Involved in regulating membrane fusion at the tonoplast and the prevacuolar compartment. 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, VCL1, VPS18 and VPS33, which in HOPS further associates with VPS39 and VPS41 and in CORVET with VPS3. A number of isoforms are produced. According to EST sequences. Expressed in roots, leaves, stems, siliques and flowers. Belongs to the STXBP/unc-18/SEC1 family.