Adaptor protein Skap-hom, a homolog of Skap55, which interacts with actin and with ADAP (adhesion and degranulation promoting adapter protein) undergoes tyrosine phosphorylation in response to plating of bone marrow-derived macrophages on fibronectin. Skap-hom has an N-terminal coiled-coil conformation that is involved in homodimer formation, a central PH domain and a C-terminal SH3 domain that associates with ADAP. The Skap-hom PH domain regulates intracellular targeting; its interaction with the DM domain inhibits Skap-hom actin-based ruffles in macrophages and its binding to 3'-phosphoinositides reverses this autoinhibition. The Skap-hom PH domain binds PI[3,4]P2 and PI[3,4,5]P3, but not to PI[3]P, PI[5]P, or PI[4,5]P2. Skap2 is a downstream target of Heat shock transcription factor 4 (HSF4) and functions in the regulation of actin reorganization during lens differentiation. It is thought that SKAP2 anchors the complex of tyrosine kinase adaptor protein 2 (NCK20/focal adhesion to fibroblast growth factor receptors at the lamellipodium in lens epithelial cells. Skap2 has an N-terminal coiled-coil conformation which interacts with the SH2 domain of NCK2, a central PH domain and a C-terminal SH3 domain that associates with ADAP (adhesion and degranulation promoting adapter protein)/FYB (the Fyn binding protein). Skap2 PH domain binds to membrane lipids. Skap adaptor proteins couple receptors to cytoskeletal rearrangements. Src kinase-associated phosphoprotein of 55 kDa (Skap55)/Src kinase-associated phosphoprotein 1 (Skap1), Skap2, and Skap-hom have an N-terminal coiled-coil conformation, a central PH domain and a C-terminal SH3 domain. Their PH domains bind 3'-phosphoinositides as well as directly affecting targets such as in Skap55 where it directly affecting integrin regulation by ADAP and NF-kappaB activation or in Skap-hom where the dimerization and PH domains comprise a 3'-phosphoinositide-gated molecular switch that controls ruffle formation. 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.
Jiyao W et al.(2023). "The conserved domain database in 2023.",