Fpk1/2 kinases regulate cellular sphingoid long‐chain base abundance and alter cellular resistance to LCB elevation or depletion

Sphingolipids are a family of eukaryotic lipids biosynthesized from sphingoid long‐chain bases ( LCB s). Sphingolipids are an essential class of lipids, as their depletion results in cell death. However, acute LCB supplementation is also toxic; thus, proper cellular LCB levels should be maintained. To characterize the “sphingolipid‐signaling intercross,” we performed a kinome screening assay in which budding yeast protein kinase‐knockout strains were screened for resistance to ISP ‐1, a potent inhibitor of LCB biosynthesis. Here, one pair of such DIR ( d eletion‐mediated I SP ‐1 r esistance) genes, FPK1 and FPK2 , was further characterized. Cellular LCB levels increased in the fpk1/2∆ strain, which was hypersensitive to phytosphingosine ( PHS ), a major LCB species of yeast cells. Concomitantly, this strain acquired resistance to ISP ‐1. Fpk1 and Fpk2 were involved in two downstream events; that is, ISP ‐1 uptake due to aminophospholipid flippase and LCB degradation due to LCB 4 expression. RSK 3, which belongs to the p90‐S6K subfamily, was identified as a functional counterpart of Fpk1/2 in mammalian cells as the RSK3 gene functionally complemented the ISP ‐1‐resistant phenotype of fpk1/2∆ cells.