Structure of FocB - a member of a family of transcription factors regulating fimbrial adhesin expression in uropathogenic Escherichia coli

In uropathogenic Escherichia coli, UPEC, different types of fimbriae are expressed to mediate interactions with host tissue. FocB belongs to the PapB family of transcription factors involved in the regulation of fimbriae gene clusters. Recent findings suggest that members from this family of proteins may form homomeric or heteromeric complexes and exert both positive and negative effects on the transcription of fimbriae genes. To elucidate the detailed function of FocB, we have determined its crystal structure at 1.4 angstrom resolution. FocB is an all alpha-helical protein with a helix-turn-helix motif. Interestingly, conserved residues important for DNA-binding are located not in the postulated recognition helix of the motif, but in the preceding helix. Results from protein-DNA-binding studies suggest that FocB interacts with the minor groove of its cognate DNA target, which is indicative of a DNA interaction that is unusual for this motif. FocB crystallizes in the form of dimers. Packing interactions in the crystals give two plausible dimerization interfaces. Conserved residues, known to be important for protein oligomerization, are present at both interfaces, suggesting that both sites could play a role in a functional FocB protein.