GLUCAGON GENE G3 ENHANCER - EVIDENCE THAT ACTIVITY DEPENDS ON COMBINATION OF AN ISLET-SPECIFIC FACTOR AND A WINGED HELIX PROTEIN

The peptide hormone glucagon is expressed in A cells of the pancreatic islets due to an interaction between multiple regulatory elements wit hin the 5'-flanking region of its gene directing glucagon gene transcr iption. An A-cell-specific enhancer-like element in the rat glucagon g ene, G3, contains two domains, both of which are necessary for G3 acti vity. Domain A of the G3 element comprises a sequence motif, PISCES, t hat is also found in control elements of the rat insulin I and somatos tatin genes exhibiting cell-specific transcriptional activities distin ct from G3. In this study, the nuclear proteins binding to domain B of G3 were characterized. In electrophoretic mobility shift assays using nuclear extracts from a glucagon-producing islet cell line, it was ob served that the binding specificity of G3-domain-B-binding proteins is related to that of winged helix proteins supporting the hypothesis th at the proteins binding to domain B of G3 may belong to the winged hel ix protein family of transcription factors. The overexpression of a do minant-negative winged helix protein mutant (derived from HNF-3) virtu ally abolished the transcriptional activity of G3 in a glucagon-expres sing islet cell line. These results suggest that the unique A-cell-spe cific basal transcriptional activity of the glucagon G3 element depend s on a combination of at least two proteins, the islet specific PISCES -binding protein and a more widely expressed winged helix protein.