ISLET-SPECIFIC PROTEINS INTERACT WITH THE INSULIN-RESPONSE ELEMENT OFTHE GLUCAGON GENE

Glucagon gene expression is negatively regulated by insulin at the tra nscriptional level. G3, a DNA control element located in the 5'-flanki ng sequence of the rat glucagon gene mediates the inhibition of transc ription, which occurs in response to insulin. We show here that two is let-specific protein complexes C1A and C1B, bind to the A domain of G3 , which is critical for the insulin response. These two complexes bind to overlapping sequences of the A domain and display Very similar bin ding specificities. Point mutations in the A domain that affect bindin g of C1A and C1B result in both decreased G3 enhancer activity and ins ulin-mediated inhibitory effects with a close correlation between dimi nution of binding and function. One of the two complexes, C1A, is simi lar or identical to B1, a protein complex interacting with the upstrea m promoter element of the glucagon gene, G(1), implicated in the A cel l-specific expression of the glucagon gene. Our data indicate that isl et-specific proteins are involved in glucagon gene regulation by insul in.