The highly conserved DNA-binding domains of A-, B- and c-Myb differ with respect to DNA-binding, phosphorylation and redox properties.
In the Myb family, as in other families of transcription factors sharing similar DNA-binding domains (DBDs), diversity of function is believed to rely mainly on the less conserved parts of the proteins and on their distinct patterns of expression. However, small conserved differences between DBDs of individual members could play a role in fine-tuning their function. We have compared the highly conserved DBDs of the three vertebrate Myb proteins (A-, B- and c-Myb) and found distinct functional differences. While A- and c-Myb behaved virtually identically in a variety of DNA-binding assays, B-Myb formed complexes of comparatively lower stability, rapidly dissociating under competitive conditions and showing less tolerance to binding site variations. The three protein domains also differed as substrates for protein kinases. Whereas PKA in theory should target the DBDs of A- and c-Myb, but not B-Myb, only c-Myb was phosphorylated by PKA. CK2 phosphorylated all three proteins, although on different sites in the N-terminal region. Finally, B-Myb was remarkably sensitive to cysteine-directed oxidation compared to the other Myb proteins. Our data suggest that the small differences that have evolved between individual Myb family members lead to clear differences in DBD properties even if their sequence recognition remains the same.