Protein quality control in the ER: balancing the ubiquitin chequebook

Abstract

In this thesis, work is discussed that addresses the machinery held responsible for substrate ubiquitylation, a novel method to block the UPS by expression of a ‘hyperactive’ DUB, and the discovery of a cytosolic chaperone that is required for the dislocation reaction. Protein quality control in the ER selects dysfunctional proteins and targets them for destruction by the ubiquitin proteasome system before they can escape to the secretory pathway. Despite the identification of many proteins involved, the exact mechanisms of key steps in the process remain to be more accurately defined. This thesis presents work that sheds light on the role of the ubiquitin machinery in ER dislocation. It is shown, that the working of a membrane-anchored E2 enzyme critically depends on its interaction with the dislocation complex within the plane of the ER membrane. Once ubiquitylated, the misfolded protein recruits the p97 protein complex to initiate extraction from the ER. This step critically depends on a deubiquitylating activity present at the p97 complex itself. Finally, to allow a smooth exit, misfolded ER proteins associate with cytosolic chaperones.