A GENETIC SCREEN FOR AMINOPHOSPHOLIPID TRANSPORT MUTANTS IDENTIFIES THE PHOSPHATIDYLINOSITOL 4-KINASE, STT4P, AS AN ESSENTIAL COMPONENT IN PHOSPHATIDYLSERINE METABOLISM

In an effort to understand molecular mechanisms of intracellular lipid transport, we have focused upon specific events required for de novo aminophospholipid synthesis in the yeast Saccharomyces cerevisiae. A g enetic system for examining the steps between phosphatidylserine (PtdS er) synthesis in the endoplasmic reticulum and its transport to and de carboxylation by PtdSer decarboxylase 2 in the Golgi/vacuole has been developed. We have isolated a mutant, denoted pstB1, that accumulates PtdSer and has diminished phosphatidylethanolamine formation despite n ormal PtdSer decarboxylase 2 activity. The lesion in PtdSer metabolism is consistent with a defect in interorganelle lipid transport. A geno mic DNA clone that complements the mutation was isolated, and sequenci ng revealed that the clone contains the STT4 gene, encoding a phosphat idylinositol 4-kinase. The pstB1 mutant exhibits a defect in Stt4p-typ e phosphatidylinositol Li-kinase activity, and direct gene replacement indicates that STT4 is the defective gene in the mutant. Creation of an STT4 null allele (stt4 Delta::HIS3) demonstrates the gene is essent ial. These results provide evidence that implicates phosphoinositides in the regulation of intracellular aminophospholipid transport.