Bacterial cell division spirals into control.

Abstract

Directing proteins to specific subcellular addresses is a central problem in cell biology. Historically, perhaps because of their general lack of compartmentalized organelles, bacteria were viewed as relatively uniform at the subcellular level. However, with advances in fluorescence microscopy came the realization that bacteria, like their eukaryotic counterparts, segregate their proteins to different cellular regions (1). Recent studies have extended this principle by demonstrating that bacteria not only localize intracellular proteins to distinct regions of the cell, but that within these regions they can also organize proteins into discrete ordered structures. For example, the actin homologs MreB and Mbl form spirals along the long axis of Bacillus subtilis (2), and the cytokinetic tubulin homolog FtsZ localizes to a ring that goes through a spiral intermediate to reposition itself during B. subtilis sporulation (3). The work of Shih et al. (4) in this issue of PNAS demonstrates that the ability of intracellular proteins to organize into ordered structures is not exclusive to bacterial cytoskeletal elements. The authors make the exciting observation that proteins that were once thought to diffusely oscillate between the cell poles are in fact present in ordered spirals. Almost all cells divide at a highly reproducible location within the cell. This regularity requires the cell to first find its appropriate division site and then localize its division machinery to that site. This process is perhaps best characterized in the rod-shaped bacterium, Escherichia coli , where three proteins, MinC, MinD, and MinE, collaborate to position the cell division protein FtsZ at midcell (5, 6). FtsZ is a cytosolic tubulin homolog that can polymerize in vitro and assembles into a ring-like structure (the Z ring) in vivo (7, 8). The Z ring assembles at the middle of the cell, which is also the future division site, and together …