Hermansky-Pudlak syndrome (HPS) is a genetic disorder characterized by defects in the formation and function of lysosome-related organelles such as melanosomes. HPS in humans or mice is caused by mutations in any of 15 genes, five of which encode subunits of biogenesis of lysosome-related organelles complex (BLOC)-1, a protein complex with no known function.Here, we show that BLOC-1 functions in selective cargo exit from early endosomes toward melanosomes. BLOC-1-deficient melanocytes accumulate the melanosomal protein tyrosinase-related protein-1 (Tyrp1), but not other melanosomal proteins, in endosomal vacuoles and the cell surface due to failed biosynthetic transit from early endosomes to melanosomes and consequent increased endocytic flux. The defects are corrected by restoration of the missing BLOC-1 subunit. Melanocytes from HPS model mice lacking a different protein complex, BLOC-2, accumulate Tyrp1 in distinct downstream endosomal intermediates, suggesting that BLOC-1 and BLOC-2 act sequentially in the same pathway. By contrast, intracellular Tyrp1 is correctly targeted to melanosomes in melanocytes lacking another HPS-associated protein complex, adaptor protein (AP)-3. The results indicate that melanosome maturation requires at least two cargo transport pathways directly from early endosomes to melanosomes, one pathway mediated by AP-3 and one pathway mediated by BLOC-1 and BLOC-2, that are deficient in several forms of HPS. INTRODUCTIONHermansky-Pudlak syndrome (HPS) is a genetic disorder characterized by hypopigmentation, prolonged bleeding, and sometimes ceroid accumulation, lung fibrosis, and/or immune defects leading to premature death Wei, 2006). HPS or a similar disorder in mice results from mutations in any of at least 15 genes (Wei, 2006). All of these genes are ubiquitously expressed, but their mutation in HPS affects mainly the generation and function of selected tissuespecific lysosome-related organelles (LROs;Bonifacino, 2004;Di Pietro and Dell'Angelica, 2005). Those LROs that are most severely affected in all forms of HPS-pigment cell melanosomes, platelet dense granules, and lung lamellar bodies-are unique in that they coexist with bona fide lysosomes in their respective cell types (Dell'Angelica et al., 2000;Marks and Seabra, 2001). The 15 known HPS-associated genes have been identified, and although the products of most are thought to participate in trafficking events that are uniquely required to form this class of LRO, the function of only a few is understood in detail.The genes disrupted in human HPS-7 (Li et al., 2003) and HPS-8 (Morgan et al., 2006) and in the mouse HPS models pallid, muted, reduced pigmentation (rp), cappuccino, and sandy encode five of the eight known subunits of a stable protein complex known as biogenesis of lysosome-related organelles complex (BLOC)-1 (Falcon-Perez et al., 2002;Moriyama and Bonifacino, 2002;Ciciotte et al., 2003;Li et al., 2003;Gwynn et al., 2004;Starcevic and Dell'Angelica, 2004). To date, no specific subcellular function has been assigned...

Cargo partitioning into intralumenal vesicles (ILVs) of multivesicular endosomes underlies such cellular processes as receptor downregulation, viral budding, and biogenesis of lysosome-related organelles such as melanosomes. We show that the melanosomal protein Pmel17 is sorted into ILVs by a mechanism that is dependent upon lumenal determinants and conserved in non-pigment cells. Pmel17 targeting to ILVs does not require its native cytoplasmic domain or cytoplasmic residues targeted by ubiquitylation and, unlike sorting of ubiquitylated cargo, is insensitive to functional inhibition of Hrs and ESCRT complexes. Chimeric protein and deletion analyses indicate that two N-terminal lumenal subdomains are necessary and sufficient for ILV targeting. Pmel17 fibril formation, which occurs during melanosome maturation in melanocytes, requires a third lumenal subdomain and proteolytic processing that itself requires ILV localization. These results establish an Hrs- and perhaps ESCRT-independent pathway of ILV sorting by lumenal determinants and a requirement for ILV sorting in fibril formation.

The epithelium of the urinary bladder must maintain a highly impermeable barrier despite large variations in urine volume during bladder filling and voiding. To study how the epithelium accommodates these volume changes, we mounted bladder tissue in modified Ussing chambers and subjected the tissue to mechanical stretch. Stretching the tissue for 5 h resulted in a 50% increase in lumenal surface area (from approximately 2900 to 4300 microm(2)), exocytosis of a population of discoidal vesicles located in the apical cytoplasm of the superficial umbrella cells, and release of secretory proteins. Surprisingly, stretch also induced endocytosis of apical membrane and 100% of biotin-labeled membrane was internalized within 5 min after stretch. The endocytosed membrane was delivered to lysosomes and degraded by a leupeptin-sensitive pathway. Last, we show that the exocytic events were mediated, in part, by a cyclic adenosine monophosphate, protein kinase A-dependent process. Our results indicate that stretch modulates mucosal surface area by coordinating both exocytosis and endocytosis at the apical membrane of umbrella cells and provide insight into the mechanism of how mechanical forces regulate membrane traffic in non-excitable cells.

