ARHGAP family genes encode Rho/Rac/Cdc42-like GTPase activating proteins with RhoGAP domain. The ARHGAP members here all have a PH domain upstream of their C-terminal RhoGAP domain. Some have additional N-terminal SH3 and WW domains. The members here include: ARHGAP9, ARHGAP12, ARHGAP15, and ARHGAP27. ARHGAP27 and ARHGAP12 shared the common-domain structure, consisting of SH3, WW, PH, and RhoGAP domains. The PH domain of ArhGAP9 employs a non-canonical phosphoinositide binding mechanism, a variation of the spectrin- Ins(4,5)P2-binding mode, that gives rise to a unique PI binding profile, namely a preference for both PI(4,5)P2 and the PI 3-kinase products PI(3,4,5)P3 and PI(3,4)P2. This lipid binding mechanism is also employed by the PH domain of Tiam1 and Slm1. 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.
Comment:the canonical binding site is delimited by the beta1- beta2 and beta3- beta4 loops; the non-canonical binding site bordered by the beta1- beta2 and beta5- beta6 loops
Jiyao W et al.(2023). "The conserved domain database in 2023.",