Coated vesicles contain a phosphatidylinositol kinase

When coated vesicles (CVs) are incubated with [gamma-32P]ATP, radioactivity is rapidly incorporated into a compound identified by thin layer chromatography as phosphatidylinositol 4-phosphate. This activity has been identified in CVs isolated from bovine brain as well as from rat liver and chick embryo skeletal muscle. Phosphatidylinositol (PI) kinase is not separated from CVs during agarose electrophoresis, which produces CVs of greater than 95% purity, indicating that the activity present does not derive from contamination. The specific activity of these highly purified CVs was demonstrated to be approximately twice that of synaptic plasma membranes, further ruling out contamination from this source. The PI kinase remains associated with the vesicle upon removal of clathrin and its associated proteins and is solubilized by nonionic detergents, suggesting it is an integral membrane protein. The authors have been unable to demonstrate the formation of significant amounts of phosphatidylinositol 4,5-bisphosphate in any of the CV preparations. In the presence of exogenous PI, activity is stimulated, with maximal phosphorylation occurring at 0.1 mM. The enzyme appears to be maximally stimulated by 200 mM MgCl2 and 1 mM ATP and is most active at pH 7.25. Calculations indicate that, under optimal conditions, approximately 25 molecules of PIP are produced per CV within 60 s, suggesting that these structures may play an important role in cellular PI metabolism