Signaling complexes of the FERM domain-containing protein GRSP1 bound to ARF exchange factor GRP1

GRP1 is a member of a family of proteins that contain a coiled-coil region, a Sec7 homology domain with guanosine nucleotide exchange activity for the ARF GTP-binding proteins, and a pleckstrin homology domain at the C termin us. The pleckstrin homology domain of GRP1 binds phosphatidylinositol (3,4, 5) trisphosphate and mediates the translocation of GRP1 to the plasma membr ane upon agonist stimulation of PI 3-kinase activity. Using a P-32-labeled GRP1 probe to screen a mouse brain cDNA expression library, we isolated a c DNA clone encoding a GRP1-binding partner (GRSP1) that exists as two differ ent splice variants in brain and lung. The GRSP1 protein contains a FERM pr otein interaction domain as well as two coiled coil domains and may therefo re function as a scaffolding protein. Mapping experiments revealed that the interaction of GRP1 and GRSP1 occurs through the coiled coil domains in th e two proteins. Immunodepletion experiments indicate that virtually all of the endogenous GRSP1 protein exists as a complex with GRP1 in lung. When co -expressed in Chinese hamster ovary cells expressing the human insulin rece ptor, both proteins display a diffuse, cytoplasmic localization. Acute tran slocation and co-localization of GRSP1 and GRP1 to ruffles in the plasma me mbrane was evident after insulin stimulation. These results identify GRSP1 as a novel member of GRP1 signaling complexes that are acutely recruited to plasma membrane ruffles in response to insulin receptor signaling.