scission and uncoating. This network is initially assembled around adaptor protein (AP) appendage domains, which are protein interaction hubs. Using crystallography, we show that FxDxF and WVxF peptide motifs from synaptojanin bind to distinct subdomains on alpha-appendages, called 'top' and 'side' sites. Appendages use both these sites to interact with their binding partners in vitro and in vivo. Occupation of both sites simultaneously results in high-affinity reversible interactions with lone appendages (e.g. eps15 and epsin1). Proteins with multiple copies of only one type of motif bind multiple appendages and so will aid adaptor clustering. These clustered alpha(appendage)-hubs have altered properties where they can sample many different binding partners, which in turn can interact with each other and indirectly with clathrin. In the final coated vesicle, most appendage binding partners are absent and thus the functional status of the appendage domain as an interaction hub is temporal and transitory giving directionality to vesicle assembly.

An Ear-Core Interaction Regulates the Recruitment of the AP-3 Complex to Membranes

AP-3 is a heterotetrameric adaptor involved in the biogenesis of lysosome-related organelles. The function of AP-3 as an adaptor relies on its ability to bind to membranes in an Arf-dependent fashion and to recognize sorting signals in the cytosolic tails of the transmembrane cargo. Here, we report an interdomain interaction involving the ear domain of the delta subunit and the sigma3 subunit of AP-3. This interaction interferes with the binding of AP-3 to Arf but not to dileucine-based sorting signals. As a consequence, the delta-ear inhibits the recruitment of AP-3 to membranes both in vitro and in vivo and impairs the sorting of lysosomal membrane proteins. These observations suggest a new regulatory mechanism for the recruitment of AP-3 to membranes involving delta-ear-sigma3 interactions.

Biochemical characterization of the coating mechanism of the endosomal donor compartment of synaptic vesicles

The heterotetrameric adaptor protein complex, AP-3, sorts proteins to both the endosome/lysosome and the synaptic vesicles. We have characterized the recruitment of pure AP-3 complex and ADP-ribosylation factor (ARF) onto the endosomal donor compartments that give rise to synaptic vesicles. We demonstrated that endosomes become heavier in a sucrose gradient after incubation with rat brain cytosol and a nonhydrolyzable GTP analog, GTPgammaS. This process requires a small GTPase, ARF-1. Furthermore, the endosomal coating is specific for AP-3 but not the AP-2 complex. This process requires only two soluble proteins AP-3 and ARF, with the recruitment of AP-3 being saturable at about 30 nM. These results establish that the synaptic vesicle's donor membrane is coated with AP-3 before vesiculation, in a coat-protein-specific and dose-dependent fashion.

Localization of the AP-3 adaptor complex defines a novel endosomal exit site for lysosomal membrane proteins

The adaptor protein (AP) 3 adaptor complex has been implicated in the transport of lysosomal membrane proteins, but its precise site of action has remained controversial. Here, we show by immuno-electron microscopy that AP-3 is associated with budding profiles evolving from a tubular endosomal compartment that also exhibits budding profiles positive for AP-1. AP-3 colocalizes with clathrin, but to a lesser extent than does AP-1. The AP-3- and AP-1-bearing tubular compartments contain endocytosed transferrin, transferrin receptor, asialoglycoprotein receptor, and low amounts of the cation-independent mannose 6-phosphate receptor and the lysosome-associated membrane proteins (LAMPs) 1 and 2. Quantitative analysis revealed that of these distinct cargo proteins, only LAMP-1 and LAMP-2 are concentrated in the AP-3-positive membrane domains. Moreover, recycling of endocytosed LAMP-1 and CD63 back to the cell surface is greatly increased in AP-3-deficient cells. Based on these data, we propose that AP-3 defines a novel pathway by which lysosomal membrane proteins are transported from tubular sorting endosomes to lysosomes.

Location of auxilin within a clathrin cage

The Dna J homologue, auxilin, acts as a co-chaperone for Hsc70 in the uncoating of clathrin-coated vesicles during endocytosis. Biochemical studies have aided understanding of the uncoating mechanism but until now there was no structural information on how auxilin interacts with the clathrin cage. Here we have determined the three-dimensional structure of a complex of auxilin with clathrin cages by cryo-electron microscopy and single particle analysis. We show that auxilin forms a discrete shell of density on the inside of the clathrin cage. Peptide competition assays confirm that a candidate clathrin box motif in auxilin, LLGLE, can bind to a clathrin construct containing the beta-propeller domain and also displace the well-characterised LLNLD clathrin box motif derived from the beta-adaptin hinge region. The means by which auxilin could both aid clathrin coat assembly and displace clathrin from AP2 during uncoating is discussed.

