INHIBITION OF PLANT PLASMA-MEMBRANE PHOSPHOINOSITIDE PHOSPHOLIPASE-C BY THE ACTIN-BINDING PROTEIN, PROFILIN

A key event in signal transduction in many eukaryotes is activation of polyphosphoinositide-specific phospholipase C (PIC). This enzyme hydr olyses the plasma membrane-associated lipid, phosphatidylinositol(4,5) bisphosphate (PtdIns(4,5)P-2) which leads to the production of the two second messenger molecules: inositol(1,4,5)trisphosphate (Ins(1,4,5)P -3) and 1,2-diacylglycerol (DG). In plants, an enzyme which functional ly resembles mammalian PIC is known to exist in the plasma membrane, b ut little is understood about how its activity is regulated. The recen t discovery of several plant proteins with 30-40% homology to the mamm alian actin- and phosphoinositide-binding protein, profilin, has promp ted an investigation as to whether these proteins (plant profilins) ar e able to interact with polyphosphoinositides and, if so, whether such interactions have physiological relevance for signal transduction via the plant phosphoinositide system. In this study it is demonstrated t hat a direct and highly specific interaction does exist between plant profilin and polyphosphoinositides and that these interactions dramati cally affect the ability of plant plasma membrane phosphoinositide pho spholipase C to utilize phosphoinositides for second messenger product ion. These data are the first to demonstrate a functional role of plan t profilin in controlling polyphosphoinositide turnover and also provi de the first evidence for a direct effect of an actin-binding protein on a membrane-associated signalling enzyme. These findings indicate a novel mechanism for control of plant phosphoinositide turnover, and su ggest a possible link between plant cell activation, second messenger production and modulation of cyto-skeletal dynamics.