Involved in DNA mismatch repair in slow-growing cells. Acts as a chaperone during the assembly of the 26S proteasome, specifically of the base subcomplex of the 19S regulatory complex (RC). Interacts with RPT1, RPT2, RPT3, RPT5, RPT6, RPN1 and RPN2. Part of transient complex (BP1) containing HSM3, RPT1, RPT2 and RPT5 formed during the assembly of the 26S proteasome. Present with 468 molecules/cell in log phase SD medium. Belongs to the proteasome subunit S5B/HSM3 family.