The building BLOC(k)s of lysosomes and related organelles

Hermansky-Pudlak syndrome defines a group of genetic disorders characterized by defects in organelles of the endosomal-lysosomal system, most notably melanosomes and platelet-dense granules. About a dozen genes have been implicated in the pathogenesis of the disease in humans and mice. Most of these genes encode novel polypeptides that are not conserved in unicellular eukaryotes. Recent studies have revealed that these polypeptides are stable components of at least three distinct, ubiquitously expressed protein complexes, named biogenesis of lysosome-related organelles complex (BLOC)-1, -2 and -3. These findings provide a framework for studies on the function of these proteins and the pathogenesis of Hermansky-Pudlak syndrome.