Characterization of the adaptor-related protein complex, AP-3

Neutrophil elastase and proteinase 3 trafficking routes in myelomonocytic cells

Neutrophil elastase (NE) and proteinase 3 (PR3) differ in intracellular localization, which may reflect different trafficking mechanisms of the precursor forms when synthesized at immature stages of neutrophils. To shed further light on these mechanisms, we compared the trafficking of precursor NE (proNE) and precursor PR3 (proPR3). Like proNE [1], proPR3 interacted with CD63 upon heterologous co-expression in COS cells but endogenous interaction was not detected although cell surface proNE/proPR3/CD63 were co-endocytosed in myelomonocytic cells. Cell surface proNE/proPR3 turned over more rapidly than cell surface CD63 consistent with processing/degradation of the pro-proteases but recycling of CD63. Colocalization of proNE/proPR3/CD63 with clathrin and Rab 7 suggested trafficking through coated vesicles and late endosomes. Partial caveolar trafficking of proNE/CD63 but not proPR3 was suggested by colocalization with caveolin-1. Blocking the C-terminus of proNE/proPR3 by creating a fusion with FK506 binding protein inhibited endosomal re-uptake of proNE but not proPR3 indicating "pro(C)"-peptide-dependent structural/conformational requirements for proNE but not for proPR3 endocytosis. The NE aminoacid residue Y199 of a proposed NE sorting motif that interacts with AP-3 [2] was not required for proNE processing, sorting or endocytosis in rat basophilic leukemia (RBL) cells expressing heterologous Y199-deleted proNE; this suggests operation of another AP-3-link for proNE targeting. Our results show intracellular multi-step trafficking to be different between proNE and proPR3 consistent with their differential subcellular NE/PR3 localization in neutrophils.

Characterization and prediction of protein nucleolar localization sequences

Although the nucleolar localization of proteins is often believed to be mediated primarily by non-specific retention to core nucleolar components, many examples of short nucleolar targeting sequences have been reported in recent years. In this article, 46 human nucleolar localization sequences (NoLSs) were collated from the literature and subjected to statistical analysis. Of the residues in these NoLSs 48% are basic, whereas 99% of the residues are predicted to be solvent-accessible with 42% in α-helix and 57% in coil. The sequence and predicted protein secondary structure of the 46 NoLSs were used to train an artificial neural network to identify NoLSs. At a true positive rate of 54%, the predictor’s overall false positive rate (FPR) is estimated to be 1.52%, which can be broken down to FPRs of 0.26% for randomly chosen cytoplasmic sequences, 0.80% for randomly chosen nucleoplasmic sequences and 12% for nuclear localization signals. The predictor was used to predict NoLSs in the complete human proteome and 10 of the highest scoring previously unknown NoLSs were experimentally confirmed. NoLSs are a prevalent type of targeting motif that is distinct from nuclear localization signals and that can be computationally predicted.

Building a fly eye: terminal differentiation events of the retina, corneal lens, and pigmented epithelia

In the past, vast differences in ocular structure, development, and physiology throughout the animal kingdom led to the widely accepted notion that eyes are polyphyletic, that is, they have independently arisen multiple times during evolution. Despite the dissimilarity between vertebrate and invertebrate eyes, it is becoming increasingly evident that the development of the eye in both groups shares more similarity at the genetic level than was previously assumed, forcing a reexamination of eye evolution. Understanding the molecular underpinnings of cell type specification during Drosophila eye development has been a focus of research for many labs over the past 25 years, and many of these findings are nicely reviewed in Chapters 1 and 4. A somewhat less explored area of research, however, considers how these cells, once specified, develop into functional ocular structures. This review aims to summarize the current knowledge related to the terminal differentiation events of the retina, corneal lens, and pigmented epithelia in the fly eye. In addition, we discuss emerging evidence that the different functional components of the fly eye share developmental pathways and functions with the vertebrate eye.

Multiple roles of the vesicular-SNARE TI-VAMP in post-Golgi and endosomal trafficking

SNARE (Soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins are the core machinery of membrane fusion. Vesicular SNAREs (v-SNAREs) interact with their target SNAREs (t-SNAREs) to form SNARE complexes which mediate membrane fusion. Here we review the basic properties and functions of the v-SNARE TI-VAMP/VAMP7 (Tetanus neurotoxin insensitive-vesicle associated membrane protein). TI-VAMP interacts with its t-SNARE partners, particularly plasmalemmal syntaxins, to mediate membrane fusion and with several regulatory proteins especially via its amino-terminal regulatory Longin domain. Partners include AP-3, Hrb/(Human immunodeficiency virus Rev binding) protein, and Varp (Vps9 domain and ankyrin repeats containing protein) and regulate TI-VAMP's function and targeting. TI-VAMP is involved both in secretory and endocytic pathways which mediate neurite outgrowth and synaptic transmission, plasma membrane remodeling and lysosomal secretion.

Herpes simplex virus type 2 tegument protein UL56 relocalizes ubiquitin ligase Nedd4 and has a role in transport and/or release of virions

Background

The ubiquitin system functions in a variety of cellular processes including protein turnover, protein sorting and trafficking. Many viruses exploit the cellular ubiquitin system to facilitate viral replication. In fact, herpes simplex virus (HSV) encodes a ubiquitin ligase (E3) and a de-ubiquitinating enzyme to modify the host's ubiquitin system. We have previously reported HSV type 2 (HSV-2) tegument protein UL56 as a putative adaptor protein of neuronal precursor cell-expressed developmentally down-regulated 4 (Nedd4) E3 ligase, which has been shown to be involved in protein sorting and trafficking.

Results

In this study, we visualized and characterized the dynamic intracellular localization of UL56 and Nedd4 using live-cell imaging and immunofluorescence analysis. UL56 was distributed to cytoplasmic vesicles, primarily to the trans-Golgi network (TGN), and trafficked actively throughout the cytoplasm. Moreover, UL56 relocalized Nedd4 to the vesicles in cells transiently expressing UL56 and in cells infected with HSV-2. We also investigated whether UL56 influenced the efficiency of viral replication, and found that extracellular infectious viruses were reduced in the absence of UL56.

Conclusion

These data suggest that UL56 regulates Nedd4 and functions to facilitate the cytoplasmic transport of virions from TGN to the plasma membrane and/or release of virions from the cell surface.

