GRF beta, a novel regulator of calcium signaling, is expressed in pancreatic beta cells and brain

By screening for genes expressed differentially in pancreatic beta cells, w e have isolated a cDNA encoding GRF beta, a novel 178-amino acid protein wh ose N terminus is identical to that of GRF1, a calcium-dependent guanine nu cleotide exchange factor, and whose C terminus is unrelated to known protei ns. We show that both GRF1 and GRF beta are expressed selectively in beta c ell lines, pancreatic islet cells and brain. Treatment of beta cell lines ( beta TC1 and HIT) with calcium ionophore led to a significant elevation in activity of the Ras signal transduction pathway, as determined by phosphory lation of extracellular signal-related kinase (ERK). Transfection of beta c ells with a plasmid encoding a dominant negative variant of GRF1 led to 70% reduction in ERK phosphorylation, consistent with a role for GRF1 in calci um-dependent Ras signaling in these cells. To examine the possible function of GRF beta, cultured cells were transfected with a GRF beta expression ve ctor. This led to a significant reduction in both GRF1-dependent ERK phosph orylation and AP1-dependent reporter gene activity. The results suggest tha t GRF1 plays a role in mediating calcium-dependent signal transduction in b eta cells and that GRF beta represents a novel dominant negative modulator of Ras signaling.