The X-ray structure of the DNA-binding domain from the Saccharomyces cerevisiae cell-cycle transcription factor Mbp1 at 2.1 angstrom resolution

The structure of the DNA-binding domain of the Saccharomyces cerevisiae cell-cycle transcription factor Mbp1 has been solved using the multi-wavelength anomalous diffraction (MAD) technique on crystals of seleno-methionyl protein and refined at 2.1 Angstrom resolution. The molecule is globular, consisting of a twisted, six-stranded beta-barrel that is packed against a loose bundle of four alpha-helices. Two of the beta-strands in combination with two of the helices form a structure characteristic of the DNA-binding motif found in the CAP family of helix-turn-helix transcription factors. In Mbp1, this beta(2) alpha(2) motif is associated with regions of both positive electrostatic potential and sequence conservation within the Mbp1/Swi4 family, suggesting a role in DNA-binding in these proteins. A combination of structural and biochemical data further indicate a similarity to HNF3 gamma/fork head, a member of the family of ''winged'' helix-turn-helix proteins. We propose a model for DNA-binding involving a recognition helix in the major groove, phosphodiester backbone interactions through the beta-hairpin and further base and/or phosphate interactions mediated by a C-terminal, positively charged loop. (C) 1997 Academic Press Limited.