Sterol sensing domain (SSD) containing proteins are required for lipid regulation, and are conserved among different organisms. 3-hydroxy-3-methyglutaryl-CoA reductase (HMGR) is a key enzyme for sterol synthesis that contains an SSD. In both mammals and yeast, HMGR undergoes regulated degradation in response to feedback of the mevalonate pathway. The configuration of HMGR changes when regulated by mevalonate molecules, which targets the protein for regulated degradation in the endoplasmic reticulum. The N-terminus of HMGR is necessary and sufficient for regulated degradation. The SSD of HMGR ranges from 5 out of 8 transmembrane spans of the N- terminus, and the SSD of HMGR is conserved between many organisms. In this work we investigated the role of the SSD in regulation of HMGR in Saccharomyces cerevisiae. We made 30 mutations in highly conserved residues of the SSD of budding yeast HMGR, and examined their phenotypes with respect to HMGR regulated degradation. To do this we used flow cytometry to measure HMGR levels in response to pharmacological manipulations of the mevalonate pathway that either increase or decrease degradation signals. We found that the SSD is involved in sensing FPP-derived molecule and oxysterol molecules. The SSD also contributed to the destabilization of HMGR during regulated degradation of the protei
