The proteasome regulatory particle (RP) initiates the degradation of ubiquitin-protein conjugates by unfolding the substrate and translocating it into the proteasome core particle (CP) to be degraded. Here we show that in yeast three proteins are found associated with RP but not RP-CP holoenzyme: Nas6, Rpn14, and Hsm3. Mutations in these genes confer proteasome loss of function phenotypes, despite their virtual absence from holoenzyme. These effects result from deficient RP assembly. Thus, Nas6, Rpn14, and Hsm3 are RP chaperones. The RP contains six ATPases–the Rpt proteins–and each RP chaperone binds to the C-terminal domain of a specific Rpt. We show in an accompanying study that RP assembly is templated through the Rpt C-termini, apparently by their insertion into binding pockets in the CP. Thus, RP chaperones may regulate proteasome assembly by directly restricting the accessibility of Rpt C-termini to the CP. In addition, competition between the CP and RP chaperones for Rpt engagement may explain the release of RP chaperones as proteasomes mature.
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