Plays a role in vesicle-mediated protein trafficking to lysosomal compartments including the endocytic membrane transport and autophagic pathways. Believed to act as a core component of the putative HOPS and CORVET endosomal tethering complexes which are 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. The HOPS complex is proposed to be recruited to Rab7 on the late endosomal membrane and to regulate late endocytic, phagocytic and autophagic traffic towards lysosomes. The CORVET complex is proposed to function as a Rab5 effector to mediate early endosome fusion probably in specific endosome subpopulations. Required for recruitment of VPS33A to the HOPS complex. Required for fusion of endosomes and autophagosomes with lysosomes; the function is dependent on its association with VPS33A but not VPS33B. The function in autophagosome-lysosome fusion implicates STX17 but not UVRAG. Core component of at least two putative endosomal tethering complexes, the homotypic fusion and vacuole protein sorting (HOPS) complex and the class C core vacuole/endosome tethering (CORVET) complex. Their common core is composed of the class C Vps proteins VPS11, VPS16, VPS18 and VPS33A, which in HOPS further associates with VPS39 and VPS41 and in CORVET with VPS8 and TGFBRAP1 (PubMed:25266290). Interacts with RAB5C (PubMed:25266290). Interacts with STX17, MON1B (By similarity). Associates with adapter protein complex 3 (AP-3) and clathrin:AP-3 complexes (PubMed:21411634). Colocalizes with AP-3, clathrin, Rab5 and Rab7b (PubMed:21411634). Cytoplasmic, peripheral membrane protein associated with early endosomes and late endosomes/lysosomes. Belongs to the VPS16 family.