ABSTRACT
HMG-CoA reductase (HMGR) undergoes regulated degradation as part of feedback control of the sterol pathway. In yeast the Hmg2 isozyme of HMGR is keyed to levels of the 20 carbon pathway intermediate geranylgeranyl pyrophosphate (GGPP): increasing levels of GGPP causes more efficient degradation by the HRD pathway, thus bring about feedback regulation of HMGR in response to pathway flux. The HRD pathway is a conserved quality control pathway critical ER-associated degradation of a wide variety of misfolded ER proteins. To understand this regulatory axis, we have explored to role and action of GGPP in this process. We have found that GGPP is highly potent as a regulatory molecule. In an in vitro limited proteolysis assay, GGPP caused a change in Hmg2 folding state that is required for regulated degradation, and functions in this regard in the low to mid nanomolar region of concentration. The structural effect of GGPP is required for regulated degradation of Hmg2, and is absent in a variety of stabilized or non-regulated mutations of the protein. Consistent with its high potency, the effects of GGPP are highly specific; other closely related molecules are 100s of times less potent or completely ineffective in affecting Hmg2 structure. Strikingly, two close GGPP analogues, 2F-GGPP and GGSPP are completely inactive at all concentrations tested. Furthermore, the GGSPP molecule is in fact an antagonist of the GGPP effects on Hmg2 both in vivo and in vitro. The effects of GGPP on Hmg2 structure and degradation are reversed by a variety of chemical chaperones, indicating that GGPP causes selective misfolding of Hmg2. Taken together, 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 highly specific for the GGPP structure. Consistent with this, the Hmg2 protein forms mulitmers, and this association is not altered by the presence or absence of GGPP. We propose that this “allosteric misfolding” may be a widely used tactic of biological regulation, with potential for development of small molecules that similarly cause selective misfolding to bring about desired destruction of a pharmaceutical target. We suggest that this type of “misfolding by allostery” be called mallostery, to include both the ideas of misfolding and allosteric regulation in a single portmanteau.