The BLOC-1 complex promotes endosomal maturation by recruiting the Rab5 GTPase-activating protein Msb3

Membrane microcompartments of the early endosomes serve as a sorting and signaling platform, where receptors are either recycled back to the plasma membrane or forwarded to the lysosome for destruction. In metazoan cells, three complexes, termed BLOC-1 to -3, mediate protein sorting from the early endosome to lysosomes and lysosome-related organelles. We now demonstrate that BLOC-1 is an endosomal Rab-GAP (GTPase-activating protein) adapter complex in yeast. The yeast BLOC-1 consisted of six subunits, which localized interdependently to the endosomes in a Rab5/Vps21-dependent manner. In the absence of BLOC-1 subunits, the balance between recycling and degradation of selected cargoes was impaired. Additionally, our data show that BLOC-1 is both a Vps21 effector and an adapter for its GAP Msb3. BLOC-1 and Msb3 interacted in vivo, and both mutants resulted in a redistribution of active Vps21 to the vacuole surface. We thus conclude that BLOC-1 controls the lifetime of active Rab5/Vps21 and thus endosomal maturation along the endocytic pathway.