Superstructure formation by RodZ hexamers of Shigella sonnei maintains the rod shape of bacilli

The rod shape of bacilli is maintained by bacterial cytoskeletal protein MreB, an actin homolog that acts in concert with the inner membrane protein RodZ. We previously reported RodZ binds RNA to control the posttranscriptional regulation of invE (virB), which controls the type III secretion system essential for the virulence of Shigella. Here, we show that purified RodZ forms "superstructures" of high molecular mass that dissociate into a midsized "basal complex" in the presence of nonionic detergent, or to a monomer in the presence of dithiothreitol. We used mass spectrometry to show that the basal complex was a hexamer. Electrophoresis mobility shift assays combined with gel filtration detected the RNA-binding activity in fractions containing molecules larger than the basal hexamer. The superstructure was consistently detected with MreB in crude cell lysates of S. sonnei that were fractionated using gel filtration. Immunofluorescence microscopy using two different super-resolution settings showed that wild-type RodZ was distributed in cells as separate dots. Consistent with the superstructure comprising homohexamers, majority of the dots distributed among areas of discrete values. In addition, simultaneous immunodetection of MreB provided the first evidence of colocalization with RodZ as larger patch like signals. These findings indicate that native RodZ forms clusters of various sizes, which may correspond to a superstructure comprising multiple hexamers required for the RNA-binding activity.