Transcriptome and proteome analysis of Chinese hamster ovary cells under low temperature and butyrate treatment

Recombinant Chinese hamster ovary (CHO) cells selected for high productivity are capable of secreting immunoglobulin G (IgG) molecules at a level that rivals plasma cells in vivo. Following butyrate treatment at 33 degrees C, further increases in productivity are observed. To better understand the mechanisms by which this increased productivity is incurred, the transcriptional response of an antibody-producing cell line undergoing these treatments was investigated using oligo-DNA microarrays. Using distance calculations, more than 900 genes were identified as kinetically differentially expressed between the butyrate-treated 33 degrees C culture and the untreated culture. Furthermore, transcript levels of the heavy and light chain IgG genes increased following treatment. Using stable isotope labeling (SILAC), the secretion rate of IgG was investigated by tracking the decay of the isotope label upon switching to unlabeled medium. Both treated and untreated cultures exhibited very similar IgG secretion kinetics. In contrast, the intracellular IgG content was found to be elevated following treatment. This result suggests that increased productivity under treatment is attributable to elevated cellular secretory capacity, rather than shorter holding times in the secretory pathway. This hypothesis is further supported by the results of gene set enrichment analysis (GSEA), which revealed that elements of the secretory pathway, including Golgi apparatus, cytoskeleton protein binding and small GTPase-mediated signal transduction are enriched and thus may play a role in the increased recombinant protein production observed under butyrate treatment at 33 degrees C.

Two patients with Hermansky Pudlak syndrome type 2 and novel mutations in AP3B1

Hermansky Pudlak syndrome type 2 (HPS2) is a rare disorder associated with mutations in the Adaptor Protein 3 (AP-3) complex, which is involved in sorting transmembrane proteins to lysosomes and related organelles. We now report 2 unrelated subjects with HPS2 who show a characteristic clinical phenotype of oculocutaneous albinism, platelet and T-lymphocyte dysfunction and neutropenia. The subjects were homozygous for different deletions within AP3B1 (g.del180242-180866, c.del153-156), which encodes the AP-3beta3A subunit, resulting in frame shifts and introduction of nonsense substitutions (p.E693fsX13, p.E52fsX11). In the subject with p.E693fsX13, this resulted in expression of a truncated variant beta3A protein. Cytotoxic T-lymphocyte (CTL) clones from both study subjects showed increased cell-surface expression of CD63 and reduced cytotoxicity. Platelets showed impaired aggregation and reduced uptake of (3)H-serotonin. These findings are consistent with CTL granule and platelet dense granule defects, respectively. This report extends the clinical and laboratory description of HPS2.

The white gene of Drosophila melanogaster encodes a protein with a role in courtship behavior

The white gene of Drosophila melanogaster has been extensively studied, yet it is still not understood how its ectopic overexpression induces male-male courtship. To investigate the cellular basis of this behavior, we examined the sexual behavior of several classes of mutants. We find that male-male courtship is seen not only in flies overexpressing the white gene, but also in mutants expected to have mislocalized White protein. This finding confirms that mislocalizing White transporter in the cells in which it is normally expressed will produce male-male courtship behaviors; the courtship behavior is not an indirect consequence of aberrant physiological changes elsewhere in the body. Male-male courtship is also seen in some mutants with altered monoamine metabolism and deficits in learning and memory, but can be distinguished from that produced by White mislocalization by its reduced intensity and locomotor activity. Double mutants overexpressing white and with mutations in genes for serotonergic neurons suggest that male-male courtship produced by mislocalizing White may not be mediated exclusively by serotonergic neurons. We also find decreased olfactory learning in white mutants and in individuals with mutations in the genes for White's binding partners, brown and scarlet. Finally, in cultured Drosophila and mammalian cells, the White transporter is found in the endosomal compartment. The additional genes identified here as being involved in male-male courtship increase the repertoire of mutations available to study sexual behavior in Drosophila.

WNK4 diverts the thiazide-sensitive NaCl cotransporter to the lysosome and stimulates AP-3 interaction

With-no-lysine kinase 4 (WNK4) inhibits electroneutral sodium chloride reabsorption by attenuating the cell surface expression of the thiazide-sensitive NaCl cotransporter (NCC). The underlying mechanism for this effect remains poorly understood. Here, we explore how WNK4 affects the trafficking of NCC through its interactions with intracellular sorting machinery. An analysis of NCC cell surface lifetime showed that WNK4 did not alter the net rate of cotransporter internalization. In contrast, direct measurements of forward trafficking revealed that WNK4 attenuated the rate of NCC surface delivery, inhibiting the anterograde movement of cotransporters traveling to the plasma membrane from the trans-Golgi network. The response was paralleled by a dramatic reduction in NCC protein abundance, an effect that was sensitive to the lysosomal protease inhibitor leupeptin, insensitive to proteasome inhibition, and attenuated by endogenous WNK4 knockdown. Subcellular localization studies performed in the presence of leupeptin revealed that WNK4 enhanced the accumulation of NCC in lysosomes. Moreover, NCC immunoprecipitated with endogenous AP-3 complexes, and WNK4 increased the fraction of cotransporters that associate with this adaptor, which facilitates cargo transport to lysosomes. WNK4 expression also increased LAMP-2-positive lysosomal content, indicating that the kinase may act by a general AP-3-dependent mechanism to promote cargo delivery into the lysosomal pathway. Taken together, these findings indicate that WNK4 inhibits NCC activity by diverting the cotransporter to the lysosome for degradation by way of an AP-3 transport carrier.

A data-mining approach to rank candidate protein-binding partners-The case of biogenesis of lysosome-related organelles complex-1 (BLOC-1)

The study of protein-protein interactions is a powerful approach to uncovering the molecular function of gene products associated with human disease. Protein-protein interaction data are accumulating at an unprecedented pace owing to interactomics projects, although it has been recognized that a significant fraction of these data likely represents false positives. During our studies of biogenesis of lysosome-related organelles complex-1 (BLOC-1), a protein complex involved in protein trafficking and containing the products of genes mutated in Hermansky-Pudlak syndrome, we faced the problem of having too many candidate binding partners to pursue experimentally. In this work, we have explored ways of efficiently gathering high-quality information about candidate binding partners and presenting the information in a visually friendly manner. We applied the approach to rank 70 candidate binding partners of human BLOC-1 and 102 candidates of its counterpart from Drosophila melanogaster. The top candidate for human BLOC-1 was the small GTPase encoded by the RAB11A gene, which is a paralogue of the Rab38 and Rab32 proteins in mammals and the lightoid gene product in flies. Interestingly, genetic analyses in D. melanogaster uncovered a synthetic sick/lethal interaction between Rab11 and lightoid. The data-mining approach described herein can be customized to study candidate binding partners for other proteins or possibly candidates derived from other types of 'omics' data.

