Crystal structure of the N-terminal domain of MukB: a protein involved in chromosome partitioning

Background: The 170 kDa protein MukB has been implicated in ATP-dependent c hromosome partitioning during cell division in Escherichia coli MukB shares its dimeric structure and domain architecture with the ubiquitous family o f SMC (structural maintenance of chromosomes) proteins that facilitate simi lar functions. The N-terminal domain of MukB carries a putative Walker A nu cleotide-binding region and the C-terminal domain has been shown to bind to DNA. Mutant phenotypes and a domain arrangement similar to motor proteins that move on microtubules led to the suggestion that MukB might be a motor protein acting on DNA. Results: We have cloned, overexpressed and crystallized a 26 kDa protein co nsisting of 227 N-terminal residues of MukB from E. coli. The structure has been solved using multiple anomalous dispersion and has been refined to 2. 2 Angstrom resolution. The N-terminal domain of MukB has a mixed alp fold w ith a central six-stranded antiparallel beta sheet. The putative nucleotide -binding loop, which is part of an unexpected helix-loop-helix motif, is ex posed on the surface and no nucleotide-binding pocket could be detected. Conclusions: The N-terminal domain of MukB has no similarity to the kinesin family of motor proteins or to any other nucleotide-binding protein. Toget her with the finding of the exposed Walker A motif this observation support s a model in which the N- and C-terminal domains come together in the dimer of MukB to form the active site. Conserved residues on one side of the mol ecule delineate a region of the N-terminal domain that is likely to interac t with the C-terminal domain.