General Receptor for Phosphoinositides-1-like Pleckstrin homology (PH) domain
GRP1/cytohesin3 and the related proteins ARNO (ARF nucleotide-binding site opener)/cytohesin-2 and cytohesin-1 are ARF exchange factors that contain a pleckstrin homology (PH) domain thought to target these proteins to cell membranes through binding polyphosphoinositides. The PH domains of all three proteins exhibit relatively high affinity for PtdIns(3,4,5)P3. Within the Grp1 family, diglycine (2G) and triglycine (3G) splice variants, differing only in the number of glycine residues in the PH domain, strongly influence the affinity and specificity for phosphoinositides. The 2G variants selectively bind PtdIns(3,4,5)P3 with high affinity,the 3G variants bind PtdIns(3,4,5)P3 with about 30-fold lower affinity and require the polybasic region for plasma membrane targeting. These ARF-GEFs share a common, tripartite structure consisting of an N-terminal coiled-coil domain, a central domain with homology to the yeast protein Sec7, a PH domain, and a C-terminal polybasic region. The Sec7 domain is autoinhibited by conserved elements proximal to the PH domain. GRP1 binds to the DNA binding domain of certain nuclear receptors (TRalpha, TRbeta, AR, ER, but not RXR), and can repress thyroid hormone receptor (TR)-mediated transactivation by decreasing TR-complex formation on thyroid hormone response elements. ARNO promotes sequential activation of Arf6, Cdc42 and Rac1 and insulin secretion. Cytohesin acts as a PI 3-kinase effector mediating biological responses including cell spreading and adhesion, chemotaxis, protein trafficking, and cytoskeletal rearrangements, only some of which appear to depend on their ability to activate ARFs. 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.",