The SM protein Car/Vps33A regulates SNARE-mediated trafficking to lysosomes and lysosome-related organelles

The SM proteins Vps33A and Vps33B are believed to act in membrane fusions in endosomal pathways, but their specific roles are controversial. In Drosophila, Vps33A is the product of the carnation (car) gene. We generated a null allele of car to test its requirement for trafficking to different organelles. Complete loss of car function is lethal during larval development. Eye-specific loss of Car causes late, light-independent degeneration of photoreceptor cells. Earlier in these cells, two distinct phenotypes were detected. In young adults, autophagosomes amassed indicating that their fusion with lysosomes requires Car. In eye discs, endocytosed receptors and ligands accumulate in Rab7-positive prelysosomal compartments. The requirement of Car for late endosome-to-lysosome fusion in imaginal discs is specific as early endosomes are unaffected. Furthermore, lysosomal delivery is not restored by expression of dVps33B. This specificity reflects the distinct pattern of binding to different Syntaxins in vitro: dVps33B predominantly binds the early endosomal Avl and Car to dSyntaxin16. Consistent with a role in Car-mediated fusion, dSyntaxin16 is not restricted to Golgi membranes but also present on lysosomes.

Human immunodeficiency virus type-1 gag and host vesicular trafficking pathways

The Gag protein of HIV-1 directs the particle assembly process. Gag recruits components of the cellular vesicular trafficking machinery in order to traverse the cytoplasm of the cell and reach the particle assembly site. The plasma membrane is the primary site of particle assembly in most cell types, while in macrophages an unusual intracellular membrane-bound compartment bearing markers of late endosomes and the plasma membrane is the predominant assembly site. Plasma membrane specificity of assembly may be directed by components of lipid rafts and the cytoplasmic leaflet component PI(4,5)P(2). Recent work has highlighted the role of adaptor protein complexes, protein sorting and recycling pathways, components of the multivesicular body, and cellular motor proteins in facilitating HIV assembly and budding. This review presents an overview of the relevant vesicular trafficking pathways and describes the individual components implicated in interactions with Gag.

Mu2 adaptin facilitates but is not essential for synaptic vesicle recycling in Caenorhabditis elegans

Synaptic vesicles must be recycled to sustain neurotransmission, in large part via clathrin-mediated endocytosis. Clathrin is recruited to endocytic sites on the plasma membrane by the AP2 adaptor complex. The medium subunit (μ2) of AP2 binds to cargo proteins and phosphatidylinositol-4,5-bisphosphate on the cell surface. Here, we characterize the apm-2 gene (also called dpy-23), which encodes the only μ2 subunit in the nematode Caenorhabditis elegans. APM-2 is highly expressed in the nervous system and is localized to synapses; yet specific loss of APM-2 in neurons does not affect locomotion. In apm-2 mutants, clathrin is mislocalized at synapses, and synaptic vesicle numbers and evoked responses are reduced to 60 and 65%, respectively. Collectively, these data suggest AP2 μ2 facilitates but is not essential for synaptic vesicle recycling.

Sorting of lysosomal proteins

Lysosomes are composed of soluble and transmembrane proteins that are targeted to lysosomes in a signal-dependent manner. The majority of soluble acid hydrolases are modified with mannose 6-phosphate (M6P) residues, allowing their recognition by M6P receptors in the Golgi complex and ensuing transport to the endosomal/lysosomal system. Other soluble enzymes and non-enzymatic proteins are transported to lysosomes in an M6P-independent manner mediated by alternative receptors such as the lysosomal integral membrane protein LIMP-2 or sortilin. Sorting of cargo receptors and lysosomal transmembrane proteins requires sorting signals present in their cytosolic domains. These signals include dileucine-based motifs, DXXLL or [DE]XXXL[LI], and tyrosine-based motifs, YXXØ, which interact with components of clathrin coats such as GGAs or adaptor protein complexes. In addition, phosphorylation and lipid modifications regulate signal recognition and trafficking of lysosomal membrane proteins. The complex interaction of both luminal and cytosolic signals with recognition proteins guarantees the specific and directed transport of proteins to lysosomes.

The tetraspanin CD63 is involved in granule targeting of neutrophil elastase

Targeting mechanisms of neutrophil elastase (NE) and other luminal proteins stored in myeloperoxidase (MPO)–positive secretory lysosomes/primary granules of neutrophils are unknown. These granules contain an integral membrane protein, CD63, with an adaptor protein-3–dependent granule delivery system. Therefore, we hypothesized that CD63 cooperates in granule delivery of the precursor of NE (proNE). Supporting this hypothesis, an association was demonstrated between CD63 and proNE upon coexpression in COS cells. This also involved augmented cellular retention of proNE requiring intact large extracellular loop of CD63. Furthermore, depletion of CD63 in promyelocytic HL-60 cells with RNA interference or a CD63 mutant caused reduction of cellular NE. However, the proNE steady-state level was similar to wild type in CD63-depleted clones, making it feasible to examine possible effects of CD63 on NE trafficking. Thus, depletion of CD63 led to reduced processing of proNE into mature NE and reduced constitutive secretion. Furthermore, CD63-depleted cells showed a lack of morphologically normal granules, but contained MPO-positive cytoplasmic vacuoles with a lack of proNE and NE. Collectively, our data suggest that granule proteins may cooperate in targeting; CD63 can be involved in ER or Golgi export, cellular retention, and granule targeting of proNE before storage as mature NE.

A dileucine signal situated in the C-terminal tail of the lysosomal membrane protein p40 is responsible for its targeting to lysosomes

Transport of newly synthesized lysosomal membrane proteins from the TGN (trans-Golgi network) to the lysosomes is due to the presence of specific signals in their cytoplasmic domains that are recognized by cytosolic adaptors. p40, a hypothetical transporter of 372 amino acids localized in the lysosomal membrane, contains four putative lysosomal sorting motifs in its sequence: three of the YXXphi-type (Y(6)QLF, Y(106)VAL, Y(333)NGL) and one of the [D/E]XXXL[L/I]-type (EQERL(360)L(361)). To test the role of these motifs in the biosynthetic transport of p40, we replaced the most critical residues of these consensus sequences, the tyrosine residue or the leucine-leucine pair, by alanine or alanine-valine respectively. We analysed the subcellular localization of the mutated p40 proteins in transfected HeLa cells by confocal microscopy and by biochemical approaches (subcellular fractionation on self-forming Percoll density gradients and cell surface biotinylation). The results of the present study show that p40 is mistargeted to the plasma membrane when its dileucine motif is disrupted. No role of the tyrosine motifs could be put forward. Taken together, our results provide evidence that the sorting of p40 from the TGN to the lysosomes is directed by the dileucine EQERL(360)L(361) motif situated in its C-terminal tail.

