INP51, A YEAST INOSITOL POLYPHOSPHATE B-PHOSPHATASE REQUIRED FOR PHOSPHATIDYLINOSITOL 4,5-BISPHOSPHATE HOMEOSTASIS AND WHOSE ABSENCE CONFERS A COLD-RESISTANT PHENOTYPE

Sequence analysis of Saccharomyces cerevisiae chromosome IX identified a 946 amino acid open reading frame (YIL002C), designated here as INP 51, that has carboxyl-and amino-terminal regions similar to mammalian inositol polyphosphate 5-phosphatases and to yeast SAC1. This two-doma in primary structure resembles the mammalian 5-phosphatase, synaptojan in. We report that Inp51p is associated with a particulate fraction an d that recombinant Inp51p exhibits intrinsic phosphatidylinositol 4,5- bisphosphate B-phosphatase activity. Deletion of INP51 (inp51) results in a ''cold-tolerant'' phenotype, enabling significantly faster growt h at temperatures below 15 degrees C as compared with a parental strai n. Complementation analysis of an inp51 mutant strain demonstrates tha t the cold tolerance is strictly due to loss of 5-phosphatase catalyti c activity. Furthermore, deletion of PLC1 in an inp51 mutant does not abrogate cold tolerance, indicating that Plc1p-mediated production of soluble inositol phosphates is not required. Cells lacking INP51 have a 2-4-fold increase in levels of phosphatidylinositol 4,5-bisphosphate and inositol 1,4,5-trisphosphate, whereas cells overexpressing Inp51p exhibit a 35% decrease in levels of phosphatidylinositol 4,5-bisphosp hate. We conclude that INP51 function is critical for proper phosphati dylinositol 4,5-bisphosphate homeostasis. In addition, we define a nov el role for a 5-phosphatase loss of function mutant that improves the growth of cells at colder temperatures without alteration of growth at normal temperatures, which may have useful commercial applications.