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 hydrolyses the plasma membrane-associated lipid, phosphatidylinositol(4,5)bisphosphate (Ptdlns(4,5)P₂) which leads to the production of the two second messenger molecules: inositol(1,4,5)trisphosphate (Ins(1,4,5)P₃) and 1,2-diacylglycerol (DG). In plants, an enzyme which functionally resembles mammalian PIC is known to exist in the plasma membrane, but little is understood about how its activity is regulated. The recent discovery of several plant proteins with 30–40% homology to the mammalian actin- and phosphoinositide-binding protein, profilin, has prompted an investigation as to whether these proteins (plant profilins) are 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 that a direct and highly specific interaction does exist between plant profilin and polyphosphoinositides and that these interactions dramatically affect the ability of plant plasma membrane phosphoinositide phospholipase C to utilize phosphoinositides for second messenger production. These data are the first to demonstrate a functional role of plant profilin in controlling polyphosphoinositide turnover and also provide 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 suggest a possible link between plant cell activation, second messenger production and modulation of cytoskeletal dynamics.