1. Laboratory of Frontier Science, Core Technology and Research Center, Tokyo Metropolitan Institute of Medical Science, Bunkyo-ku, Tokyo 113-8613, Japan
2. Institute of Molecular Biology and Physiology, University of Copenhagen, 13 Universitetsparken, DK 2100 Copenhagen, Denmark
3. National Institute of Advanced Industrial Science and Technology, Biological Information Research Center, Kohtoh-ku, Tokyo 135-0064, Japan
4. PRESTO, Japan Science and Technology Agency, Kawaguchi, Saitama 332-0012, Japan

Correspondence to: Keiji Tanaka¹Shigeo Murata^1,4 Correspondence and requests for materials should be addressed to S.M. (Email: smurata@rinshoken.or.jp) or K.T. (Email: tanakak@rinshoken.or.jp). The sequences for human PAC1 and PAC2 have been deposited in GenBank under accession numbers BR000236 and BR000237, respectively.

Abstract

The 26S proteasome is a multisubunit protease responsible for regulated proteolysis in eukaryotic cells^{1, 2}. It comprises one catalytic 20S proteasome and two axially positioned 19S regulatory complexes³. The 20S proteasome is composed of 28 subunits arranged in a cylindrical particle as four heteroheptameric rings, _{1–71–71–71–7} (refs 4, 5), but the mechanism responsible for the assembly of such a complex structure remains elusive. Here we report two chaperones, designated proteasome assembling chaperone-1 (PAC1) and PAC2, that are involved in the maturation of mammalian 20S proteasomes. PAC1 and PAC2 associate as heterodimers with proteasome precursors and are degraded after formation of the 20S proteasome is completed. Overexpression of PAC1 or PAC2 accelerates the formation of precursor proteasomes, whereas knockdown by short interfering RNA impairs it, resulting in poor maturation of 20S proteasomes. Furthermore, the PAC complex provides a scaffold for -ring formation and keeps the -rings competent for the subsequent formation of half-proteasomes. Thus, our results identify a mechanism for the correct assembly of 20S proteasomes.

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