PARTIALLY PURIFIED RHOA-STIMULATED PHOSPHOLIPASE-D ACTIVITY SPECIFICALLY BINDS TO PHOSPHATIDYLINOSITOL 4,5-BISPHOSPHATE

Phosphatidylinositol 4,5-bisphosphate (PIP2) is absolutely required fo r the ADP-ribosylation factor-stimulated phospholipase D (PLD) activit y. In the present study, partially purified rat brain PLD was found to be activated by another PLD activator, RhoA, when PIP2, but not other acidic phospholipids, was included in vesicles comprising phosphatidy lethanolamine (PE) and the PLD substrate phosphatidylcholine (PC) (PE/ PC vesicles), demonstrating the absolute requirement of PIP2 for the R hoA-stimulated PLD activation, too. It is interesting that the RhoA-de pendent PLD activity in the partially purified preparation was drastic ally decreased after the preparation was incubated with and separated from PE/PC vesicles containing PIP2. The PLD activity was extracted by higher concentrations of NaCl from the vesicles containing PIP2 that were incubated with and then separated from the partially purified PLD preparation. These results demonstrate that RhoA-dependent PLD binds to PE/PC vesicles with PIP2. The degree of binding of the RhoA-depende nt PLD activity to the vesicles was totally dependent on the amount of PIP2 in the vesicles and correlated well with the extent of the enzym e activation. Furthermore, it was found that a recombinant peptide of the pleckstrin homology domain of beta-adrenergic receptor kinase fuse d to glutathione S-transferase, which specifically binds to PIP2, inhi bited the PIP2-stimulated, RhoA-dependent PLD activity in a concentrat ion-dependent manner. From these results, it is concluded that in vitr o rat brain PLD translocates to the vesicles containing PIP2, owing to its specific interaction with PIP2, to access its substrate PC, there by catalyzing the hydrolysis of PC. PLD appears to localize exclusivel y on plasma membranes of cells and tissues. An aminoglycoside, neomyci n, that has high affinity for PIP2 effectively extracted the RhoA-depe ndent PLD activity from rat brain membranes. This indicates that PIP2 serves as an anchor to localize PLD on plasma membranes in vivo.