Protein Folding Quality Control in the Endoplasmic Reticulum in Budding Yeast

Abstract

Endoplasmic reticulum (ER) is the first membrane compartment of secretory pathway in eukaryotic cells. Newly synthesized proteins are translocated into ER lumen, and they are screened by endoplasmic reticulum quality control (ERQC) system. Only correctly folded and functional proteins can be sorted out to Golgi and later membrane compartments. Misfolded proteins are retained in the ER and turned over by a mechanism conserved from yeast to human known as endoplasmic reticulum-associated protein degradation (ERAD). While the mammalian system is less understood, the ERAD mechanism in yeast is explained in more detail, and it is shown to be centered on two membrane associated E3 ubiquitin ligases: Hrd1p and Doa10p. Previous studies suggested that Hrd1p ubiquitinates misfolded luminal proteins and membrane proteins with luminal lesions, while Doa10p targets membrane proteins with misfolded cytosolic domain. But how exactly the two ERAD E3s detects these lesions remains elusive.

In this thesis, I have used Saccharomyces cerevisiae as a model organism to study the quality control of two classes of ER luminal proteins ? N-linked glycoproteins and non-glycosylated proteins, both of which are ERAD substrates and degraded by Hrd1p when misfolded.