Getting organized — how bacterial cells move proteins and DNA

In recent years, bacterial cells have been shown to be spatially organized by localized protein complexes and dynamic cytoskeletal filaments.Localized protein complexes are frequently based on integral membrane proteins. Their specific subcellular localization occurs by diffusion and capture or targeted membrane insertion.Dynamic cytoskeletal filaments serve as scaffolds that determine the localization of other proteins and provide directionality and force for macromolecular transport processes.Actin homologues of the MreB family assemble into helical cables that line the inner face of the cytoplasmic membrane. They show dynamic subcellular localization patterns and function in the positioning of proteins that are involved in cell-wall biosynthesis.Plasmid segregation is achieved by three-component partitioning systems. One of the components is an actin homologue, a tubulin homologue or a Walker-type ATPase that assembles into dynamic cytoskeletal filaments. The other two components establish centromere-like nucleoprotein complexes that mediate the attachment of plasmids to these filaments and regulate the partitioning process.Bacterial chromosomes have a conserved circular arrangement within the cell, with the loci arrayed in a linear order that reflects their position on the chromosomal DNA. The subcellular location of each locus is determined during DNA segregation.Unlike in eukaryotes, bacteria segregate their chromosomes while DNA replication is in progress. The chromosomal-origin regions are partitioned by an active mechanism that might involve cytoskeletal structures that are formed by the dynamic assembly of Walker-type ATPases.Bacteria have evolved at least three independent mechanisms to define the site of cell division — the Min system, nucleoid occlusion, and the MipZ and ParB system. In all cases, an inhibitor of cell division is dynamically localized within the cell so that cytokinesis is restricted to a region at mid-cell between the two sister nucleoids.