HOPS and Vps13 Have Antagonistic Roles in Regulating Lipid Droplet Morphology.

Lipid droplets are dynamic organelles that store neutral lipids (mostly triacylglycerols and sterol esters) and are bound by a single phospholipid monolayer. The formation and breakdown of the droplets are important for cellular metabolism; however, the molecular mechanisms regulating their biogenesis and turnover are still obscure. In this study, we used Erg6-yeGFP as a lipid droplet marker, isolated 1100 temperature-sensitive mutants, and observed lipid droplet morphology in these mutants by fluorescence microscopy. Six mutants with abnormal lipid droplet morphology were obtained and next-generation sequencing identified responsible mutations in genes encoding subunits of HOPS complex or its binding partner Ypt7. Therefore, the genetic screen indicated that the HOPS complex plays a critical role in maintaining lipid droplet morphology. In the absence of Vps11, vacuoles were fragmented with dot-like structures and degradation of lipid droplet surface protein Erg6 (under glucose starvation condition) decreased dramatically. A genetic screen for mutations that could rescue the abnormal lipid droplet morphology in Δvps11 by fluorescence microscopy identified two independent mutations in vps1302. Vps1301 and Vps1302 are paralogs that function as intermembrane lipid transfer proteins. Deletion of either vps1301 or vps1302 rescued the abnormal lipid droplet morphology in Δvps11 as well.