Proteasome chaperone dimer PSMG3:PSMG4 binds the PSMA5 (alpha-5) subunit of the 20S core particle (20S CP) outer ring (alpha-ring) (reviewed in Tomko and Hochstrasser 2013). Using human recombinant proteins, it was established that the PSMG3:PSMG4 heterodimer interacts most strongly with PSMA5, and weakly with PSMA7 (alpha-4) and PSMA1 (alpha-6) (Satoh et al. 2019). The existing experimental evidence implies that the PSMG3:PSMG4 heterodimer serves as molecular matchmaker for the assembly of the PSMA4:PSMA5:PSMA6 subcomplex during formation of the outer ring (Satoh et al. 2019), similarly to findings previously reported in yeast (Takagi et al. 2014).