Summary Mouse coat color mutants have led to the identification of more than 120 genes that encode proteins involved in all aspects of pigmentation, from the regulation of melanocyte development and differentiation to the transcriptional activation of pigment genes, from the enzymatic formation of pigment to the control of melanosome biogenesis and movement [Bennett and Lamoreux (2003)Pigment Cell Res. 16, 333]. One of the more perplexing of the identified mouse pigment genes is encoded at the Silver locus, first identified by Dunn and Thigpen [(1930) J. Heredity21, 495] as responsible for a recessive coat color dilution that worsened with age on black backgrounds. The product of the Silver gene has since been discovered numerous times in different contexts, including the initial search for the tyrosinase gene, the characterization of major melanosome constituents in various species, and the identification of tumor‐associated antigens from melanoma patients. Each discoverer provided a distinct name: Pmel17, gp100, gp95, gp85, ME20, RPE1, SILV and MMP115 among others. Although all its functions are unlikely to have yet been fully described, the protein clearly plays a central role in the biogenesis of the early stages of the pigment organelle, the melanosome, in birds, and mammals. As such, we will refer to the protein in this review simply as pre‐melanosomal protein (Pmel). This review will summarize the structural and functional aspects of Pmel and its role in melanosome biogenesis.

Biogenesis of the ribbon-like membrane network of the mammalian Golgi requires membrane tethering by the conserved GRASP domain in GRASP65 and GRASP55, yet the tethering mechanism is not fully understood. Here, we report the crystal structure of the GRASP55 GRASP domain, which revealed an unusual arrangement of two tandem PDZ folds that more closely resemble prokaryotic PDZ domains. Biochemical and functional data indicated that the interaction between the ligand-binding pocket of PDZ1 and an internal ligand on PDZ2 mediates the GRASP self-interaction, and structural analyses suggest that this occurs via a unique mode of internal PDZ ligand recognition. Our data uncover the structural basis for ligand specificity and provide insight into the mechanism of GRASP-dependent membrane tethering of analogous Golgi cisternae.Golgi biogenesis involves membrane tethering by two multifunctional proteins, GRASP65 and GRASP55, which are differentially localized to cis and medial Golgi cisternae, respectively. GRASP65 is associated with cis Golgi cisternae via both binding to the cis-localized golgin GM130 and insertion of its myristoylated N terminus (1, 2). GRASP55 is primarily on medial Golgi cisternae and binds medial-localized golgin-45 and other proteins and is myristoylated and palmitoylated (3, 4). Each protein contains a conserved N-terminal GRASP domain that mediates self-association, which results in the formation of homotypic tethering complexes that link analogous cisternae in adjacent ministacks (5-8).The GRASP region is predicted to contain two PDZ-like domains (4, 9). PDZ domains are ubiquitous globular proteinprotein interaction modules featuring a hydrophobic binding groove, which interacts with the C terminus of its target ligand, although recognition of internal sequences has also been observed (10, 11). Although recent work supports the presence of PDZ domains within the GRASP module (8), secondary structure predictions indicate significant mismatches to the typical organization of ␤-strands and ␣-helices found in eukaryotic PDZs.Toward elucidating the structural mechanism of GRASPmediated tethering, we solved the crystal structure of the GRASP domain of GRASP55. Although the GRASP domain was indeed composed of two PDZ domains, the domains were circularly permuted, resulting in overall folds that were structurally more similar to prokaryotic PDZs. This unusual arrangement of a metazoan PDZ revealed that the key ␤2 strands of the binding grooves lay outside of the previously predicted PDZ-like regions. Significantly, an internal ligand sequence mapped in GRASP65 that binds to its first PDZ domain formed a surface projection that appeared to fit inside a deep depression within the PDZ1-binding pocket. Taken together, these data suggest a unique internal PDZ ligand interaction on opposite sides of the molecule and imply a multimeric tethering mechanism that mediates Golgi biogenesis. MATERIALS AND METHODSConstructs-GFP-ActA (8) was cloned into GRASP55 pCS-2 (12) using XbaI. To generate His-tagged GRASP55, GR...

Formation of the ribbon-like membrane network of the Golgi apparatus depends on GM130 and GRASP65, but the mechanism is unknown. We developed an in vivo organelle tethering assaying in which GRASP65 was targeted to the mitochondrial outer membrane either directly or via binding to GM130. Mitochondria bearing GRASP65 became tethered to one another, and this depended on a GRASP65 PDZ domain that was also required for GRASP65 self-interaction. Point mutation within the predicted binding groove of the GRASP65 PDZ domain blocked both tethering and, in a gene replacement assay, Golgi ribbon formation. Tethering also required proximate membrane anchoring of the PDZ domain, suggesting a mechanism that orientates the PDZ binding groove to favor interactions in trans. Thus, a homotypic PDZ interaction mediates organelle tethering in living cells.

No abstract

No abstract