Plays a role in vesicle-mediated protein trafficking to lysosomal compartments including the endocytic membrane transport pathways. Believed to act as a core component of the putative HOPS endosomal tethering complex which is proposed to be involved in the Rab5-to-Rab7 endosome conversion probably implicating MON1A/B, and via binding SNAREs and SNARE complexes to mediate tethering and docking events during SNARE-mediated membrane fusion (By similarity). May be involved in vesicle trafficking to the hepatocyte apical membrane and play a role in development of the intra-hepatic biliary tree. May target endosomes to the pigment granule in melanocytes. Essential for early embryonic development. Component of the homotypic fusion and vacuole protein sorting (HOPS) complex (By similarity). Cytoplasmic, peripheral membrane protein associated with early endosomes and late endosomes/lysosomes. Mutants display hepatomegaly associated with large, vesicle-filled hepatocytes. They also have defects in pigmentation with reduced numbers of melanophores and no iridophores. Belongs to the VPS18 family.