Adaptins

Clathrin has long been known to provide the structural basis for vesicle budding from the plasma membrane during endocytosis, but how is clathrin targeted specifically to some cellular membranes and not others? The answer seems to lie in the adaptors--protein complexes whose shape resembles the head of Mickey Mouse--which seem to be required both for clathrin-coat assembly and for sequestering specific receptors by interacting with their cytoplasmic domains. In this article, Margaret Robinson describes what is currently known about these versatile proteins.

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.

Conserved Structural Motifs in Intracellular Trafficking Pathways

The formation of coated vesicles is a fundamental step in many intracellular trafficking pathways. COPI and clathrin represent two important and distinct sets of vesicle coating machinery, involved primarily in mediating intra-Golgi and endocytic transport, respectively. Here we identify an important functional region at the carboxyl terminus of the gamma subunit of the COPI complex (gammaCOP) and describe the X-ray crystal structure of this domain at 2.3 A resolution. This domain of gammaCOP exhibits unexpected structural similarity to the carboxyl-terminal appendage domains of the alpha and beta subunits of the AP2 adaptor proteins, integral components of clathrin-coated vesicles. The remarkable structural conservation exhibited by the gammaCOP appendage domain, coupled with functional data and primary sequence analysis, supports a model of COPI function with significant structural and mechanistic parallels to vesicular transport by the clathrin/AP2 system.

Mechanisms of membrane deformation

Membrane traffic requires the generation of high-curvature lipid-bound transport carriers represented by tubules and vesicles. The mechanisms through which membranes are deformed has gained much recent attention. A major advance has been the demonstration that direct interactions between cytosolic proteins and lipid bilayers are important in the acquisition of membrane curvature. Rather than being driven only by the formation of membrane-associated structural scaffolds, membrane deformation requires physical perturbation of the lipid bilayer. A variety of proteins have been identified that directly bind and deform membranes. An emerging theme in this process is the importance of amphipathic peptides that partially penetrate the lipid bilayer.

The beta2-adaptin clathrin adaptor interacts with the mitotic checkpoint kinase BubR1

The adaptor AP2 is a heterotetrameric complex that associates with clathrin and regulatory proteins to mediate rapid endocytosis from the plasma membrane. Here, we report the identification of the mitotic checkpoint kinase BubR1 as a novel binding partner of beta2-adaptin, one of the AP2 large subunits. Using two-hybrid experiments and in vitro binding assays, we show that beta2-adaptin binds to BubR1 through its amino-terminal beta2-'trunk' domain, while the beta2-binding region of BubR1 maps to the carboxy-terminal kinase domain. Subcellular immunolocalization studies suggest that the interaction between BubR1 and beta2-adaptin could take place in the cytosol at any time during the cell cycle. In addition, we found that BubR1 and the BubR1-related kinase, Bub1, also bind to beta-adaptins of other AP complexes. Together, these results support a model in which the mitotic checkpoint kinases BubR1 and BuB1, by binding to beta-adaptins, may play novel roles in the regulation of vesicular intracellular traffic.

The endocytic protein alpha-Adaptin is required for numb-mediated asymmetric cell division in Drosophila

During asymmetric cell division in Drosophila sensory organ precursor cells, the Numb protein localizes asymmetrically and segregates into one daughter cell, where it influences cell fate by repressing signal transduction via the Notch receptor. We show here that Numb acts by polarizing the distribution of alpha-Adaptin, a protein involved in receptor-mediated endocytosis. alpha-Adaptin binds to Numb and localizes asymmetrically in a Numb-dependent fashion. Mutant forms of alpha-Adaptin that no longer bind to Numb fail to localize asymmetrically and cause numb-like defects in asymmetric cell division. Our results suggest a model in which Numb influences cell fate by downregulating Notch through polarized receptor-mediated endocytosis, since Numb also binds to the intracellular domain of Notch.