Protein networks supporting AP-3 function in targeting lysosomal membrane proteins

The AP-3 adaptor complex targets selected transmembrane proteins to lysosomes and lysosome-related organelles. We reconstituted its preferred interaction with liposomes containing the ADP ribosylation factor (ARF)-1 guanosine triphosphatase (GTPase), specific cargo tails, and phosphatidylinositol-3 phosphate, and then we performed a proteomic screen to identify new proteins supporting its sorting function. We identified approximately 30 proteins belonging to three networks regulating either AP-3 coat assembly or septin polymerization or Rab7-dependent lysosomal transport. RNA interference shows that, among these proteins, the ARF-1 exchange factor brefeldin A-inhibited exchange factor 1, the ARF-1 GTPase-activating protein 1, the Cdc42-interacting Cdc42 effector protein 4, an effector of septin-polymerizing GTPases, and the phosphatidylinositol-3 kinase IIIC3 are key components regulating the targeting of lysosomal membrane proteins to lysosomes in vivo. This analysis reveals that these proteins, together with AP-3, play an essential role in protein sorting at early endosomes, thereby regulating the integrity of these organelles.

Identification of novel genes that modify phenotypes induced by Alzheimer's beta-amyloid overexpression in Drosophila

Sustained increases in life expectancy have underscored the importance of managing diseases with a high incidence in late life, such as various neurodegenerative conditions. Alzheimer's disease (AD) is the most common among these, and consequently significant research effort is spent on studying it. Although a lot is known about the pathology of AD and the role of beta-amyloid (Abeta) peptides, the complete network of interactions regulating Abeta metabolism and toxicity still eludes us. To address this, we have conducted genetic interaction screens using transgenic Drosophila expressing Abeta and we have identified mutations that affect Abeta metabolism and toxicity. These analyses highlight the involvement of various biochemical processes such as secretion, cholesterol homeostasis, and regulation of chromatin structure and function, among others, in mediating toxic Abeta effects. Several of the mutations that we identified have not been linked to Abeta toxicity before and thus constitute novel potential targets for AD intervention. We additionally tested these mutations for interactions with tau and expanded-polyglutamine overexpression and found a few candidate mutations that may mediate common mechanisms of neurodegeneration. Our data offer insight into the toxicity of Abeta and open new areas for further study into AD pathogenesis.

What is the function of neuronal AP-3?

Neurotransmission requires the proper organization and rapid recycling of synaptic vesicles. Rapid retrieval has been suggested to occur either by kiss-and-stay or kiss-and-run mechanisms, whereas classical recycling is mediated by clathrin-dependent endocytosis. Molecular coats are key components in the selection of cargos, AP-2 (adaptor protein 2) playing a prominent role in synaptic vesicle endocytosis. Another coat protein, AP-3, has been implicated in synaptic vesicle biogenesis and in the generation of secretory and lysosomal-related organelles. In the present review, we will particularly focus on the recent data concerning the recycling of synaptic vesicles and the function of AP-3 and the v-SNARE (vesicular soluble N-ethylmaleimide-sensitive fusion protein-attachment protein receptor) TI-VAMP (tetanus neurotoxin-insensitive vesicle-associated membrane protein) in these processes. We propose that AP-3 plays an important regulatory role in neurons which contributes to the basal and stimulated exocytosis of synaptic vesicles.

Involvement of beta3A subunit of adaptor protein-3 in intracellular trafficking of receptor-like protein tyrosine phosphatase PCP-2

PCP-2 is a human receptor-like protein tyrosine phosphatase and a member of the MAM domain family cloned in human pancreatic adenocarcinoma cells. Previous studies showed that PCP-2 directly interacted with beta-catenin through the juxtamembrane domain, dephosphorylated beta-catenin and played an important role in the regulation of cell adhesion. Recent study showed that PCP-2 was also involved in the repression of beta-catenin-induced transcriptional activity. Here we describe the interactions of PCP-2 with the beta3A subunit of adaptor protein (AP)-3 and sorting nexin (SNX) 3. These protein complexes were detected using the yeast two-hybrid assay with the juxtamembrane and membrane-proximal catalytic domain of PCP-2 as "bait" Both AP-3 and SNX3 are molecules involved in intracellular trafficking of membrane receptors. The association between the beta3A subunit of AP-3 and PCP-2 was further confirmed in mammalian cells. Our results suggested a possible mechanism of intracellular trafficking of PCP-2 mediated by AP-3 and SNX3 which might participate in the regulation of PCP-2 functions.

Characterization of AP3B2_v2, a novel splice variant of human AP3B2

A novel splice variant of human AP3B2, named AP3B2_v2, was isolated through the large-scale sequencing analysis of a human fetal brain cDNA library. The AP3B2_v2 cDNA is 1171 bp in length. Sequence analysis revealed AP3B2_v2 missed 22 exons that existed in AP3B2_v1, leading to a different putative protein. The deduced proteins were 145 amino acids (designated as AP3B2_v2) and 1082 amino acids (AP3B2_v1) in length, sharing the C-terminal 145 amino acids. RT-PCR analysis showed that human AP3B2_v2 were expressed in several human adult tissues analyzed. The expression levels of AP3B2_v2 were relatively high in brain and testis. In contrast, low levels of expression were detected in kidney, pancreas, spleen, thymus, prostate, ovary and small intestine.

Thepinkgene encodes theDrosophilaorthologue of the human Hermansky–Pudlak syndrome 5 (HPS5) gene

Hermansky–Pudlak syndrome (HPS) consists of a set of human autosomal recessive disorders, with symptoms resulting from defects in genes required for protein trafficking in lysosome-related organelles such as melanosomes and platelet dense granules. A number of human HPS genes and rodent orthologues have been identified whose protein products are key components of 1 of 4 different protein complexes (AP-3 or BLOC-1, -2, and -3) that are key participants in the process. Drosophila melanogaster has been a key model organism in demonstrating the in vivo significance of many genes involved in protein trafficking pathways; for example, mutations in the “granule group” genes lead to changes in eye colour arising from improper protein trafficking to pigment granules in the developing eye. An examination of the chromosomal positioning of Drosophila HPS gene orthologues suggested that CG9770, the Drosophila HPS5 orthologue, might correspond to the pink locus. Here we confirm this gene assignment, making pink the first eye colour gene in flies to be identified as a BLOC complex gene.

