Two type II phosphatidylinositol 4-kinases function sequentially in tubule-mediated cargo delivery from early endosomes to melanosomes

Melanosomes are pigment cell-specific lysosome-related organelles in which melanin pigments are synthesized and stored. Melanosome maturation requires delivery of melanogenic cargoes via tubular transport carriers that emanate from early endosomes and that require BLOC-1 for their formation. Here we show that phosphatidylinositol-4-phosphate (PtdIns4P) and the type II PtdIns-4-kinases (PI4KII and PI4KII{beta}) support BLOC-1-dependent tubule formation to regulate melanosome biogenesis. Depletion of PI4KII or PI4KII{beta} with shRNAs in melanocytes reduced melanin content and misrouted BLOC-1-dependent cargoes to late endosomes/lysosomes. Genetic epistasis, cell fractionation, and quantitative live-cell imaging analyses show that PI4KII and PI4KII{beta} function sequentially and non-redundantly downstream of BLOC-1 during tubule elongation towards melanosomes by generating local pools of PtdIns4P. The data demonstrate that both type II PtdIns-4-kinases are necessary for efficient BLOC-1-dependent tubule elongation and subsequent melanosome contact and content delivery during melanosome biogenesis. The independent functions of PtdIns-4-kinases in tubule extension are likely downstream of redundant functions in BLOC-1-dependent tubule initiation. SUMMARYContents are delivered from early endosomal intermediates to maturing melanosomes through tubular transport carriers. Zhu et al show that two type II phosphatidylinositol kinases, PI4KII and PI4KII{beta}, sequentially generate phosphatidylinositol-4-phosphate during tubule initiation and elongation for ultimate melanosome content delivery.