The Sterol-sensing Domain (SSD) Directly Mediates Signal-regulated Endoplasmic Reticulum-associated Degradation (ERAD) of 3-Hydroxy-3-methylglutaryl (HMG)-CoA Reductase Isozyme Hmg2

Deeba Pourmand,Talib Davis,Chandra L Theesfeld,Randolph Y Hampton,Renee M Garza

doi:10.1074/jbc.m111.244798

Deeba Pourmand, Talib Davis + Show 3 more

Open Access

https://doi.org/10.1074/jbc.m111.244798

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Abstract

The sterol-sensing domain (SSD) is a conserved motif in membrane proteins responsible for sterol regulation. Mammalian proteins SREBP cleavage-activating protein (SCAP) and HMG-CoA reductase (HMGR) both possess SSDs required for feedback regulation of sterol-related genes and sterol synthetic rate. Although these two SSD proteins clearly sense sterols, the range of signals detected by this eukaryotic motif is not clear. The yeast HMG-CoA reductase isozyme Hmg2, like its mammalian counterpart, undergoes endoplasmic reticulum (ER)-associated degradation that is subject to feedback control by the sterol pathway. The primary degradation signal for yeast Hmg2 degradation is the 20-carbon isoprene geranylgeranyl pyrophosphate, rather than a sterol. Nevertheless, the Hmg2 protein possesses an SSD, leading us to test its role in feedback control of Hmg2 stability. We mutated highly conserved SSD residues of Hmg2 and evaluated regulated degradation. Our results indicated that the SSD was required for sterol pathway signals to stimulate Hmg2 ER-associated degradation and was employed for detection of both geranylgeranyl pyrophosphate and a secondary oxysterol signal. Our data further indicate that the SSD allows a signal-dependent structural change in Hmg2 that promotes entry into the ER degradation pathway. Thus, the eukaryotic SSD is capable of significant plasticity in signal recognition or response. We propose that the harnessing of cellular quality control pathways to bring about feedback regulation of normal proteins is a unifying theme for the action of all SSDs.

Highlights

In the regulation of mammalian sterol pathway activity, a protein transmembrane region known as the sterol-sensing domain (SSD)3 is required for sterols to alter both the transcription of sterol-relevant genes and the stability of 3-hydroxy-3methylglutaryl (HMG)-CoA reductase (HMGR), one of the rate-determining enzymes of the sterol pathway [6]
We included mutations reported to function in mammalian SSD proteins, including the YIYF to AAAA mutant that rendered mammalian HMG-CoA reductase (HMGR) insensitive to regulated degradation [14] and three mutants, Y298C, L315F, and D428A, that make mammalian SREBP cleavage-activating protein (SCAP) insensitive to sterols [11, 12, 30]
In earlier studies of Hmg2 [28], we found that scrambling a five-amino acid region, TFYSA, brought about constitutive degradation that was unresponsive to the farnesyl pyrophosphate (FPP)-derived signal [25]

Summary

Introduction

In the regulation of mammalian sterol pathway activity, a protein transmembrane region known as the sterol-sensing domain (SSD)3 is required for sterols to alter both the transcription of sterol-relevant genes and the stability of 3-hydroxy-3methylglutaryl (HMG)-CoA reductase (HMGR), one of the rate-determining enzymes of the sterol pathway [6]. We wondered to what extent the highly conserved SSD is involved in regulated degradation of Hmg2 caused by the nonsterol FPP-derived signal. A subset of the SSD mutants altered the response of Hmg2 degradation rate to oxysterols, whereas preserving the response to the main FPP-derived signal.

Results

Conclusion