Of the three critical enhancer elements that mediate ␤-cell-specific and glucose-responsive expression of the insulin gene, only the identity of the transcription factor binding to the RIPE3b element (RIPE3b1) has remained elusive. Using a biochemical purification approach, we have identified the RIPE3b1 factor as a mammalian homologue of avian MafA/L-Maf (mMafA). The avian MafA is a cell-type determination factor that expressed ectopically can trigger lens differentiation program, but no mammalian homologue of avian MafA has previously been identified. Here, we report cloning of the human mafA (hMafA) and demonstrate that it can specifically bind the insulin enhancer element RIPE3b and activate insulingene expression. In addition, mMafA has a very restrictive cellular distribution and is selectively expressed in pancreatic ␤ but not in ␣ cells. We suggest that mMafA has an essential role in the function and differentiation of ␤-cells and thus may be associated with the pathophysiological origins of diabetes. Pancreatic ␤-cells synthesize and secrete insulin, which is essential for the maintenance of normal metabolism. Thus, a reduction in the functional mass of pancreatic ␤-cells results in diabetes. ␤-cell-specific expression of the insulin gene is regulated by transcription factors binding to three conserved insulin enhancer elements [E1 (Ϫ100 to Ϫ91 bp), A3 (Ϫ201 to Ϫ196 bp) and RIPE3b (Ϫ126 to Ϫ101 bp)] (1-5). Two of the three factors binding to these elements, PDX-1 and BETA2, have been identified and found to have profound roles in regulating pancreatic development and the differentiation of ␤-cells (6-13). Thus, transcription factors regulating insulin gene expression are key mediators of development, differentiation, and function of ␤-cells. Hence, identification and characterization of insulin gene transcription factors is critical for understanding the pathophysiology of diabetes.Pancreatic ␤-cell-specific insulin gene expression results from the expression of a unique combination of PDX-1, BETA2, and RIPE3b1 factors in this cell type. The transcription factor PDX-1 is expressed in pancreatic ␤-cells and has a heterogeneous expression pattern in other pancreatic cell types and in the duodenum (6)(7)(8)(9)(10)14). BETA2 is expressed in all pancreatic endocrine cell types, some intestinal endocrine cells, and the brain (11-13). The cellular distribution of RIPE3b-binding activity has been characterized by electrophoretic mobility-shift assay (EMSA) with nuclear extracts from both insulin-producing and non-insulin-producing cell lines (2,3,15,16). Two specific RIPE3b-binding complexes have been identified:(i) RIPE3b1 detected only in pancreatic ␤-cell lines, and (ii) the RIPE3b2-binding complex detected in all cell lines examined. The binding of these transcription factors to the A3, E1, and RIPE3b elements also regulates glucose-mediated alterations in insulin gene expression (3,(17)(18)(19). Our earlier studies demonstrated that the binding activity of the RIPE3b1 activator was induced in response to a...

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