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The base of the proteasome regulatory particle exhibits chaperone-like activity

Abstract

Protein substrates of the proteasome must apparently be unfolded and translocated through a narrow channel to gain access to the proteolytic active sites of the enzyme. Protein folding in vivo is mediated by molecular chaperones. Here, to test for chaperone activity of the proteasome, we assay the reactivation of denatured citrate synthase. Both human and yeast proteasomes stimulate the recovery of the native structure of citrate synthase. We map this chaperone-like activity to the base of the regulatory particle of the proteasome, that is, to the ATPase-containing assembly located at the substrate-entry ports of the channel. Denatured but not native citrate synthase is bound by the base complex. Ubiquitination of citrate synthase is not required for its binding or refolding by the base complex of the proteasome. These data suggest a model in which ubiquitin–protein conjugates are initially tethered to the proteasome by specific recognition of their ubiquitin chains; this step is followed by a nonspecific interaction between the base and the target protein, which promotes substrate unfolding and translocation.

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Figure 1: Interaction of human proteasomes with non-native citrate synthase.
Figure 2: Proteasome-mediated inhibition of citrate synthase aggregation.
Figure 3: Interaction of non-native citrate synthase with proteasomal subcomplexes.
Figure 4: Interaction of yeast proteasomes (y26S) and yeast base–20S (ybase–20S) complexes with non-native citrate synthase.
Figure 5: Formation of complexes containing denatured (dCS) but not native (nCS) citrate synthase with yeast proteasomes (y26S) and the base–20S proteasome particle (ybase–20S).

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Acknowledgements

We thank R. Gali and C. Larsen for providing us with the base–20S particle of the yeast proteasome; D. Zantopf and G. Grelle for technical support; and J. Buchner and W. Dubiel for citrate-synthase-specific and S10a-specific antibodies. This work was supported by grants from the Deutsche Forschungsgemeinschaft (to M. S., Schm 884/2-1; and P.-M. K., Kl 427 8-2/8-3), from the NIH (to D. F., GM43601) and from the Medical Foundation Charles Kings Trust and the J. and A. Taub Research Fund (to M. G.).

Correspondence and requests for materials should be addressed to M.S.

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Correspondence to Marion Schmidt.

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Braun, B., Glickman, M., Kraft, R. et al. The base of the proteasome regulatory particle exhibits chaperone-like activity. Nat Cell Biol 1, 221–226 (1999). https://doi.org/10.1038/12043

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