MOLECULAR AND ENZYMATIC CHARACTERIZATION OF 3 PHOSPHOINOSITIDE-SPECIFIC PHOSPHOLIPASE-C ISOFORMS FROM POTATO

Many cellular responses to stimulation of cell-surface receptors by ex tracellular signals are transmitted across the plasma membrane by hydr olysis of phosphatidylinositol-4,5-bisphosphate (PIP2), which is cleav ed into diacylglycerol and inositol-1,4,5-trisphosphate by phosphoinos itide-specific phospholipase C (PI-PLC). We present structural, bioche mical, and RNA expression data for three distinct PI-PLC isoforms, StP LC1, StPLC2, and StPLC3, which were cloned from a guard cell-enriched tissue preparation of potato (Solanum tuberosum) leaves. All three enz ymes contain the catalytic X and Y domains, as well as C-2-like domain s also present in all PI-PLCs. Analysis of the reaction products obtai ned from PIP2 hydrolysis unequivocally identified these enzymes as gen uine PI-PLC isoforms. Recombinant StPLCs showed an optimal PIP2-hydrol yzing activity at 10 mu M Ca2+ and were inhibited by Al3+ in equimolar amounts. In contrast to PI-PLC activity in plant plasma membranes, ho wever, recombinant enzymes could not be activated by Mg2+. All three s tplc genes are expressed in various tissues of potato, including leave s, flowers, tubers, and roots, and are affected by drought stress in a gene-specific manner.