Structural basis for the activation of 20S proteasomes by 11S regulators

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Abstract

Most of the non-lysosomal proteolysis that occurs in eukaryotic cells is performed by a nonspecific and abundant barrel-shaped complex called the 20S proteasome1. Substrates access the active sites, which are sequestered in an internal chamber, by traversing a narrow opening2 (α-annulus) that is blocked in the unliganded 20S proteasome by amino-terminal sequences of α-subunits3. Peptide products probably exit the 20S proteasome through the same opening. 11S regulators (also called PA26 (ref. 4), PA28 (ref. 5) and REG6,7) are heptamers4,8,9 that stimulate 20S proteasome peptidase activity in vitro and may facilitate product release in vivo. Here we report the co-crystal structure of yeast 20S proteasome with the 11S regulator from Trypanosoma brucei4 (PA26). PA26 carboxy-terminal tails provide binding affinity by inserting into pockets on the 20S proteasome, and PA26 activation loops induce conformational changes in α-subunits that open the gate separating the proteasome interior from the intracellular environment. The reduction in processivity expected for an open conformation of the exit gate may explain the role of 11S regulators in the production of ligands for major histocompatibility complex class I molecules10,11.

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Figure 1: Structure of the PA26–proteasome complex.
Figure 2: Structure of PA26.
Figure 3: Conformational change in 20S proteasome α-subunits.
Figure 4: Mechanism of binding.
Figure 5: Mechanism of activation.

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Acknowledgements

We thank P. Brown, N.-L. Chan, B. Howard and M. Mathews for assistance with data collection; D. Finley for Saccharomyces cerevisae strain SUB459, K. Alexander for assistance with protein purification; and W. Sundquist and M. Rechesteiner for comments on the manuscript. This work was supported by grants from the NIH and based on research conducted at the Stanford Synchrotron Radiation Laboratory (SSRL), which is funded by the Department of Energy (BES, BER) and the NIH (NCRR, NIGMS). Preliminary structure determination was performed with data collected from small crystals at Beamline X25 of the National Synchrotron Light Source, which is supported by the Office of Basic Energy Research of the US Department of Energy and by the NIH.

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Correspondence to Christopher P. Hill.

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