Mass Spectrometry Analysis of the Native Protein Complex Containing Actinin-4 in Prostate Cancer Cells

Actinin-4 was originally identified as an actin-binding protein associated with cell motility and cancer invasion and metastasis. However, actinin-4 forms complexes with a large number of different partner proteins and is speculated to have several distinct functions depending on its partner. The level of actinin-4 expression was found to be significantly lower in prostate cancer cells than in non-cancerous basal cells, and restoration of actinin-4 expression inhibited cell proliferation by prostate cancer cell line 22RV1. Immunoprecipitation and mass spectrometry analysis revealed that actinin-4 forms native complexes with several partner proteins in 22RV1 cells, including with beta/gamma-actin, calmodulin, the clathrin heavy chain, non-muscular myosin heavy chain, heterogeneous nuclear ribonucleoprotein A1, and Ras-GTPase-activating protein SH3 domain-binding protein. Clathrin is a coat protein that covers the internalized membrane pit that forms during early endocytosis. We found that other clathrin-related and unrelated cargo proteins, including dynamin, adaptin-delta, beta subunit of neuronal adaptin-like protein, and p47A, also interact with actinin-4. Immunofluorescence microscopy revealed that dynamin and clathrin co-localized with actinin-4 at the sites of membrane ruffling, and transfection of actinin-4 cDNA facilitated the transport of transferrin into perinuclear endosomes. Endocytosis terminates signaling evoked by cell surface receptors and regulates the recycling of receptors and ligands. We identified a panel of proteins whose expression and/or subcellular localization was regulated by actinin-4 by performing organelle fractionation and ICAT-LC-MS/MS. The decreased expression of actinin-4 protein in prostate cancer cells may cause aberrations in the intracellular trafficking of various cell surface molecules and contribute to carcinogenesis.

The Drosophila Pigmentation Gene pink (p) Encodes a Homologue of Human Hermansky-Pudlak Syndrome 5 (HPS5)

Lysosome-related organelles comprise a group of specialized intracellular compartments that include melanosomes and platelet dense granules (in mammals) and eye pigment granules (in insects). In humans, the biogenesis of these organelles is defective in genetic disorders collectively known as Hermansky-Pudlak syndrome (HPS). Patients with HPS-2, and two murine HPS models, carry mutations in genes encoding subunits of adaptor protein (AP)-3. Other genes mutated in rodent models include those encoding VPS33A and Rab38. Orthologs of all of these genes in Drosophila melanogaster belong to the 'granule group' of eye pigmentation genes. Other genes associated with HPS encode subunits of three complexes of unknown function, named biogenesis of lysosome-related organelles complex (BLOC)-1, -2 and -3, for which the Drosophila counterparts had not been characterized. Here, we report that the gene encoding the Drosophila ortholog of the HPS5 subunit of BLOC-2 is identical to the granule group gene pink (p), which was first studied in 1910 but had not been identified at the molecular level. The phenotype of pink mutants was exacerbated by mutations in AP-3 subunits or in the orthologs of VPS33A and Rab38. These results validate D. melanogaster as a genetic model to study the function of the BLOCs.

Neutrophil elastase sorting involves plasma membrane trafficking requiring the C-terminal propeptide

The primary granules/secretory lysosomes of neutrophils store mature neutrophil elastase (NE) as a luminal protein after proteolytic removal of N-terminal and C-terminal pro-peptides from a proform of NE. The N-terminal pro-peptide prevents premature activation that might be toxic to the cell, but the C-terminal pro-peptide has no defined function. In this study, we investigated the role of the C-terminal pro-peptide in trafficking of NE by expressing, in rat basophilic leukemia (RBL) cells, both wild-type NE and the mutant NE/Delta248-267, which lacks the C-terminal pro-peptide. Both transfected proteins were found to be targeted to secretory lysosomes. In addition, results from antibody ligation and cell-surface biotinylation indicated that proform of NE was targeted to the plasma membrane, and then subjected to endocytosis. The results were supported by the detection of targeting of the proform to the plasma membrane followed by internalization both in RBL cells and normal granulopoietic precursor cells. Targeting of NE to the plasma membrane required the C-terminal pro-peptide as NE/Delta248-267 expressed in RBL cells bypassed plasma membrane trafficking. Our results indicate targeting of a population of NE to the plasma membrane and internalization dependent on the C-terminal NE pro-peptide.

Loss of AP-3 function affects spontaneous and evoked release at hippocampal mossy fiber synapses

Synaptic vesicle (SV) exocytosis mediating neurotransmitter release occurs spontaneously at low intraterminal calcium concentrations and is stimulated by a rise in intracellular calcium. Exocytosis is compensated for by the reformation of vesicles at plasma membrane and endosomes. Although the adaptor complex AP-3 was proposed to be involved in the formation of SVs from endosomes, whether its function has an indirect effect on exocytosis remains unknown. Using mocha mice, which are deficient in functional AP-3, we identify an AP-3-dependent tetanus neurotoxin-resistant asynchronous release that can be evoked at hippocampal mossy fiber (MF) synapses. Presynaptic targeting of the tetanus neurotoxin-resistant vesicle soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) tetanus neurotoxin-insensitive vesicle-associated membrane protein (TI-VAMP) is lost in mocha hippocampal MF terminals, whereas the localization of synaptobrevin 2 is unaffected. In addition, quantal release in mocha cultures is more frequent and more sensitive to sucrose. We conclude that lack of AP-3 results in more constitutive secretion and loss of an asynchronous evoked release component, suggesting an important function of AP-3 in regulating SV exocytosis at MF terminals.

Lipidomic strategies to study structural and functional defects of ABC-transporters in cellular lipid trafficking

The majority of the human ATP-binding cassette (ABC)-transporters function in cellular lipid trafficking and in the regulation of membrane lipid composition associating their dysfunction with human disease phenotypes related to sterol, phospholipid and fatty acid homeostasis. Based on findings from monogenetic disorders, animal models, and in vitro systems, major clues on the expression, function and cellular localization of human ABC-transporters have been gained. Here we review novel lipidomic technologies including quantitative mRNA expression monitoring by realtime RT-PCR and DNA-microarrays, lipid mass spectrometry, cellular fluorescence imaging and flow cytometry as promising tools to further define regulatory networks, lipid species patterns and subcellular domains important for ABC-transporter-mediated lipid trafficking.

Regulation of large dense-core vesicle volume and neurotransmitter content mediated by adaptor protein 3

Adaptor protein 3 (AP-3) is a vesicle-coat protein that forms a heterotetrameric complex. Two types of AP-3 subunits are found in mammalian cells. Ubiquitous AP-3 subunits are expressed in all tissues of the body, including the brain. In addition, there are neuronal AP-3 subunits that are thought to serve neuron-specific functions such as neurotransmitter release. In this study, we show that overexpression of neuronal AP-3 in mouse chromaffin cells results in a striking decrease in the neurotransmitter content of individual vesicles (quantal size), whereas deletion of all AP-3 produces a dramatic increase in quantal size; these changes were correlated with alterations in dense-core vesicle size. AP-3 appears to localize in the trans-Golgi network and possibly immature secretory vesicles, where it may be involved in the formation of neurosecretory vesicles.

