AP-3: What color's your coat?

Signal-mediated delivery of proteins in several intracellular trafficking pathways requires the adaptor complexes AP-1, AP-2 and AP-3. Adaptors are heterotetrameric protein complexes that serve in cargo sorting through binding to di-leucine or tyrosine-based signals present in the cytoplasmic tails of cargo proteins. Adaptors team up with cytoplasmic coat proteins to effect inclusion of cargo into transport vesicles. AP-1 associates with vesicles budding from the trans-side of the Golgi complex, and AP-2 with endocytic vesicles budding from the plasma membrane. Both AP-1 and AP-2 cooperate with clathrin, a complex of three heavy chains and three light chains, which polymerizes to form the scaffold of the coat. The AP-3 adaptor mediates selective transport to lysosomes and lysosome-related organelles and has important biological roles in organisms as diverse as humans, flies, mice and yeast. In yeast, AP-3 function is required for the ALP (alkaline phosphatase) pathway to the vacuole. Mutations in AP-3 subunits in mice result in coat color defects and bleeding disorders and, in Drosophila, result in defects in eye pigmentation. In humans, mutations in the AP-3 β3A subunit cause an inherited disorder, Hermansky–Pudlak syndrome (HPS), in which patients show deficiencies in skin and eye pigmentation and a complete lack of dense granules in platelets, resulting in impaired blood clotting.