Yeast ceramide synthases, Lag1 and Lac1, have distinct substrate specificity

Lag1 was the first l ongevity a ssurance g ene, discovered in Saccharomyces cerevisiae . Lag1 is a ceramide synthase and its homolog, Lac1, has a similar enzymatic function but no role in aging. Lag1 and Lac1 lay in an enzymatic branchpoint of the sphingolipid pathway that is interconnected by the activity of the C4-hydroxylase, Sur2. By uncoupling the enzymatic branch point and using lipidomic mass spectrometry, metabolic labeling and in vitro assays we show that Lag1 preferentially synthesizes phyto-sphingolipids. Using photo-bleaching experiments we show that the Lag1 protein is uniquely required for the establishment of a lateral diffusion barrier in the nuclear envelope, which depends on phytoceramide. Given the role of this diffusion barrier in the retention of aging factors in the mother cell, we suggest that the different specificities of the two ceramide synthases, and the specific effect of Lag1 on asymmetrical inheritance, may explain why Δlag1 cells have an increased lifespan while Δlac1 cells do not.