Physiological Roles of Clathrin Adaptor AP Complexes: Lessons from Mutant Animals

Clathrin-associated adaptor protein (AP) complexes play a key role in the transport of proteins, by regulating the formation of transport vesicles as well as cargo selection, between organelles of the post-Golgi network, namely, the trans-Golgi network (TGN), endosomes, lysosomes and the plasma membrane. Evidence has been accumulating for the physiological importance of AP complexes. Deficiency in AP-1A or AP-2 results in embryonic lethality in mice, indicating that these AP complexes are essential for normal development of embryos in mammals. In contrast, mutations in the genes encoding subunits of AP-3A cause an autosomal recessive disorder, Hermansky-Pudlak syndrome in human and its disease models in mice. Knockout mice for the neuron-specific AP-3B suffer from epileptic seizure. Further studies on the physiological and pathological aspects of AP complexes will not only be beneficial for better understanding of developmental biology and medical sciences, but also deepen our insight into the molecular mechanisms of vesicular traffic.

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.

Identification of a homozygous deletion in the AP3B1 gene causing Hermansky-Pudlak syndrome, type 2

We report on the molecular etiology of an unusual clinical phenotype associating congenital neutropenia, thrombocytopenia, developmental delay, and hypopigmentation. Using genetic linkage analysis and targeted gene sequencing, we defined a homozygous genomic deletion in AP3B1, the gene encoding the beta chain of the adaptor protein-3 (AP-3) complex. The mutation leads to in-frame skipping of exon 15 and thus perturbs proper assembly of the heterotetrameric AP-3 complex. Consequently, trafficking of transmembrane lysosomal proteins is aberrant, as shown for CD63. In basal keratinocytes, the incorporated immature melanosomes were rapidly degraded in large phagolysosomes. Despite distinct ultramorphologic changes suggestive of aberrant vesicular maturation, no functional aberrations were detected in neutrophil granulocytes. However, a comprehensive immunologic assessment revealed that natural killer (NK) and NKT-cell numbers were reduced in AP-3-deficient patients. Our findings extend the clinical and molecular phenotype of human AP-3 deficiency (also known as Hermansky-Pudlak syndrome, type 2) and provide further insights into the role of the AP-3 complex for the innate immune system.

Characterization of three novel imprinted snoRNAs from mouse Irm gene

Most, if not all, of snoRNAs in mammals are intron-encoded, implying the expressional and functional relativeness between the snoRNA and their hosts. By computational analysis of an intron database extracted from 65 known mouse imprinted genes, three novel orphan box C/D snoRNAs were identified from Irm gene which is maternally expressed and related to human disorders. The snoRNAs were positively detected and found to express in all the mouse tissues except kidney. The imprinted snoRNAs exhibit stringent structures, but quite variable in locations at their host introns, suggesting their maturation probably through a splicing independent manner. We characterized Irm as a new kind of snoRNA host gene which has no protein-coding capacity and no 5'TOP structure in its mRNA. The newly identified snoRNAs appear mouse-specific, however, their function remains to be elucidated.

Insights into the biogenesis of lysosome-related organelles from the study of the Hermansky-Pudlak syndrome

Lysosome-related organelles (LROs) are a family of cell-type-specific organelles that include melanosomes, platelet dense bodies, and cytotoxic T cell granules. The name, LRO, recognizes the fact that all of these organelles contain subsets of lysosomal proteins in addition to cell-type-specific proteins. The recent identification of genetic disorders that cause combined defects in several of these organelles indicates that they share common biogenetic pathways. Studies of one of these disorders, the Hermansky-Pudlak syndrome (HPS), have provided helpful insights into the molecular machinery involved in LRO biogenesis. HPS is a genetically heterogeneous disorder caused by mutations in any of 7 genes in humans and 15 genes in mice. These genes encode subunits of 4 multi-protein complexes named AP-3, BLOC-1, BLOC-2 and BLOC-3, in addition to miscellaneous components of the general protein trafficking machinery. The AP-3 complex is a coat protein involved in vesicle formation and cargo selection in the endosomal-lysosomal system. One of these cargo molecules is the melanosomal enzyme, tyrosinase, the missorting of which may explain the defective melanosomes in AP-3-deficient humans and mice. The function of the BLOC complexes is unknown, although they are thought to mediate either vesicle tethering/fusion or cytoplasmic dispersal of LROs. Further studies of these complexes should contribute to the elucidation of the mechanisms of LRO biogenesis and the pathogenesis of HPS.

HIV-1 Nef stabilizes AP-1 on membranes without inducing ARF1-independent de novo attachment

HIV-1 Nef affects the trafficking of numerous cellular proteins to optimize viral replication and evade host defenses. The adaptor protein (AP) complexes, which form part of the cytoplasmic coat of endosomal vesicles, are key cellular co-factors for Nef. Nef binds these complexes and alters their physiologic cycle of attachment and release from membranes. Specifically, while AP-1 normally becomes cytosolic when attachment events are blocked by inhibition of the GTPase cycle of ADP-ribosylation factor-1 (ARF1), the complex remains membrane-associated in Nef-expressing cells. To investigate the mechanism of this effect, we used a permeabilized cell system to detect the de novo attachment of exogenous AP-1 to endosomal membranes. Nef did not mediate de novo attachment independently of ARF1, despite its ability to maintain the association of AP-1 with endosomal membranes when the activity of ARF1 was blocked. We conclude that Nef stabilizes AP complexes on endosomal membranes after ARF1-dependent attachment. This stabilization may facilitate coat formation and stimulate the trafficking of multiple cellular proteins.

Functions of adaptor protein (AP)-3 and AP-1 in tyrosinase sorting from endosomes to melanosomes

Specialized cells exploit adaptor protein complexes for unique post-Golgi sorting events, providing a unique model system to specify adaptor function. Here, we show that AP-3 and AP-1 function independently in sorting of the melanocyte-specific protein tyrosinase from endosomes to the melanosome, a specialized lysosome-related organelle distinguishable from lysosomes. AP-3 and AP-1 localize in melanocytes primarily to clathrin-coated buds on tubular early endosomes near melanosomes. Both adaptors recognize the tyrosinase dileucine-based melanosome sorting signal, and tyrosinase largely colocalizes with each adaptor on endosomes. In AP-3-deficient melanocytes, tyrosinase accumulates inappropriately in vacuolar and multivesicular endosomes. Nevertheless, a substantial fraction still accumulates on melanosomes, concomitant with increased association with endosomal AP-1. Our data indicate that AP-3 and AP-1 function in partially redundant pathways to transfer tyrosinase from distinct endosomal subdomains to melanosomes and that the AP-3 pathway ensures that tyrosinase averts entrapment on internal membranes of forming multivesicular bodies.

