The 26S proteasome is a multisubunit protease responsible for regulated proteolysis in eukaryotic cells1,2. 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, α1–7β1–7β1–7α1–7 (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 α-ring formation and keeps the α-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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We thank Y. Murakami for the ornithine decarboxylase degradation assay system, K. Furuyama for technical support, and D. Finley for comments on the manuscript. This work was supported by grants from the Japanese Science and Technology Agency (to S.M.), the Ministry of Education, Science and Culture of Japan (to S.M. and K.T.) and the New Energy and Industrial Technology Development Organization (to T.N.). Y.H. was supported by the Japanese Society for the Promotion of Science.
The sequences for human PAC1 and PAC2 have been deposited in GenBank under accession numbers BR000236 and BR000237, respectively. Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.
Supplementary Figures S1–S4. (PDF 2132 kb)
Text to accompany the above Supplementary Figures. (PDF 82 kb)
Additional discussion of results from the study. (PDF 127 kb)
Description of additional methods used in this study. (PDF 81 kb)
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