Cloning and characterization of SCS7 which encodes a protein with a cytochrome B₅-like domain fused to a hydroxylase/desaturase domain that is required for synthesis of inositolphoephoceramides in S. cerevisiae

SCS 7 is a gene that is required for the conversion of the inoaitolphosphoceramide species IPC-B to the more hydrophilic IPC-C species. IPC-B has been reported to differ from IPC-C by a hydroxylation. Since SCS7 gene encodes a protein having a cytochrome bj domain and a domain that resembles the family of cytochrome b5-dependent O2 and iron utilizing enzymes that catalyze desaturation or hydroxylation of fatty acids and sterols, we believe SCSVp is the IPC-B hydroxylase. Failure to convert IPC-B to IPC-C results in suppression of the Ca3+-sensitivity of csgl and csg2 mutants that fail to synthesize the mannosylated sphingolipids (MIPC and M(IP)jC). Suppression is also conferred by increased conversion of IPC-C to IPC-D as well as by reduction in activity of serine palmitoyl-transferase. Our results indicate that increased synthesis of IPC-C {and perhaps mislocalization of IPC-C) results in the Ca!-sensitivity of the csg mutants.