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T-lymphoma invasion and metastasis 1 and 2 Pleckstrin Homology (PH) domain, N-terminal domain Tiam1 activates Rac GTPases to induce membrane ruffling and cell motility while Tiam2 (also called STEF (SIF (still life) and Tiam1 like-exchange factor) contributes to neurite growth. Tiam1/2 are Dbl-family of GEFs that possess a Dbl(DH) domain with a PH domain in tandem. DH-PH domain catalyzes the GDP/GTP exchange reaction in the GTPase cycle and facillitating the switch between inactive GDP-bound and active GTP-bound states. Tiam1/2 possess two PH domains, which are often referred to as PHn and PHc domains. The DH-PH tandem domain is made up of the PHc domain while the PHn is part of a novel N-terminal PHCCEx domain which is made up of the PHn domain, a coiled coil region(CC), and an extra region (Ex). PHCCEx mediates binding to plasma membranes and signalling proteins in the activation of Rac GTPases. The PH domain resembles the beta-spectrin PH domain, suggesting non-canonical phosphatidylinositol binding. CC and Ex form a positively charged surface for protein binding. There are 2 motifs in Tiam1/2-interacting proteins that bind to the PHCCEx domain: Motif-I in CD44, ephrinBs, and the NMDA receptor and Motif-II in Par3 and JIP2.Neither of these fall in the PHn domain. 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.",