Component of the vacuolar transporter chaperone (VTC) complex, which plays a role in vacuolar membrane fusion. Required for LMA1 release prior to membrane fusion (PubMed:11823419). Binds inositol hexakisphosphate (Ins6P) and similar inositol polyphosphates, such as 5-diphospho-inositol pentakisphosphate (5-InsP7); these are important intracellular signaling molecules (PubMed:27080106). Inositol polyphosphate binding promotes vacuolar polyphosphate synthesis (PubMed:27080106). The VTC complex is an integral membrane heterooligomer composed of VTC1, VTC2, VTC3 and VTC4. The complex interacts with the v-SNARE NYV1 and with the V(0) subunit of V-ATPase VPH1. By low phosphate. The SPX domain has very high affinity for inositol polyphosphates, such as myo-inositol hexakisphosphate and 5-diphospho-myo-inositol pentakisphosphate (5-InsP7), and moderate affinity for inorganic pyrophosphate. Its affinity for inorganic phosphate is tow to three orders of magnitude lower. Present with 2630 molecules/cell in log phase SD medium. Belongs to the VTC2/3 family.