A Repeated Coiled-Coil Interruption in the Escherichiacoli Condensin MukB

MukB, a divergent structural maintenance of chromosomes (SMC) protein, is important for chromosome segregation and condensation in Escherichia coli and other γ-proteobacteria. MukB and canonical SMC proteins share a common five-domain structure in which globular N- and C-terminal regions combine to form an ATP-binding-cassette-like ATPase domain. This ATPase domain is connected to a central, globular dimerization domain by a long antiparallel coiled coil. The structures of both globular domains have been solved recently. In contrast, little is known about the coiled coil, in spite of its clear importance for SMC function.Recently, we identified interacting regions on the N- and C-terminal halves of the MukB coiled coil through photoaffinity cross-linking experiments. On the basis of these low-resolution experimental constraints, phylogenetic data, and coiled-coil prediction analysis, we proposed a preliminary model in which the MukB coiled coil is divided into multiple segments. Here, we use a disulfide cross-linking assay to detect paired residues on opposite strands of MukB's coiled coil. This method provides accurate register data and demonstrates the presence of at least five coiled-coil segments in this domain. Moreover, these studies show that the segments are interrupted by a repeated, unprecedented deviation from canonical coiled-coil structure. These experiments provide a sufficiently detailed view of the MukB coiled coil to allow rational manipulation of this region for the first time, opening the door for structure–function studies of this domain.

Graphical AbstractFigure optionsDownload high-quality image (145 K)Download as PowerPoint slideResearch Highlights
► MukB is a bacterial condensin involved in proper chromosome segregation.
► This protein contains a long, segmented, and structurally uncharacterized coiled coil.
► Disulfide cross-linking was used to identify intrafacial a–d′ pairs along the coiled coil.
► Five segments were defined, each disrupted by a novel, repeated coiled-coil deviation.
► These data provide a model at sufficient resolution for rational structure–function studies.