Many bacteria show polar characteristics, such as the positioning of a flagellum at one end of the cell. However, the mechanisms involved in establishing this polarity are poorly understood. A recent study from Lucy Shapiro and colleagues has made important progress in understanding these processes, identifying a master regulator of polarity in Caulobacter crescentus.

The C. crescentus life cycle involves two different cell types, both with specialized structures located at one pole of the cell. Several proteins that are involved in the development of these structures and that have corresponding polar distributions have been identified, providing useful markers of polarity. Shapiro and colleagues made use of these features to investigate whether the actin-like protein MreB is required for polarization in C. crescentus.

MreB has a distinctive localization pattern, forming a spiral structure that extends along the length of C. crescentus cells. By analogy to eukaryotic actin, MreB molecules might have an intrinsic polarity, so the spirals they form could be used for the asymmetric localization of molecules required for the development of polar structures. To test this, the authors analysed the effect of MreB depletion on the distribution of four signalling proteins — PleC, DivJ, CckA and DivK — that are required for polar development in C. crescentus. Depletion of MreB abolished the polar foci that are usually formed by all four proteins at certain points in the cell cycle, consistent with a role for MreB as a global regulator of polarity.

Importantly, MreB seems to be actively required for specifying polarity, rather than having a passive role in protein localization. Unlike CckA and DivK, which form foci at both poles of C. crescentus cells, PleC and DivJ are asymmetrically distributed at certain points in the cell cycle, localizing to only one pole. When MreB was depleted and then re-expressed, although polar foci of PleC and DivJ were restored, these were located at the wrong pole in ∼50% of cells. This indicates that when MreB is depleted, cells lose all memory of their initial polarity, so that when MreB is re-expressed polarity becomes randomized. MreB must therefore be required for the initial decision that determines which pole of the cell is which.

This study shows intriguing similarities between the establishment of polarity in C. crescentus and the corresponding processes in eukaryotic cells, in which actin has a central role. It will be interesting to see whether similar mechanisms operate in other bacteria.