Structural Determinants of Ligand-Mediated Hmg2 Mallostery

The sterol pathway is a conserved eukaryotic process that produces essential metabolites. Hmg-CoA reductase (HMGR) is a key rate limiting enzyme of the sterol pathway and is the most successful clinical target for managing cholesterol levels. However, determination of the structural features that are essential for its regulation remains elusive. Because of the conservation between mammals and yeast, for many years we have studied the structural features of a yeast Hmg2 isozyme of HMGR. Hmg2 is regulated by the mevalonate pathway isoprenoid GGPP. GGPP acts similarly to an allosteric ligand and appears to bind to a specific site on Hmg2, causing Hmg2 to undergo reversible misfolding. Misfolded Hmg2 is then recognized by the protein quality control mechanism known as the HRD pathway. Due to its similarity to allosteric regulation we have termed this ligand-dependent misfolding mallostery. In this work, we systematically probe each residue of the Hmg2 lumenal domain as well as assess the transmembrane domain and lumenal domain as independent domains and use in vivo and in vitro techniques to assess what regions are important for GGPP response. We identified stabilizing mutations within each loop of the lumenal domain but did not find mutants that were completely unresponsive to GGPP. When assessing the transmembrane domain and lumenal domain as two independent structures, we found that only the transmembrane domain responded to GGPP specifically in vitro. Our results suggest that GGPP binding occurs within the Hmg2 transmembrane domain and that the loops in the lumenal domain are required for transducing a mallosteric response to GGPP. Taken together, these data suggest a new model of Hmg2 regulation.