Osmoadaptation-dependent activity of microsomal HMG-CoA reductase in the extremely halotolerant black yeast Hortaea werneckii is regulated by ubiquitination

We have investigated regulation of HMG-CoA reductase (HMGR) in one of the most salt-tolerant fungi, Hortaea werneckii, under different salinities and at the level of protein degradation. Two different HwHMGR isoenzymes were identified, specific to mitochondria and endoplasmic reticulum: HwHmg1 and HwHmg2, respectively. The activity of microsomal HwHmg2 is highest under hypo-saline and extremely hyper-saline conditions, and down-regulated under optimal growth conditions. We show that this is due to intense ubiquitination and proteasomal degradation of HwHmg2. The activity of the truncated mitochondrial HwHmg1 is constant under different growth conditions, suggesting an osmoadaptation-directed fate for mevalonate utilization in H. werneckii.