Rho GTPase-activating proteins (RhoGAPs or ARHGAPs) bind to Rho proteins and enhance the hydrolysis rates of bound GTP. ARHGAP9 functions as a GAP for Rac and Cdc42, but not for RhoA []. It negatively regulates cell migration and adhesion []. It also acts as a docking protein for the MAP kinases Erk2 and p38alpha, and may facilitate cross-talk between the Rho GTPase and MAPK pathways to control actin remodeling []. It contains SH3, WW, Pleckstin homology (PH), and RhoGAP domains. This entry represents the SH3 domain found in ARHGAP9.
Slm1 is a component of the target of rapamycin complex 2 (TORC2) signaling pathway. It plays a role in the regulation of actin organization and is a target of sphingolipid signaling during the heat shock response []. Slm1 contains a single PH domain that binds PtdIns(4,5)P2, PtdIns(4)P, and dihydrosphingosine 1-phosphate (DHS-1P). Slm1 possesses two binding sites for anionic lipids. The non-canonical binding site of the PH domain of Slm1 is used for ligand binding, and it is proposed that beta-spectrin, Tiam1 and ArhGAP9 also have this type of phosphoinositide binding site []. PH domains have diverse functions, but in general are involved in targeting proteins to the appropriate cellular location or in the interaction with a binding partner []. They share little sequence conservation, but all have a common fold, which is electrostatically polarized. Less than 10% of PH domains bind phosphoinositide phosphates (PIPs) with high affinity and specificity []. PH domains are distinguished from other PIP-binding domains by their specific high-affinity binding to PIPs with two vicinal phosphate groups: PtdIns(3,4)P2, PtdIns(4,5)P2 or PtdIns(3,4,5)P3 which results in targeting some PH domain proteins to the plasma membrane []. A few display strong specificity in lipid binding. Any specificity is usually determined by loop regions or insertions in the N terminus of the domain, which are not conserved across all PH domains. PH domains are found in cellular signaling proteins such as serine/threonine kinase, tyrosine kinases, regulators of G-proteins, endocytotic GTPases, adaptors, as well as cytoskeletal associated molecules and in lipid associated enzymes [].
