The structure of the DNA-binding domain of the Saccharomyces cerevisiae cell-cycle transcription factor Mbp1 has been solved using the multiwavelength anomalous diffraction (MAD) technique on crystals of selenomethionyl protein and refined at 2.1 A 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 beta2/alpha2 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 HNF3gamma/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.
Copyright 1997 Academic Press Limited.