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Src kinase-associated phosphoprotein 1 Pleckstrin homology (PH) domain Adaptor protein Skap1 (also called Skap55/Src kinase-associated phosphoprotein of 55 kDa) and its partner, ADAP (adhesion and degranulation promoting adapter protein) help reorganize the cytoskeleton and/or promote integrin-mediated adhesion upon immunoreceptor activation. Skap1 is also involved in T Cell Receptor (TCR)-induced RapL-Rap1 complex formation and LFA-1 activation. Skap1 has an N-terminal coiled-coil conformation which is proposed to be involved in homodimer formation, a central PH domain and a C-terminal SH3 domain that associates with ADAP. The Skap1 PH domain plays a role in controlling integrin function via recruitment of ADAP-SKAP complexes to integrins as well as in controlling the ability of ADAP to interact with the CBM signalosome and regulate NF-kappaB. SKAP1 is necessary for RapL binding to membranes in a PH domain-dependent manner and the PI3K pathway. Skap adaptor proteins couple receptors to cytoskeletal rearrangements. Skap55/Skap1, Skap2, and Skap-homology (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.",