Mutations in Hnf-1α are the most common Mendelian cause of diabetes mellitus. To elucidate the molecular function of a mutational hotspot, we cocrystallized human HNF-1α 83-279 with a high-affinity promoter and solved the structure of the complex. Two identical protein molecules are bound to the promoter. Each contains a homeodomain and a second domain structurally similar to POU-specific domains that was not predicted on the basis of amino acid sequence. Atypical elements in both domains create a stable interface that further distinguishes HNF-1α from other flexible POU-homeodomain proteins. The numerous diabetes-causing mutations in HNF-1α thus identified a previously unrecognized POU domain which was used as a search model to identify additional POU domain proteins in sequence databases.
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We thank Andrzej Krolewski and Ron Shigeta for helpful discussions, Aneel Aggarwal (Mount Sinai School of Medicine) for coordinates for Pit-1 complexes prior to deposition, the staff at APS (19-ID) and NSLS (X12C and X4A) for help with data collection, and Marco Pontoglio (Insitut Pasteur) for assistance in setting up the transcriptional reporter assay. Supported by NIH grant R01 DK43123 (S.E.S.), post-doctoral fellowships from the Juvenile Diabetes Foundation (Y.-I.C.) and the Mary K. Iacocca Foundation (Y.-I.C., S.D.), and a Burroughs Wellcome Fund Scholar Award in Experimental Therapeutics (S.E.S.).