ESCRT-III and ER-PM contacts maintain lipid homeostasis.

Eukaryotic cells are compartmentalized into organelles by intracellular membranes. While the organelles are distinct, many of them make intimate contact with one another. These contacts were first observed in the 1950's, but only recently have the functions of these contact sites begun to be understood. In yeast, the ER makes extensive inter-membrane contacts with the plasma membrane (PM), covering approximately 40% of the PM. Many functions of ER-PM contacts have been proposed, including: non-vesicular lipid trafficking, ion transfer, and as signaling hubs. Surprisingly, cells that lack ER-PM contacts grow well, indicating that alternative pathways may be compensating for the loss of ER-PM contact. In order to better understand the function of ER-PM contact sites we used saturating transposon mutagenesis to identify synthetic lethal mutants in a yeast strain lacking ER-PM contact sites. The strongest hits were components of the ESCRT complexes. The synthetic lethal mutants have low levels of some lipid species but accumulate free fatty acids and lipid droplets. We found that only ESCRT-III components are synthetic lethal, indicating that Vps4 and other ESCRT complexes do not function in this pathway. These data suggest that ESCRT-III proteins and ER-PM contact sites act in independent pathways to maintain lipid homeostasis.