Inositol hexakisphosphate-mediated regulation of glutamate receptors in rat brain sections

D-myo-inositol 1,2,3,4,5,6-hexakisphosphate (InsP6), one of the most abundant inositol phosphates within cells, has been proposed to play a key role in vesicle trafficking and receptor compartmentalization. In the present study, we used in vitro receptor autoradiography, subcellular fractionation, and immunoblotting to investigate its effects on alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) and N-methyl-D-aspartate (NMDA) receptors. Qualitative and quantitative analysis of 3H-AMPA binding indicated that incubation of frozen-thawed brain sections with InsP6 at 35 degrees C enhanced AMPA receptor binding in several brain regions, with maximal increases in the hippocampus and cerebellum. Moreover, saturation kinetics demonstrated that InsP6-induced augmentation of AMPA binding was due to an increment in the maximal number of AMPA binding sites. At the immunological level, Western blots performed on crude mitochondrial/synaptic (P2) fractions revealed that InsP6 (but not InsP5 and InsP3) treatment increased glutamate receptor (GluR)1 and GluR2 subunits of AMPA receptors, an effect that was associated with concomitant reductions in microsomal (P3) fractions. Interestingly, the InsP6-induced modulation of AMPA receptor binding was blocked at room temperature, and pretreatment with heparin also dampered its action on both AMPA receptor binding and GluR subunits. These effects of InsP6 appear to be specific to AMPA receptors, as neither 3H-glutamate binding to NMDA receptors nor levels of NR1 and NR2A subunits in P2 and P3 fractions were affected. Taken together, our data strongly suggest that InsP6 specifically regulates AMPA receptor distribution, possibly through a clathrin-dependent process.

Adaptor protein-2 exhibits alpha 1 beta 1 or alpha 6 beta 1 integrin-dependent redistribution in rhabdomyosarcoma cells

Downregulation of several signaling pathways, such as those stimulated by growth factor receptors, occurs by internalization of signaling receptors through clathrin-coated pits. The first step in internalization or endocytosis is interaction with AP-2, which results in coated pit formation by assembly of clathrin to AP-2. Changes in endocytosis are reflected in the distribution of AP-2 molecules at the cell surface. Integrins are receptors which mediate attachment to the extracellular matrix and also stimulate numerous intracellular signaling pathways; however, it is not known how signaling through integrins is terminated or downregulated. Endocytosis through clathrin-coated pits offers an attractive mechanism for this. This work explores the relationship between AP-2 and beta(1) integrins. RD cells grown for 24 h on collagen or laminin exhibit a redistribution of AP-2 to the cell periphery relative to those grown on fibronectin or polylysine. The total AP-2 protein levels in the cells are unaffected. Blocking alpha(1)beta(1) integrin ligand binding on collagen prevents this redistribution fully. On laminin where alpha(1)beta(1) and alpha(6)beta(1) integrins are engaged, both receptors must be simultaneously blocked to prevent AP-2 redistribution, confirming that the redistribution depends on the specific engagement of the receptors. Immunofluorescence reveals that the majority of alpha(1)beta(1) integrins colocalize with alpha(6)beta(1) integrins in linear structures identified as focal adhesions. A separate fraction of alpha(1)beta(1) integrins colocalize with AP-2 in coated pits. Interestingly, alpha(6)beta(1) integrins are not located in coated pits, demonstrating that integrin colocalization with AP-2 is not necessary to induce redistribution of AP-2.

Subunit H of the V-ATPase involved in endocytosis shows homology to beta-adaptins

The vacuolar ATPase (V-ATPase) is a multisubunit enzyme that facilitates the acidification of intracellular compartments in eukaryotic cells and plays an important role in receptor-mediated endocytosis, intracellular trafficking processes, and protein degradation. In this study we show that the C-terminal fragment of 350 residues of the regulatory subunit H (V1H) of the V-ATPase shares structural and functional homologies with the beta-chains of adaptor protein complexes. Moreover, the fragment is similar to a region in the beta-subunit of COPI coatomer complexes, which suggests the existence of a shared domain in these three different families of proteins. For beta-adaptins, this fragment binds to cytoplasmic di-leucine-based sorting motifs such as in HIV-1 Nef that mediate endocytic trafficking. Expression of this fragment in cells blocks the internalization of transmembrane proteins, which depend on di-leucine-based motifs, whereas mutation of the consensus sequence GEY only partly diminishes the recognition of the sorting motif. Based on recent structural analysis, our results suggest that the di-leucine-binding domain consists of a HEAT or ARM repeat protein fold.

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.

