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Son of Sevenless (SOS) Pleckstrin homology (PH) domain SOS is a Ras guanine nucleotide exchange factor. SOS is thought to transmit signals from activated receptor tyrosine kinases to the Ras signaling pathway. SOS contains a histone domain, Dbl-homology (DH), a PH domain, Rem domain, Cdc25 domain, and a Grb2 binding domain. The SOS PH domain binds to phosphatidylinositol-4,5-bisphosphate (PIP2) and phosphatidic acid (PA). SOS is dependent on Ras binding to the allosteric site via its histone domain for both a lower level of activity (Ras GDP) and maximal activity (Ras GTP). The DH domain blocks the allosteric Ras binding site in SOS. The PH domain is closely associated with the DH domain and the action of the DH-PH unit gates a reciprocal interaction between Ras and SOS. The C-terminal proline-rich domain of SOS binds to the adapter protein Grb2 which localizes the Sos protein to the plasma membrane and diminishes the negative effect of the C-terminal domain on the guanine nucleotide exchange activity of the CDC25-homology domain of SOS. 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.",