SOLUTION STRUCTURE OF THE B-MYB DNA-BINDING DOMAIN - A POSSIBLE ROLE FOR CONFORMATIONAL INSTABILITY OF THE PROTEIN IN DNA-BINDING AND CONTROL OF GENE-EXPRESSION

Double- and triple-resonance heteronuclear NMR spectroscopy have been used to determine the high-resolution solution structure of the minima l B-Myb DNA-binding domain (B-MybR2R3) and to characterize the specifi c complex formed with a synthetic DNA fragment corresponding to the My b target site on the Myb-regulated gene tom-1. B-MybR2R3 is shown to c onsist of two independent protein domains (R2 and R3) joined by a shor t linker, which have strikingly different tertiary structures despite significant sequence similarities, In addition, the C-terminal region of B-Myb R2 is confirmed to have a poorly defined structure, reflectin g the existence of multiple conformations in slow to intermediate exch ange. This contrasts with the tertiary structure reported for c-MybR2R 3, in which both R2 and R3 have the same fold and the C-terminal regio n of R2 forms a stable, well-defined helix [Ogata, K., et al. (1995) N at. Struct. Biol, 2, 309-320]. The NMR data suggest there are extensiv e contacts between B-MybR2R3 and its DNA target site in the complex an d are consistent with a significant conformational change in the prote in on binding to DNA, with one possibility being the formation of a st able helix in the C-terminal region of R2. In addition, conformational heterogeneity identified in R2 of B-MybR2R3 bound to the tom-1-A targ et site may play an important role in the control of gene expression b y Myb proteins.