Backbone dynamics, amide hydrogen exchange, and resonance assignments of the DNA methylphosphotriester repair domain of Escherichia coli Ada using NMR

The 10 kDa amino-terminal fragment of Escherichia coli Ada protein (N-Ada10) repairs methyl phosphotriesters in DNA and possesses a tightly bound zinc ion. The complete resonance assignments of this protein domain have been obtained using multidimensional homonuclear and heteronuclear NMR experiments. The assignments served to study the internal mobility of this protein domain via N-15 relaxation experiments. This involved the measurement of longitudinal and transverse N-15 relaxation rates, as well as the amide proton solvent exchange rates. Relaxation rates in the rotating frame, R(1 rho), of N-15 nuclei were measured at different spin-lock field strengths, leading to the detection of two slow conformational exchange processes at Gly-25 and Gln-73. For the latter, which is next to the active site of this protein domain, the characteristic time of this process was found to be around 60 mu s. The other relaxation experiments unveiled some regions of fast internal motions, faster than the overall correlation time. These motions were found in the N- and C- terminal tails, in segment 33-35 which forms the turn between beta-strands S1 and S2, and residues 47-52 located in a long loop preceding strand S3. The latter loop belongs to the potential DNA binding surface of N-Ada10. While the structure from residue 18 to residue 26 appears not well defined in the calculated structure, the relaxation experiments do not indicate higher mobility for this region. Residues at the N-terminal portion, including the first helix, the sequentially adjacent loop, and part of the second helix, exhibit internal motions close to the time scale of the overall rotational correlation time. This appears to be related to the fact that the first helix has no hydrogen bonds or salt bridges to the rest of the protein and is stabilized only by the involvement of some of its side chains in a hydrophobic core consisting of the side chains of two phenylalanines, a tryptophan, a leucine, and a valine. The four cysteines which bind the zinc show motions on different time scales ranging from microseconds to picoseconds. Thus the motions in the immediate region around the bound zinc of the DNA methyl phosphotriester repair domain are of relatively small amplitude but take place over a wide time range. On the other hand, high mobility is found in the turn connecting S1 and S2 and in the loop preceding S3, regions of the potential DNA binding surface.