FERM domain-containing protein 4A (FRMD4A) is part of the Par-3/FRMD4A/cytohesin-1 complex that activates Arf6, a central player in actin cytoskeleton dynamics and membrane trafficking, during junctional remodeling and epithelial polarization. The Par-3/Par-6/aPKC/Cdc42 complex regulates the conversion of primordial adherens junctions (AJs) into belt-like AJs and the formation of linear actin cables. When primordial AJs are formed, Par-3 recruits scaffolding protein FRMD4A which connects Par-3 and the Arf6 guanine-nucleotide exchange factor (GEF), cytohesin-1 [].FERM domain-containing protein 4B (FRMD4B, also called GRP1-binding protein, GRSP1) is a novel member of GRP1 signaling complexes that are recruited to plasma membrane ruffles in response to insulin receptor signaling. The GRSP1/FRMD4B protein contains a FERM protein domain as well as two coiled coil domains and may function as a scaffolding protein. GRP1 and GRSP1 interact through the coiled coil domains in the two proteins []. The FERM domain has a cloverleaf tripart structure composed of: (1) FERM_N (A-lobe or F1); (2) FERM_M (B-lobe, or F2); and (3) FERM_C (C-lobe or F3). The C-lobe/F3 within the FERM domain is part of the PH domain family. Like most other ERM members they have a phosphoinositide-binding site in their FERM domain. The FERM C domain is the third structural domain within the FERM domain. The FERM domain is found in the cytoskeletal-associated proteins such as ezrin, moesin, radixin, 4.1R, and merlin. These proteins provide a link between the membrane and cytoskeleton and are involved in signal transduction pathways. The FERM domain is also found in protein tyrosine phosphatases (PTPs) , the tyrosine kinases FAK and JAK, in addition to other proteins involved in signaling. This domain is structurally similar to the PH and PTB domains and consequently is capable of binding to both peptides and phospholipids at different sites [, ].
