Yeast Env9 is a Conserved Oxidoreductase Involved in Lipid Droplet Biogenesis

Interactions between organelles and trafficking of cellular materials are finely regulated to maintain cellular integrity. The complexity of such events is highlighted in the endomembrane system, which is comprised of the endoplasmic reticulum (ER), Golgi apparatus, late endosomes, lysosomes, lipid droplets, and plasma membrane. As endomembrane system regulation are conserved between yeast and human, yeast serves as an ideal model for its study. Yeast ENV9 is a novel gene involved in vesicular trafficking. Previous characterization by our lab revealed that ENV9 deletion leads to defects in a lysosomal enzyme processing, increase in vesiculated vacuoles, and caffeine sensitivity. Our bioinformatic studies show that ENV9 is conserved among eukaryotes and exhibits similarity to Retinol Dehydrogenase 12 ( RDH12 ) involved in redox reactions in visual cycle. Mutations of RDH12 have been linked to Leber Congenital Amaurosis. Using microscopic and biochemical approaches, we show localization of Env9 to lipid droplets. Here we report that ENV9 plays a role in lipid droplet morphology by positively regulating lipid droplet expansion and carbon stress‐induced lipid droplet fragmentation. Lastly, Env9 displays reductase activity towards sterol synthesis substrate 3‐hydroxy‐3‐methylglutaryl‐coenzyme A (HMG CoA) and toxic aldehyde 4‐Hydroxynonenal (4‐HNE). Point mutations in the conserved residues in the active (N146L) and cofactor‐binding sites (G23‐24A) of Env9 abolish its reductase activity, as reported in other retinol dehydrogenases. Thus, we report a novel oxidoreductase involved in lipid droplet morphology. ENV9 may serve as a model to study lipid droplet expansion, organelle interactions in endomembrane system, and retinal disease‐associated mutations. (NIH‐AREA #R15 GM85794‐02)