Protein tyrosine kinase Pyk2 is activated by a variety of G-protein-coupled receptors and by extracellular signals that elevate intracellular Ca2+ concentration. We have identified a new Pyk2 binding protein designated Pap. Pap is a multidomain protein composed of an N-terminal alpha-helical region with a coiled-coil motif, followed by a pleckstrin homology domain, an Arf-GAP domain, an ankyrin homology region, a proline-rich region, and a C-terminal SH3 domain. We demonstrate that Pap forms a stable complex with Pyk2 and that activation of Pyk2 leads to tyrosine phosphorylation of Pap in living cells. Immunofluorescence experiments demonstrate that Pap is localized in the Golgi apparatus and at the plasma membrane, where it is colocalized with Pyk2. In addition, in vitro recombinant Pap exhibits strong GTPase-activating protein (GAP) activity towards the small GTPases Arf1 and Arf5 and weak activity towards Arf6. Addition of recombinant Pap protein to Golgi preparations prevented Arf-dependent generation of post-Golgi vesicles in vitro. Moreover, overexpression of Pap in cultured cells reduced the constitutive secretion of a marker protein. We propose that Pap functions as a GAP for Arf and that Pyk2 may be involved in regulation of vesicular transport through its interaction with Pap.

The ADP-ribosylation factor (Arf) family of GTP-binding proteins are regulators of membrane traffic and the actin cytoskeleton. Both negative and positive regulators of Arf, the centaurin ␤ family of Arf GTPase-activating proteins (GAPs) and Arf guanine nucleotide exchange factors, contain pleckstrin homology (PH) domains and are activated by phosphoinositides. To understand how the activities are coordinated, we have examined the role of phosphoinositide binding for Arf GAP function using ASAP1/centaurin ␤4 as a model. In contrast to Arf exchange factors, phosphatidylinositol 4,5-bisphosphate (PtdIns-4,5-P 2 ) specifically activated Arf GAP. D3 phosphorylated phosphoinositides were less effective. Activation involved PtdIns-4,5-P 2 binding to the PH domain; however, in contrast to the Arf exchange factors and contrary to predictions based on the current paradigm for PH domains as independently functioning recruitment signals, we found the following: (i) the PH domain was dispensable for targeting to PDGF-induced ruffles; (ii) activation and recruitment could be uncoupled; (iii) the PH domain was necessary for activity even in the absence of phospholipids; and (iv) the Arf GAP domain influenced localization and lipid binding of the PH domain. Furthermore, PtdIns-4,5-P 2 binding to the PH domain caused a conformational change in the Arf GAP domain detected by limited proteolysis. Thus, these data demonstrate that PH domains can function as allosteric sites. In addition, differences from the published properties of the Arf exchange factors suggest a model in which feedforward and feedback loops involving lipid metabolites coordinate GTP binding and hydrolysis by Arf. Arf1 proteins are ubiquitous and essential GTP-binding proteins in eukaryotes. They were first identified as cofactors for cholera toxin-catalyzed ADP-ribosylation of G s (1). The relationship between this activity and the normal cellular function of Arf is not clear. The best described activity of Arf is the regulation of membrane traffic (2, 3). More recently, potential roles in reorganization of the actin cytoskeleton have been identified for Arf1 and Arf6 (4 -7). Neither the molecular mechanism by which Arf proteins regulate these pathways nor the means by which Arf itself is regulated have been delineated; however, phospholipids probably function in both capacities.The relationship between Arf and phospholipids is complex. Arf activates phospholipase D and phosphatidylinositol 4-phosphate 5-kinase, resulting in the production of phosphatidic acid (PA) and PtdIns-4,5-P 2 (8 -11). Phosphoinositides and phosphatidic acid also regulate Arf function. Arf itself binds PtdIns-P 2 (12). Phosphoinositides have a role in converting Arf-GTP to Arf-GDP by the centaurin ␤ family of Arf GAPs. Four members of this family have been identified 2 (13, 14), and one member of the family, ASAP1 (also called centaurin ␤4), was purified as an Arf GAP that is coordinately activated by PA and phosphoinositides (13). The Arf⅐PtdIns-P 2 complex was found to be the substrat...