The beta-appendages of the four adaptor-protein (AP) complexes: structure and binding properties, and identification of sorting nexin 9 as an accessory protein to AP-2

Adaptor protein (AP) complexes are essential components for the formation of coated vesicles and the recognition of cargo proteins for intracellular transport. Each AP complex exposes two appendage domains with that function to bind regulatory accessory proteins in the cytosol. Secondary structure predictions, sequence alignments and CD spectroscopy were used to relate the beta-appendages of all human AP complexes to the previously published crystal structure of AP-2. The results suggested that the beta-appendages of AP-1, AP-2 and AP-3 have similar structures, consisting of two subdomains, whereas that of AP-4 lacks the inner subdomain. Pull-down and overlay assays showed partial overlap in the binding specificities of the beta-appendages of AP-1 and AP-2, whereas the corresponding domain of AP-3 displayed a unique binding pattern. That AP-4 may have a truncated, non-functional domain was indicated by its apparent inability to bind any proteins from cytosol. Of several novel beta-appendage-binding proteins detected, one that had affinity exclusively for AP-2 was identified as sorting nexin 9 (SNX9). SNX9, which contains a phox and an Src homology 3 domain, was found in large complexes and was at least partially associated with AP-2 in the cytosol. SNX9 may function to assist AP-2 in its role at the plasma membrane.

The beta-appendages of the four adaptor-protein (AP) complexes: structure and binding properties, and identification of sorting nexin 9 as an accessory protein to AP-2

Adaptor protein (AP) complexes are essential components for the formation of coated vesicles and the recognition of cargo proteins for intracellular transport. Each AP complex exposes two appendage domains with that function to bind regulatory accessory proteins in the cytosol. Secondary structure predictions, sequence alignments and CD spectroscopy were used to relate the beta-appendages of all human AP complexes to the previously published crystal structure of AP-2. The results suggested that the beta-appendages of AP-1, AP-2 and AP-3 have similar structures, consisting of two subdomains, whereas that of AP-4 lacks the inner subdomain. Pull-down and overlay assays showed partial overlap in the binding specificities of the beta-appendages of AP-1 and AP-2, whereas the corresponding domain of AP-3 displayed a unique binding pattern. That AP-4 may have a truncated, non-functional domain was indicated by its apparent inability to bind any proteins from cytosol. Of several novel beta-appendage-binding proteins detected, one that had affinity exclusively for AP-2 was identified as sorting nexin 9 (SNX9). SNX9, which contains a phox and an Src homology 3 domain, was found in large complexes and was at least partially associated with AP-2 in the cytosol. SNX9 may function to assist AP-2 in its role at the plasma membrane.

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.

Studies of synaptic vesicle endocytosis in the nematode C. elegans

After synaptic vesicle exocytosis, synaptic vesicle proteins must be retrieved from the plasma membrane, sorted away from other membrane proteins, and reconstituted into a functional synaptic vesicle. The nematode Caenorhabditis elegans is an organism well suited for a genetic analysis of this process. In particular, three types of genetic studies have contributed to our understanding of synaptic vesicle endocytosis. First, screens for mutants defective in synaptic vesicle recycling have identified new proteins that function specifically in neurons. Second, RNA interference has been used to quickly confirm the roles of known proteins in endocytosis. Third, gene targeting techniques have elucidated the roles of genes thought to play modulatory or subtle roles in synaptic vesicle recycling. We describe a molecular model for synaptic vesicle recycling and discuss how protein disruption experiments in C. elegans have contributed to this model.

Interactions between adaptor protein-1 of the clathrin coat and microtubules via type 1a microtubule-associated proteins

The classical view suggests that adaptor proteins of the clathrin coat mediate the sorting of cargo protein passengers into clathrin-coated pits and the recruitment of clathrin into budding areas in the donor membrane. In the present study, we provide biochemical and morphological evidence that the adaptor protein 1 (AP-1) adaptor of the trans-Golgi network clathrin interacts with microtubules. AP-1 in cytosolic extracts interacted with in vitro assembled microtubules, and these interactions were inhibited by ATP depletion of the extracts or in the presence of 5'-adenylylimidodiphosphate. An overexpressed gamma-subunit of the AP-1 complex associated with microtubules, suggesting that this subunit may mediate the interaction of AP-1 with the cytoskeleton. Purified AP-1 did not interact with purified microtubules, but interaction occurred when an isolated microtubule-associated protein fraction was added to the reaction mix. The gamma-adaptin subunit of AP-1 specifically co-immunoprecipitated with a microtubule-associated protein of type 1a from rat brain cytosol. This suggests that type 1a microtubule-associated protein may mediate the association of AP-1 with microtubules in the cytoplasm. The microtubule binding activity of AP-1 was markedly inhibited in cytosol of mitotic cells. By means of its interaction with microtubule-associated proteins, we propose novel roles for AP-1 adaptors in modulating the dynamics of the cytoskeleton, the stability and shape of coated organelles, and the loading of nascent AP-1-coated vesicles onto appropriate microtubular tracks.

