Slp1-Emp65: A Guardian Factor that Protects Folding Polypeptides from Promiscuous Degradation.

Newly synthesized proteins engage molecular chaperones that assist folding. Their progress is monitored by quality control systems that target folding errors for degradation. Paradoxically, chaperones that promote folding also direct unfolded polypeptides for degradation. Hence, a mechanism was previously hypothesized that prevents the degradation of actively folding polypeptides. In this study, we show that a conserved endoplasmic reticulum (ER) membrane protein complex, consisting of Slp1 and Emp65 proteins, performs this function in the ER lumen. The complex binds unfolded proteins and protects them from degradation during folding. In its absence, approximately 20%-30% of newly synthesized proteins that could otherwise fold are degraded. Although the Slp1-Emp65 complex hosts a broad range of clients, it is specific for soluble proteins. Taken together, these studies demonstrate the vulnerability of newly translated, actively folding polypeptides and the discovery of a new proteostasis functional class we term "guardian" that protects them from degradation.