Involved in a late step in sorting of cargo proteins of the multivesicular body (MVB) for incorporation into intralumenal vesicles. The lumenal sequestrated membrane proteins are targeted into the vacuole after fusion of the endosome with the vacuole. Regulates the recruitment of VPS4 to the ESCRT-III complex, probably in conjunction with DID2, and VPS4 catalyzes the disassembly of the ESCRT-III complex. Interacts with DID2. Interacts with VPS4 (via MIT domain); the interaction prevents VPS4 oligomerization, competes with the binding of VTA1 to VSP4 and diminishes the ATPase activity of VSP4. The endosomal location appears to be dependent on DID2. Present with 4760 molecules/cell in log phase SD medium. Belongs to the IST1 family.