Clathrin interactions with C-terminal regions of the yeast AP-1 beta and gamma subunits are important for AP-1 association with clathrin coats

Heterotetrameric adaptor (AP) complexes are thought to coordinate cargo recruitment and clathrin assembly during clathrin-coated vesicle biogenesis. We have identified, and characterized the physiological significance of clathrin-binding activities in the two large subunits of the AP-1 complex in Saccharomyces cerevisiae. Using GST-fusion chromatography, two clathrin-binding sites were defined in the beta1 subunit that match consensus clathrin-binding sequences in other mammalian and yeast clathrin-binding proteins. Clathrin interactions were also identified with the C-terminal region of the gamma subunit. When introduced into chromosomal genes, point mutations in the beta1 clathrin-binding motifs, or deletion of the gamma C-terminal region, reduced association of AP-1 with clathrin in coimmunoprecipitation assays. The beta1 mutations or the gamma truncation individually produced minor effects on AP-1 distribution by subcellular fractionation. However, when beta1 and gamma mutations were combined, severe defects were observed in AP-1 association with membranes and incorporation into clathrin-coated vesicles. The combination of subunit mutations accentuated growth and alpha-factor pheromone maturation defects in chc1-ts cells, though not to the extent caused by complete loss of AP-1 activity. Our results suggest that both the beta1 and gamma subunits contribute interactions with clathrin that are important for stable assembly of AP-1 complexes into clathrin coats in vivo.

Developmental and morphological regulation of clathrin-mediated endocytosis inTrypanosoma brucei

Essentially all macromolecular communication between Trypanosoma brucei and its host is confined to vesicular trafficking events occurring at or around the flagellar pocket. The vertebrate stage bloodstream form trypomastigote exhibits an extremely high rate of endocytosis required for nutrient uptake and probably also evasion of the host immune system. However, the rate of endocytosis is very low in the procyclic vector parasite, indicating that endocytosis is subject to a marked level of developmental regulation. Previous ultrastructural studies and crude biochemical fractionations have indicated the presence of coated pits and vesicles that are analogous to clathrin coats in the bloodstream form, but not in the procyclic. However, a definitive description of the components of this coat and its molecular function in T. brucei has remained elusive. We describe the molecular cloning and initial characterisation of components of the T. brucei endocytic coats: clathrin heavy chain (TbCLH) and a β-adaptin (TbAPβ1). TbCLH is markedly upregulated in the bloodstream form compared with the procyclic, whereas TbAPβ1 is subject to more limited developmental regulation. We generated antisera against both proteins and show that the clathrin coat is tightly associated with the flagellar pocket in both major life stages. However, in bloodstream parasites TbCLH is also extensively distributed throughout the posterior end of the cell on numerous large vesicular and tubular structures. By cryoimmuno EM, clathrin is localised to collecting tubules at the flagellar pocket and is also associated with the trans-Golgi network. These EM data confirm that the electron dense coats reported on trypanosome vesicles and tubules contain clathrin. The TbAPβ1 exhibits an atypical distribution relative to previously characterised adaptins, associating not only with the trans-Golgi but also with other tubular-vesicular elements. Localisation of TbAPβ1 is also subject to developmental regulation. These data describe major endocytic coat proteins in T. brucei for the first time, and indicate stage-specific expression of the clathrin heavy chain. Modulation of clathrin expression is likely to be an important factor in the developmental regulation of endocytosis and recycling in the African trypanosome.

Clathrin

Clathrin was discovered nearly 25 years ago. Since then, a large number of other proteins that participate in the process by which clathrin-coated vesicles retrieve synaptic membranes or take up endocytic receptors have been identified. The functional relationships among these disparate components remain, in many cases, obscure. High-resolution structures of parts of clathrin, determined by X-ray crystallography, and lower-resolution images of assembled coats, determined by electron cryomicroscopy, now provide the information necessary to integrate various lines of evidence and to design experiments that test specific mechanistic notions. This review summarizes and illustrates the recent structural results and outlines what is known about coated-vesicle assembly in the context of this information.