FERM, RhoGEF and pleckstrin domain-containing protein 1 and related proteins Pleckstrin Homology (PH) domain, repeat 2
Members here include FARP1 (also called Chondrocyte-derived ezrin-like protein; PH domain-containing family C member 2), FARP2 (also called FIR/FERM domain including RhoGEF; FGD1-related Cdc42-GEF/FRG), and FARP6 (also called Zinc finger FYVE domain-containing protein 24). They are members of the Dbl family guanine nucleotide exchange factors (GEFs) which are upstream positive regulators of Rho GTPases. Little is known about FARP1 and FARP6, though FARP1 has increased expression in differentiated chondrocytes. FARP2 is thought to regulate neurite remodeling by mediating the signaling pathways from membrane proteins to Rac. It is found in brain, lung, and testis, as well as embryonic hippocampal and cortical neurons. FARP1 and FARP2 are composed of a N-terminal FERM domain, a proline-rich (PR) domain, Dbl-homology (DH), and two C-terminal PH domains. FARP6 is composed of Dbl-homology (DH), and two C-terminal PH domains separated by a FYVE domain. This hierarchy contains the second PH repeat. 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.",