title: BLOC-3 mutated in Hermansky-Pudlak syndrome is a Rab32/38 guanine nucleotide exchange factor.
abstract: Hermansky-Pudlak syndrome (HPS) is a human disease characterized by partial loss of pigmentation and impaired blood clotting. These symptoms are caused by defects in the biogenesis of melanosomes and platelet dense granules, often referred to as lysosome-related organelles. Genes mutated in HPS encode subunits of the biogenesis of lysosome-related organelles complexes (BLOCs). BLOC-1 and BLOC-2, together with the AP-3 clathrin adaptor complex, act at early endosomes to sort components required for melanin formation and melanosome biogenesis away from the degradative lysosomal pathway toward early stage melanosomes. However the molecular functions of the Hps1-Hps4 complex BLOC-3 remain mysterious. Like other trafficking pathways, melanosome biogenesis and transport of enzymes involved in pigmentation involves specific Rab GTPases, in this instance Rab32 and Rab38. We now demonstrate that BLOC-3 is a Rab32 and Rab38 guanine nucleotide exchange factor (GEF). Silencing of the BLOC-3 subunits Hps1 and Hps4 results in the mislocalization of Rab32 and Rab38 and reduction in pigmentation. In addition, we show that BLOC-3 can promote specific membrane recruitment of Rab32/38. BLOC-3 therefore defines a novel Rab GEF family with a specific function in the biogenesis of lysosome-related organelles.
authors.affiliation: Department of Biochemistry, University of Oxford, South Parks Road, Oxford OX1 3QU, UK.
authors.forename: Andreas
authors.initials: A
authors.lastname: Gerondopoulos
authors.forename: Lars
authors.initials: L
authors.lastname: Langemeyer
authors.forename: Jin-Rui
authors.initials: JR
authors.lastname: Liang
authors.forename: Andrea
authors.initials: A
authors.lastname: Linford
authors.forename: Francis A
authors.initials: FA
authors.lastname: Barr
mesh_terms: D000595:Amino Acid Sequence; D020662:Guanine Nucleotide Exchange Factors / Q000235:genetics / Q000378:metabolism*; D022861:Hermanski-Pudlak Syndrome / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D020460:Melanosomes; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D008969:Molecular Sequence Data; D009154:Mutation; D011506:Proteins / Q000235:genetics / Q000378:metabolism*; D020691:rab GTP-Binding Proteins / Q000235:genetics / Q000378:metabolism*
chemical_list: D020662:Guanine Nucleotide Exchange Factors; C102160:HPS1 protein, human; C451752:HPS4 protein, human; D008565:Membrane Proteins; D011506:Proteins; C412058:RAB38 protein, human; C466094:Rab32 protein, human; D020691:rab GTP-Binding Proteins