Proteasome activator Blm10 maintains cellular proteostatic balance and gamete quality in budding yeast

The proteasome is responsible for regulated protein degradation in eukaryotic cells. Its best characterized substrates are ubiquitinated proteins that are targeted to the 26S proteasome complex, consisting of a 19S regulatory particle (RP) capping the barrel-shaped 20S core peptidase (CP). The CP can interact with other caps, including Blm10/PA200, a nearly 250 kDa protein whose biological function is not well understood. Blm10 is upregulated during gametogenesis in budding yeast, suggestive of a natural stage-specific modulation of proteasome composition. Here, we investigate the function of Blm10 during yeast gametogenesis, identifying it as a weak activator of the proteasome that can displace the 19S RP from the CP. Due to this competition for the CP, overexpression of Blm10 can lead to attenuation of ubiquitin-dependent degradation and proteostatic defects. Cells lacking Blm10 also display markers of proteostatic stress, including Hsp104 foci and heat sensitivity, suggesting that Blm10 safeguards normal proteostatic balance. We find that Blm10 is important for producing fit gametes and ensuring full rejuvenation of aged cells following gametogenesis. Furthermore, we observe direct association of aggregate-prone proteins and protein-folding factors with immunoprecipitated Blm10-proteasomes. Finally, we discovered that 14-3-3 proteins Bmh1/2 are novel cofactors of Blm-10 bound proteasomes and describe how Blm10 controls CP gate configuration. Overall, our data suggest a role for Blm10-proteasomes in maintaining gamete proteostasis through fine-tuning of proteasome activity and prevention of protein aggregation.