Association of the Hermansky-Pudlak syndrome type-3 protein with clathrin

Background

Hermansky-Pudlak syndrome (HPS) is a disorder of lysosome-related organelle biogenesis characterized by oculocutaneous albinism and prolonged bleeding. These clinical findings reflect defects in the formation of melanosomes in melanocytes and dense bodies in platelets. HPS type-3 (HPS-3) results from mutations in the HPS3 gene, which encodes a 1004 amino acid protein of unknown function that contains a predicted clathrin-binding motif (LLDFE) at residues 172–176.

Results

Clathrin was co-immunoprecipitated by HPS3 antibodies from normal but not HPS3 null melanocytes. Normal melanocytes expressing a GFP-HPS3 fusion protein demonstrated partial co-localization of GFP-HPS3 with clathrin following a 20°C temperature block. GFP-HPS3 in which the predicted clathrin-binding domain of HPS3 was mutated (GFP-HPS3-delCBD) did not co-localize with clathrin under the same conditions. Immunoelectron microscopy of normal melanocytes expressing GFP-HPS3 showed co-localization of GFP-HPS3 with clathrin, predominantly on small vesicles in the perinuclear region. In contrast, GFP-HPS3-delCBD did not co-localize with clathrin and exhibited a largely cytoplasmic distribution.

Conclusion

HPS3 associates with clathrin, predominantly on small clathrin-containing vesicles in the perinuclear region. This association most likely occurs directly via a functional clathrin-binding domain in HPS3. These results suggest a role for HPS3 and its protein complex, BLOC-2, in vesicle formation and trafficking.

The C-terminal tail of the gp41 transmembrane envelope glycoprotein of HIV-1 clades A, B, C, and D may exist in two conformations: an analysis of sequence, structure, and function

In addition to the major ectodomain, the gp41 transmembrane glycoprotein of HIV-1 is now known to have a minor ectodomain that is part of the long C-terminal tail. Both ectodomains are highly antigenic, carry neutralizing and non-neutralizing epitopes, and are involved in virus-mediated fusion activity. However, data have so far been biologically based, and derived solely from T cell line-adapted (TCLA), B clade viruses. Here we have carried out sequence and theoretically based structural analyses of 357 gp41 C-terminal sequences of mainly primary isolates of HIV-1 clades A, B, C, and D. Data show that all these viruses have the potential to form a tail loop structure (the minor ectodomain) supported by three, β-sheet, membrane-spanning domains (MSDs). This means that the first (N-terminal) tyrosine-based sorting signal of the gp41 tail is situated outside the cell membrane and is non-functional, and that gp41 that reaches the cell surface may be recycled back into the cytoplasm through the activity of the second tyrosine-sorting signal. However, we suggest that only a minority of cell-associated gp41 molecules – those destined for incorporation into virions – has 3 MSDs and the minor ectodomain. Most intracellular gp41 has the conventional single MSD, no minor ectodomain, a functional first tyrosine-based sorting signal, and in line with current thinking is degraded intracellularly. The gp41 structural diversity suggested here can be viewed as an evolutionary strategy to minimize HIV-1 envelope glycoprotein expression on the cell surface, and hence possible cytotoxicity and immune attack on the infected cell.

An AP-1/clathrin coat plays a novel and essential role in forming the Weibel-Palade bodies of endothelial cells

Clathrin provides an external scaffold to form small 50-100-nm transport vesicles. In contrast, formation of much larger dense-cored secretory granules is driven by selective aggregation of internal cargo at the trans-Golgi network; the only known role of clathrin in dense-cored secretory granules formation is to remove missorted proteins by small, coated vesicles during maturation of these spherical organelles. The formation of Weibel-Palade bodies (WPBs) is also cargo driven, but these are cigar-shaped organelles up to 5 mum long. We hypothesized that a cytoplasmic coat might be required to make these very different structures, and we found that new and forming WPBs are extensively, sometimes completely, coated. Overexpression of an AP-180 truncation mutant that prevents clathrin coat formation or reduced AP-1 expression by small interfering RNA both block WPB formation. We propose that, in contrast to other secretory granules, cargo aggregation alone is not sufficient to form immature WPBs and that an external scaffold that contains AP-1 and clathrin is essential.

A Novel Hook-Related Protein Family and the Characterization of Hook-Related Protein 1

The spatial organization of organelles within a cell is dependent on microtubules. Recently, members of the Hook family of proteins have been proposed to function in linking organelles to microtubules. We report the identification of a completely novel protein family, the Hook-related protein (HkRP) family, from which the Hook proteins have diverged. Bioinformatic analysis of the HkRP family revealed several conserved domains, including a unique C-terminal HkRP domain. The central region of each protein is comprised of an extensive coiled-coil domain, and the N-terminus contains a putative microtubule-binding domain. This domain has been shown to bind microtubules in the Hook protein and show that the HkRP1 protein is microtubule-associated. While endogenous HkRP1 has no distinct organelle association, expression of the C-terminal membrane-binding domain suggests a function of the HkRP1 in early endosome. Ultrastructural studies reveal that expression of the C-terminal HkRP1 domain causes an accumulation of internal membranes with an electron-dense coat. Co-localization studies show a concomitant redistribution of the early endosome marker sorting-nexin 1 but not the early endosome antigen-1 (EEA1). The steady-state distribution of the epidermal growth factor receptor is also specifically disrupted by expression of the C-terminal domain. We propose that HkRP1 is involved in the process of tubulation of sorting nexin-1 positive membranes from early endosome subdomains.

AP-3 directs the intracellular trafficking of HIV-1 Gag and plays a key role in particle assembly

Gag proteins direct the process of retroviral particle assembly and form the major protein constituents of the viral core. The matrix region of the HIV-1 Gag polyprotein plays a critical role in the transport of Gag to the plasma membrane assembly site. Recent evidence indicates that Gag trafficking to late endosomal compartments, including multivesicular bodies, occurs prior to viral particle budding from the plasma membrane. Here we demonstrate that the matrix region of HIV-1 Gag interacts directly with the delta subunit of the AP-3 complex, and that this interaction plays an important functional role in particle assembly. Disruption of this interaction eliminated Gag trafficking to multivesicular bodies and diminished HIV particle formation. These studies illuminate an early step in retroviral particle assembly and provide evidence that the trafficking of Gag to late endosomes is part of a productive particle assembly pathway.

