Type I phosphatidylinositol 4-phosphate 5-kinase directly interacts with ADP-ribosylation factor 1 and is responsible for phosphatidylinositol 4,5-bisphosphate synthesis in the Golgi compartment

Phosphatidylinositol (PtdIns) 4,5-bisphosphate is involved in many aspects of membrane traffic, but the regulation of its synthesis is only partially understood. Golgi membranes contain PI 4-kinase activity and a pool of phos phatidylinositol phosphate (PIP), which is further increased by ADP-ribosyl ation factor 1 (ARF1). COS7 cells were transfected with alpha and beta form s of PI 4-kinase, and only membranes from COS7 cells transfected with PI 4- kinase beta increased their content of PIP when incubated with ARF1. PtdIns (4,5)P-2 content in Golgi membranes was nonexistent but could be increased to a small extent upon adding either cytosol or Type I or Type II PIP kinas es. However, when ARF1 was present, Ptdins(4,5)P-2 levels increased dramati cally when membranes were incubated in the presence of cytosol or Type I, b ut not Type II, PIP kinase. To examine whether ARF1 could directly activate Type I PIP 5-kinase, we used an in vitro assay consisting of phosphatidych oline-containing liposomes, ARF1, and PIP 5-kinase, ARF1 increased Type I P IP 5-kinase activity in a guanine nucleotide-dependent manner, identifying this enzyme as a direct effector for ARF1.