Cooperation of Multiple Chaperones Required for the Assembly of Mammalian 20S Proteasomes

The 20S proteasome is a catalytic core of the 26S proteasome, a central enzyme in the degradation of ubiquitin-conjugated proteins. It is composed of 14 distinct gene products that form four stacked rings of seven subunits each, α1–7β1–7β1–7α1–7. It is reported that the biogenesis of mammalian 20S proteasomes is assisted by proteasome-specific chaperones, named PAC1, PAC2, and hUmp1, but the details are still unknown. Here, we report the identification of a chaperone, designated PAC3, as a component of α rings. Although it can intrinsically bind directly to both α and β subunits, PAC3 dissociates before the formation of half-proteasomes, a process coupled with the recruitment of β subunits and hUmp1. Knockdown of PAC3 impaired α ring formation. Further, PAC1/2/3 triple knockdown resulted in the accumulation of disorganized half-proteasomes that are incompetent for dimerization. Our results describe a cooperative system of multiple chaperones involved in the correct assembly of mammalian 20S proteasomes.