Identification and characterization of a Saccharomyces cerevasiae gene, RSB1, involved in sphingoid long-chain base release

Sphingoid long-chain bases (LCBs) and long-chain base phosphates (LCBPs) act as signaling molecules in eukaryotic cells. Accumulation of LCBPs results in cell growth inhibition in yeast, although the mechanism is unknown. Here; we identified a novel yeast gene, RSB1 (resistance to sphingoid long-chain base), by screening a multicopy suppressor of the LCB-sensitive phenotype of the LCBP lyase mutant. RSB1 encodes a polypeptide of 354 amino acids with a molecular mass of 40.4 kDa. Rsb1p is predicted to be an integral membrane protein with seven transmembrane-spanning domains. We demonstrated that cells overproducing Rsb1p showed a decrease in accumulation of exogenously added sphingosine and dihydrosphingosine because of their increased release. This release was ATP-dependent, and a mutant of the predicted ATP binding motif had no activity. Substrate specificity analysis of Rsb1p demonstrated that it is active on LCBs but not on LCBPs or other hydrophobic compounds. These results suggest that Rsb1p is a transporter or flippase that translocates LCBs from the cytoplasmic side toward the extracytoplasmic side of the membrane.