GTP hydrolysis by ADP-ribosylation factor is dependent on both an ADP-ribosylation factor GTPase-activating protein and acid phospholipids.

Abstract

ADP-ribosylation factor (ARF) is a 21-kDa GTP binding protein that regulates eukaryotic membrane traffic. Both the binding and hydrolysis of GTP by ARF have been shown to be necessary for this function. However, purified mammalian ARF lacks intrinsic GTPase activity (< 0.0015 min-1). We document the presence, in bovine brain extracts, of a protein with the predicted properties for an ARF GTPase-activating protein (ARF GAP). This activity was highly dependent on phospholipids. An acid phospholipid fraction from bovine brain (containing primarily phosphatidylinositol 4,5-bisphosphate (PIP2), phosphatidylinositol 4-phosphate, phosphatidylinositol, and phosphatidylserine) had no effect on intrinsic GTPase activity of purified ARF but increased the ARF GAP activity of bovine brain homogenates about 8-fold. This dependence on acid phospholipids was retained after > 100-fold purification of ARF GAP, making it, likely, an inherent property of this reaction. PIP2 alone stimulated ARF GAP activity up to 30-fold with a half-maximal effect at 100-300 microM but had no effect on the GTPase rate of ARF alone. Phosphatidylinositol 4-phosphate was also active but had only 50% of the maximal effect and twice the EC50 of PIP2. Phosphatidylserine, phosphatidylethanolamine, phosphatidylcholine, phosphatidylinositol, and diacylglycerol either alone or in the presence of ARF GAP do not stimulate ARF GTPase activity. ARF proteins have been identified recently as regulators of phospholipase D. The product of the phospholipase D reaction, phosphatidic acid, stimulated ARF GAP approximately 5-fold and reduced the PIP2 concentration needed for GAP stimulation about 6-fold. The substrate of phospholipase D, phosphatidylcholine, inhibited ARF GAP activity, but this inhibition seen with phosphatidylcholine was partially reversed by phosphatidic acid. A feedback loop for the coordinate regulation of phospholipase D and ARF activities is proposed.