Cellular, molecular and clinical characterization of patients with Hermansky-Pudlak syndrome type 5

Hermansky-Pudlak syndrome (HPS) is a disorder of lysosome-related organelles such as melanosomes and platelet dense granules. Seven genes are now associated with HPS in humans. An accurate diagnosis of each HPS subtype has important prognostic and treatment implications. Here we describe the cellular, molecular, and clinical aspects of the recently identified HPS-5 subtype. We first analyzed the genomic organization and the RNA expression pattern of HPS5, located on chromosome 11p14, and demonstrated tissue-specific expression of at least three alternatively spliced HPS5 mRNA transcripts, coding for HPS5A and HPS5B proteins, that differ at their 5'-ends. Genetic screening of 15 unassigned HPS patients yielded six new HPS5 mutations in four patients. Clinically, our HPS-5 patients exhibited iris transillumination, variable hair and skin pigmentation, and absent platelet dense bodies, but not pulmonary fibrosis or granulomatous colitis. In two patients with homozygous missense mutations, hemizygosity was ruled out by gene-dosage multiplex polymerase chain reaction, and immunocytochemical analyses of their fibroblasts supported the HPS-5 diagnosis. Specifically, LAMP-3 distribution was restricted to the perinuclear region in HPS-5 fibroblasts, in contrast to the normal LAMP-3 distribution, which extended to the periphery. This specific intracellular vesicle distribution in fibroblasts, in combination with the clinical features, will improve the characterization of the HPS-5 subtype.

Adaptors for clathrin coats: structure and function

Clathrin-coated vesicles (CCVs) are responsible for the transport of proteins between various compartments of the secretory and endocytic systems. Clathrin forms a scaffold around these vesicles that is linked to membranes by clathrin adaptors. The adaptors simultaneously bind to clathrin and to transmembrane proteins and/or phospholipids and can also interact with each other and with other components of the CCV formation machinery. The result is a collection of proteins that can make multiple, moderate strength (microM Kd) interactions and thereby establish the dynamic regulatable networks to drive vesicle genesis at the correct time and place in the cell. This review focuses on the structure of clathrin adaptors and how these structures provide functional information on the mechanism of CCV formation.

The adaptor protein AP-4 as a component of the clathrin coat machinery: a morphological study

The four members of the AP (adaptor protein) family are heterotetrameric cytosolic complexes that are involved in the intracellular trafficking of cargo proteins between different organelles. They interact with motifs present in the cytoplasmic tails of their specific cargo proteins at different intracellular locations. While AP-1, AP-2 and AP-3 have been investigated extensively, very few studies have focused on the fourth member, AP-4. In the present study, we report on the intracellular localization of AP-4 in the MDCK (Madin-Darby canine kidney) and MelJuSo cell lines after immunogold labelling of ultrathin cryosections. We find that AP-4 is localized mainly in the Golgi complex, as well as on endosomes and transport vesicles. Interestingly, we show for the first time that AP-4 is localized with the clathrin coat machinery in the Golgi complex and in the endocytic pathway. Furthermore, we find that AP-4 is localized with the CI-MPR (cation-independent mannose 6-phosphate receptor), but not with the transferrin receptor, LAMP-2 (lysosomal-associated membrane protein-2) or invariant chain. The difference in morphology between CI-MPR/AP-4-positive vesicles and CI-MPR/AP-1-positive vesicles raises the possibility that AP-4 acts at a location different from that of AP-1 in the intracellular trafficking pathway of CI-MPR.

Leishmania major: clathrin and adaptin complexes of an intra-cellular parasite

To investigate the role of clathrin-mediated trafficking during the Leishmania lifecycle, open reading frames encoding clathrin heavy chain and the beta-adaptins, major components of the adaptor complexes, have been analysed both in silico and experimentally. The Leishmania genome encodes three beta-adaptins, which arose at a time predating speciation of these divergent trypanosomatids. Unlike Trypanosoma brucei, both clathrin heavy chain and beta-adaptin1 are constitutively expressed throughout the Leishmania life cycle. Clathrin relocalises in amastigotes relative to promastigotes, consistent with developmental alterations to the morphology of the endo-membrane system.

AP-1 and AP-3 facilitate lysosomal targeting of Batten disease protein CLN3 via its dileucine motif

CLN3 is a transmembrane protein with a predominant localization in lysosomes in non-neuronal cells but is also found in endosomes and the synaptic region in neuronal cells. Mutations in the CLN3 gene result in juvenile neuronal ceroid lipofuscinosis or Batten disease, which currently is the most common cause of childhood dementia. We have recently reported that the lysosomal targeting of CLN3 is facilitated by two targeting motifs: a dileucine-type motif in a cytoplasmic loop domain and an unusual motif in the carboxyl-terminal cytoplasmic tail comprising a methionine and a glycine separated by nine amino acids (Kyttala, A., Ihrke, G., Vesa, J., Schell, M. J., and Luzio, J. P. (2004) Mol. Biol. Cell 15, 1313-1323). In the present study, we investigated the pathways and mechanisms of CLN3 sorting using biochemical binding assays and immunofluorescence methods. The dileucine motif of CLN3 bound both AP-1 and AP-3 in vitro, and expression of mutated CLN3 in AP-1- or AP-3-deficient mouse fibroblasts showed that both adaptor complexes are required for sequential sorting of CLN3 via this motif. Our data indicate the involvement of complex sorting machinery in the trafficking of CLN3 and emphasize the diversity of parallel and sequential sorting pathways in the trafficking of membrane proteins.

Genetic analysis of the neuronal and ubiquitous AP-3 adaptor complexes reveals divergent functions in brain

Neurons express adaptor (AP)-3 complexes assembled with either ubiquitous (beta3A) or neuronal-specific (beta3B) beta3 isoforms. However, it is unknown whether these complexes indeed perform distinct functions in neuronal tissue. Here, we explore this hypothesis by using genetically engineered mouse models lacking either beta3A- or beta3B-containing AP-3 complexes. Somatic and neurological phenotypes were specifically associated with the ubiquitous and neuronal adaptor deficiencies, respectively. At the cellular level, AP-3 isoforms were localized to distinct neuronal domains. beta3B-containing AP-3 complexes were preferentially targeted to neuronal processes. Consistently, beta3B deficiency compromised synaptic zinc stores assessed by Timm's staining and the synaptic vesicle targeting of membrane proteins involved in zinc uptake (ZnT3 and ClC-3). Surprisingly, despite the lack of neurological symptoms, beta3A-deficient mouse brain possessed significantly increased synaptic zinc stores and synaptic vesicle content of ZnT3 and ClC-3. These observations indicate that the functions of beta3A- and beta3B-containing complexes are distinct and divergent. Our results suggest that concerted nonredundant functions of neuronal and ubiquitous AP-3 provide a mechanism to control the levels of selected membrane proteins in synaptic vesicles.

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

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