Ligand-regulated entry into the HRD ERAD pathway: The dark side of allostery

Abstract

HMG-CoA reductase (HMGR) undergoes regulated degradation as part of feedback control of the sterol pathway. In yeast the stability of the Hmg2 isozyme of HMGR is controlled by the 20 carbon isoprenoid geranylgeranyl pyrophosphate (GGPP): increasing levels of GGPP causes more efficient degradation by the HRD pathway, allowing feedback regulation of HMGR. The HRD pathway is a conserved quality control pathway critical for the ER-associated degradation of misfolded ER proteins. We have explored the action of GGPP in HRD-dependent Hmg2 degradation. GGPP was highly potent as a regulatory molecule in vivo, and in vitro, GGPP altered Hmg2 folding at nanomolar concentrations. These effects of GGPP were absent in a variety of stabilized or non-regulated Hmg2 mutants. Consistent with its high potency, the effects of GGPP were highly specific; other closely related molecules were ineffective in altering Hmg2 structure. In fact, two close GGPP analogues, 2F-GGPP and GGSPP were completely inactive at all concentrations tested, and GGSPP was an antagonist of GGPPs effects in vivo and in vitro. The effects of GGPP on Hmg2 structure and degradation were reversed by chemical chaperones, indicating that GGPP caused selective Hmg2 misfolding. These data indicate that GGPP functions in a manner analogous to an allosteric ligand, causing Hmg2 misfolding through interaction with a reversible, specific binding site. Consistent with this, the Hmg2 protein forms mulitmers. We propose that this \"allosteric misfolding,\" or mallostery, may be a widely used tactic of biological regulation, with potential for development of small molecule pharmaceuticals that induce selective misfolding.