Abstract
Lipid phosphorylation was shown to occur on the isolated sarcoplasmic-reticulum (SR) Ca2+-transport ATPase. More than 95% of the radioactivity incorporated on incubation of the SR ATPase with [gamma-32P]ATPMg can be extracted with acidic organic solvents and was identified as 1-(3-sn-phosphatidyl)-1D-myo-inositol 4-phosphate (PtdIns4P) [Varsányi, Toelle, Heilmeyer, Dawson & Irvine (1983) EMBO J. 2, 1543-1548]. This lipid phosphorylation is only observed at nanomolar concentrations of free Ca2+; in the presence of micromolar free Ca2+ PtdIns4P disintegrates rapidly. Also, upon blockade of the kinase reaction PtdIns4P decomposes, indicating a PtdIns/PtdIns4P turnover. The PtdIns4P concentration is dependent on the free Ca2+ concentration, being half-maximal at 35 nM-Ca2+. PtdIns4P hydrolysis is catalysed by a PtdIns4P phosphomonoesterase; accordingly no diacylglycerol is formed, which would be a product of a phosphodiesteratic cleavage. Fluoride inhibits this phosphomonoesterase. Ca2+ does not influence directly either the PtdIns kinase or the PtdIns4P phosphomonoesterase. PtdIns4P forms a tight complex with the transport ATPase, from which it can be removed only by chromatography on heparin-agarose in the presence of Triton X-100. It is concluded that Ca2+ regulates the PtdIns/PtdIns4P turnover by availability of substrate, depending on the Ca2+-transport-ATPase conformation, which traps or exposes the respective lipid head groups.