The Hermansky–Pudlak syndrome (HPS) protein is part of a high molecular weight complex involved in biogenesis of early melanosomes

Abstract

Hermansky–Pudlak syndrome (HPS) is a rare autosomal recessive disorder in which oculocutaneous albinism, bleeding tendency and a ceroid-lipofuscin lysosomal storage disease result from defects of multiple cytoplasmic organelles: melanosomes, platelet dense granules and lysosomes. The HPS polypeptide, a 700 amino acid protein which is unrelated to any known proteins, is likely to be involved in the biogenesis of these different organelles. Here, we show that HPS is a non-glycosylated, non-membrane protein which is a component of two distinct high molecular weight complexes. In non-melanotic cells the HPS protein is contained almost entirely in an ∼200 kDa complex that is widely distributed throughout the cytosol. In melanotic cells the HPS protein is partitioned between this cytosolic complex and a >500 kDa complex that appears to consist of the ∼200 kDa complex in association with membranous components. Subcellular fractionation, immunofluorescence and immunoelectron microscopy studies indicate that the membrane-associated HPS complex of melanotic cells is associated with tubulovesicular structures, small non-coated vesicles, and nascent and early-stage melanosomes. These findings suggest that the HPS complex is involved in the biogenesis of early melanosomes.