Mammalian inositol polyphosphate 5-phosphatase II can compensate for the absence of all three yeast Sac1-like-domain-containing 5-phosphatases

Phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P-2] plays a complex role in generating intracellular signalling molecules, and also in regulating a ctin-binding proteins, vesicular trafficking and vacuolar fusion. Four inos itol polyphosphate 5-phosphatases (hereafter called 5-phosphatases) have be en identified in Saccharomyces cerevisiae: Inp51p, Inp52p, Inp53p and Inp54 p. Each enzyme contains a 5-phosphatase domain which hydrolyses PtdIns(4,5) P-2, forming PtdIns4P, while Inp52p and Inp53p also express a polyphosphoin ositide phosphatase domain within the Sac1-like domain. Disruption of any t wo yeast 5-phosphatases containing a Sac1-like domain results in abnormalit ies in actin polymerization, plasma membrane, vacuolar morphology and bud-s ite selection. Triple null mutant 5-phosphatase strains are non-viable. To investigate the role of PtdIns(4,5)P-2 in mediating the phenotype of double and triple 5-phosphatase null mutant yeast, we determined whether a mammal ian PtdIns(4,5)P-2 5-phosphatase, 5-phosphatase II, which lacks polyphospho inositide phosphatase activity, could correct the phenotype of triple 5-pho sphatase null mutant yeast and restore cellular PtdIns(4,5)P-2 levels to ne ar basal values. Mammalian 5-phosphatase II expressed under an inducible pr omoter corrected the growth, cell wall, vacuolar and actin polymerization d efects of the triple 5-phosphatase null mutant yeast strains. Cellular PtdI ns(4,5)P-2 levels in various 5-phosphatase double null mutant strains demon strated significant accumulation (4.5-, 3- and 2-fold for Delta inp51 Delta inp53, Delta inp51 Delta inp52 and Delta inp52 Delta inp53 double null mut ants respectively), which was corrected significantly following 5-phosphata se II expression. Collectively, these studies demonstrate the functional an d cellular consequences of PtdIns(4,5)P-2 accumulation and the evolutionary conservation of function between mammalian and yeast PtdIns(4,5)P-2 5-phos phatases.