The Hsp110 Molecular Chaperone Stabilizes Apolipoprotein B from Endoplasmic Reticulum-associated Degradation (ERAD)

Apolipoprotein B ( apoB) is the most abundant protein in low density lipoproteins and plays key roles in cholesterol homeostasis. The co- translational degradation of apoB is controlled by fatty acid levels in the endoplasmic reticulum ( ER) and is mediated by the proteasome. To define the mechanism of apoB degradation, we employed a cell- free system in which proteasome-dependent degradation is recapitulated with yeast cytosol, and we developed an apoB yeast expression system. We discovered that a yeast Hsp110, Sse1p, associates with and stabilizes apoB, which contrasts with data indicating that select Hsp70s and Hsp90s facilitate apoB degradation. However, the Ssb Hsp70 chaperones have no effect on apoB turnover. To determine whether our results are relevant in mammalian cells, Hsp110 was overexpressed in hepatocytes, and enhanced apoB secretion was observed. This study indicates that chaperones within distinct complexes can play unique roles during ER- associated degradation ( ERAD), establishes a role for Sse1/ Hsp110 in ERAD, and identifies Hsp110 as a target to lower cholesterol.