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J Biol Chem. 2009 Dec 11;284(50):34889-900. doi: 10.1074/jbc.M109.044875. Epub 2009 Oct 8.

Regulated endoplasmic reticulum-associated degradation of a polytopic protein: p97 recruits proteasomes to Insig-1 before extraction from membranes.

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  • 1Department of Molecular Genetics, University of Texas Southwestern Medical Center, Dallas, Texas 75390-9046, USA.

Abstract

Polytopic membrane proteins subjected to endoplasmic reticulum (ER)-associated degradation are extracted from membranes and targeted to proteasomes for destruction. The extraction mechanism is poorly understood. One polytopic ER protein subjected to ER-associated degradation is Insig-1, a negative regulator of cholesterol synthesis. Insig-1 is rapidly degraded by proteasomes when cells are depleted of cholesterol, and its degradation is inhibited when sterols accumulate in cells. Insig-2, a functional homologue of Insig-1, is degraded slowly, and its degradation is not regulated by sterols. Here, we report that a single amino acid substitution in Insig-2, Insig-2(L210A), causes Insig-2 to be degraded in an accelerated and sterol-regulated manner similar to Insig-1. In seeking an explanation for the accelerated degradation, we found that proteasomes bind to wild type Insig-1 and mutant Insig-2(L210A) but not to wild type Insig-2, whereas the proteins are still embedded in cell membranes. This binding depends on at least two factors, ubiquitination of Insig and association with the ATPase p97/VCP complex. These data suggest that p97 recruits proteasomes to polytopic ER proteins even before they are extracted from membranes.

PMID:
19815544
[PubMed - indexed for MEDLINE]
PMCID:
PMC2787351
Free PMC Article
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