Plays a role in vesicle-mediated protein trafficking to lysosomal compartments including the endocytic membrane transport pathways (PubMed:25273556, PubMed:26783301, PubMed:27558849). Believed to act as a component of the putative HOPS endosomal tethering complex which is proposed to be involved in the rab-5-to-rab-7 endosome conversion probably implicating sand-1, and via binding SNAREs and SNARE complexes to mediate tethering and docking events during SNARE-mediated membrane fusion (PubMed:25273556). The HOPS complex is proposed to be recruited to rab-7 on the late endosomal membrane and to regulate late endocytic, phagocytic and autophagic traffic towards lysosomes (By similarity). Within the HOPS complex, contributes to the normal development of gut granules in embryonic and adult intestinal cells (PubMed:24501423, PubMed:25273556). Required for endosome/lysosome fusion (PubMed:26783301, PubMed:27558849). Required for early embryonic development (PubMed:27558849). Probable component of the homotypic fusion and vacuole protein sorting (HOPS) complex consisting of the core class C Vps proteins vps-11, vps-16, vps-18, and which further associates with vps-33.1, vps-39 and vps-41 (PubMed:24501423, PubMed:25273556). Interacts with spe-39 (PubMed:19109425). Localizes to lysosome-related gut granules. Ubiquitously expressed at high levels in somatic tissues including the pharynx, muscles, hypodermis, neurons, coelomocytes and spermatheca (PubMed:27558849). Expressed in the intestine (PubMed:25273556, PubMed:27558849). Viable, and develop into fertile adults, but their progeny arrest before embryonic morphogenesis (PubMed:27558849). Defective endosome/lysosome size and fusion, and delayed endocytic trafficking to lysosomes in coelomocytes (PubMed:26783301, PubMed:27558849). Reduced number of gut granules in the adult intestine (PubMed:24501423, PubMed:27558849). RNAi-mediated knockdown results in a reduced number of gut granules in embryonic intestinal cells (PubMed:24501423). RNAi-mediated knockdown results in the accumulation of lysosomal proteins such as lmp-1 in early endocytic compartments, and the formation of large late endosomes/lysosomes, but with simultaneous expression of rab-5- and rab-7-positive vesicles on the basal side of gut cells (PubMed:25273556). Double RNAi-mediated knockdown together with vsp-33.2 results in defective protein trafficking to lysosomal compartments, and an irregular distribution of vesicles of various sizes throughout the gut cells. RNAi-mediated knockdown in a sand-1 mutant background results in no viable offspring (PubMed:25273556). Double RNAi-mediated knockdown together with vsp-33.1 in a sand-1 mutant background rescues the large endosome phenotype and the defective protein trafficking to lysosomal compartments in the sand-1 single mutant, but still results in lethality (PubMed:25273556). Double knockout with tbc-2 or single knockout in a constitutively active rab-5 mutant background, rescues the large endosome formation defect in coelomocytes in the individual tbc-2 single and constitutively active rab-5 mutants (PubMed:27558849). Belongs to the STXBP/unc-18/SEC1 family.