Membrane protein quality control in post‐Golgi compartments

Abstract

Elimination of non‐native polypeptides is critical for protein quality control (QC) and fundamental to preserve cellular proteostasis. The "peripheral" QC of cell surface membrane proteins (MP) remains poorly understood. To uncover the principles of MPQC in post‐Golgi compartments, we engineered chimeric CD4 molecules containing wt and temperature‐sensitive mutant l domain. The mutant was accumulated at the cell surface and unfolded at the non‐permissive temperature. Increased internalization and impaired recycling accounts for the metabolic destabilization of the unfolded chimera at the cell surface. The postendocytic sorting of the internalized chimeras was conformational‐dependent; unfolding redirected the chimera from constitutive recycling towards lysosomal degradation. Ubiquitination plays a pivotal role in the non‐native chimera degradation since; a) the chimera ubiquitination was enhanced several fold upon unfolding, 2) depletion of ubiquitin‐dependent endosomal sorting machinery (Hrs, Stam‐2 and Tsg101) delayed lysosomal delivery and 3) physical and functional association of the misfolded variant with an E3 Ub‐ligase could be identified. We propose a working model for the peripheral QC of MPs with a central role in cellular proteostasis and the etiology of conformational diseases. Supported by CIHR.