"Toward a Structural and Functional Model of the Interaction Between the Small GTPase Arf1 and its ArfGAP ASAP1 at the Membrane Surface"
ADP-ribosylation factor (Arf) GTPase-activating proteins (GAPs) are multidomain enzymes that need to bind to membranes to catalyze the hydrolysis of GTP bound to the small GTP-binding protein Arf. In cells, ASAP1 affects cell behaviors dependent on adhesions and actin, including proliferation, invasion, and metastasis of cancer cells. In vitro, binding of the Pleckstrin homology (PH) domain of the ArfGAP ASAP1 to membranes containing the phosphatidylinositol phosphate PIP(4,5)P2 is key for maximum GTP hydrolysis, but not fully understood. By combining Nuclear Magnetic Resonance, Neutron Reflectometry and Molecular Dynamics simulation, we show that binding of multiple PI(4,5)P2 molecules to the ASAP1 PH domain (i) triggers a functionally relevant allosteric conformational switch involving regions distant from the membrane interface, and (ii) maintains the PH domain in a well-defined orientation, allowing critical contacts between the newly exposed segments and an Arf1 mimic to occur. Our model provides a framework to understand how the interaction between PI(4,5)P2 and the ASAP1 PH domain at the membrane may play a role in the regulation of Arf1 and ASAP1.