Irresistible curves

The DivIVA protein helps to organize cell growth and polarity in Gram-positive bacteria by localizing specifically to the cytoplasmic membrane at the cell poles and division septum, independent of other proteins. In this issue, Lenarcic et al suggest that DivIVA localization depends on the concave geometry of the membrane at those sites. This recognition of membrane curvature is reminiscent of another bacterial protein, SpoVM, as well as eukaryotic BAR-domain proteins. Originally isolated as a regulator of cell-division site placement in Bacillus subtilis, DivIVA and its orthologues are found in a diverse array of Gram-positive bacteria (Edwards et al, 2000; Fadda et al, 2007). In Streptomyces, Corynebacterium and Mycobacterium species that grow by apical extension instead of sidewall extension, DivIVA is required for normal growth. It localizes to cell poles, celldivision septa and emerging branches, which are all areas of active growth (Fadda et al, 2007; Hempel et al, 2008; Kang et al, 2008; Letek et al, 2008). In cocci, such as Streptococcus pneumoniae that grow at their septum, DivIVA preferentially localizes there (Fadda et al, 2007). Although DivIVA localizes similarly to the division septum and cell poles of the rod-shaped bacterium Bacillus subtilis, it is dispensable for cylindrical wall growth, which is orchestrated by the actin homologue MreB. Instead, DivIVA organizes cell polarity in this species. It initially localizes to the invaginating division septum after the cell-division protein FtsZ has assembled the Z ring. DivIVA then recruits MinJ and the MinCD complex, which inhibits aberrant assembly of additional Z rings at the septum and daughter cell poles, thus spatially restricting septal-wall growth to midcell (Bramkamp et al, 2008). In addition, at early stages of B. subtilis sporulation, polarly localized DivIVA interacts with RacA, which binds a centromere-like site on the chromosome. This interaction helps to pull the chromosome poleward into the prespore before the asymmetric septum closes (Bramkamp et al, 2008). The above roles for DivIVA strongly suggest that its polar and septal localization is essential for its function (Figure 1). Importantly, this positioning of DivIVA itself is not dependent on any other known protein: when expressed in species normally lacking DivIVA, such as Escherichia coli or even the yeast Schizosaccharomyces pombe, it still localizes to the cell poles and septum (Edwards et al, 2000). This was the first hint that DivIVA localization might recognize specific membrane lipids, a physical cue, such as membrane curvature, or both. This study by Lenarcic et al addresses this hypothesis. They first showed that DivIVA targets the membrane through a predicted N-terminal amphipathic helix. When fused to GFP and expressed in B. subtilis or E. coli, this helix was sufficient for binding anywhere on the membrane, but the rest of DivIVA was required for specific localization to cell poles or division septa. Using shape mutants of E. coli, the authors then showed that DivIVA– GFP localizes preferentially to regions of sharpest concavity, consistent with the curvature hypothesis. It is unlikely that DivIVA recognizes specific phospholipids that are enriched at the septum and cell poles of B. subtilis, because DivIVA was still able to localize to these regions in mutants deficient in these lipids. These results prompted the authors to test the negative membrane-curvature hypothesis in greater detail. It is likely that multiple DivIVA molecules are involved in recognizing membrane curvature, because DivIVA oligomerizes into ‘doggy-bone’ structures in vitro (Stahlberg et al, 2004), probably through a conserved coiled-coil domain at its C terminus (Edwards et al, 2000; Lenarcic et al, 2009).