Protein Phosphatase Sit4 Affects Lipid Droplet Synthesis and Soraphen A Resistance Independent of Its Role in Regulating Elongator Dependent tRNA Modification.

The protein phosphatase Sit4 has been shown to be required for lipogenesis and resistance against the acetyl-CoA carboxylase inhibitor soraphen A. Since Sit4 is also required for biosynthesis of Elongator dependent tRNA modifications such as 5-methoxycarbonylmethyluridine (mcm⁵U), we investigated the relevance of tRNA modifications in lipogenesis and soraphen A response. While and Elongator () mutants copy defects in mcm⁵U formation and stress sensitivity, they do not share soraphen A sensitivity and low lipid droplet (LD) phenotypes. In contrast to , we found mutants to display partial soraphen A resistance and a high LD phenotype. Screening a collection of tRNA modification mutants additionally identified the tRNA pseudo-uridine synthase gene to be required for soraphen A sensitivity. Since and share high LD and soraphen A resistance phenotypes, these are likely caused by translational defects. In support of this notion, we observe overexpression of tRNAUUG suppresses lipolysis defects of mutants. Hence, the mutation results in a composite defect including tRNA modification deficiency and loss of Snf1 kinase dephosphorylation, which induce opposite effects on LD regulation. Importantly, however, the Snf1 kinase regulatory defects of the phosphatase mutant dominate over effects on LD regulation imposed by loss of the tRNA modification alone.