Characterization of sphingosine kinase (SK) activity in Saccharomyces cerevisiae and isolation of SK-deficient mutants

Sphingosine kinase (SK) catalyses the phosphorylation of sphingosine to generate sphingosine 1phosphate, which is a second messenger involved in the proliferative responses of mammalian cells. Although the yeast Saccharomyces cerevisiae has similar phosphorylated sphingoid bases which appear to be involved in growth regulation and the response to stress, SK activity had not been previously demonstrated in yeast. In this study, an in vitro system was set up to characterize yeast SK activity. Activity was detected in the cytosol at neutral pH and 37 degrees C. Yeast SK phosphorylated the sphingoid bases sphingosine, dihydrosphingosine and phytosphingosine. (D,L)-threo-dihydrosphingosine, an inhibitor of mammalian SK, did not inhibit the yeast enzyme. Unique properties of yeast SK were an optimal temperature of 43 degrees C, and in vivo activation during nutrient deprivation. Spontaneous mutants with diminished SK activity were isolated utilizing a screen for resistance to sphingosine in a sphingosine-phosphate-lyase deletion background. Abnormal growth and heat sensitivity were observed in these mutants. These findings suggest that SK may function as a stress-response protein in yeast.