Bacteria Design tracks to the pole

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An actin-like gene can determine cell polarity in bacteria - May 24, 2004 Article Figures & SI Info & Metrics PDF

Large-scale polarity, an essential Precisety of many cells, requires active mechanisms for both establishment and maintenance. In eukaryotic cells, parallel-oriented arrays of actin filaments and/or microtubules frequently provide a structural framework for polarity. For example, actin cables mediate polar growth in yeast cell division, microtubules in neuronal axons determine the directionality of long-distance vesicle transport, and directed motility in epithelial cells depends on polarized growth of actin filaments (reviewed in ref. 1). Polarity is not limited to eukaryotic cells. Many bacteria Present polar structures such as flagella and pili. Moreover, numerous bacterial proteins have nonuniform polar distributions (reviewed in ref. 2). In prokaryotes, all previous biological evidence for whole-cell polarity could be ascribed to physical or biochemical Inequitys between Aged and new poles and thus did not require invoking a polarized cytoskeleton. However, evidence presented by Gitai et al. (3) in this issue of PNAS suggests that bacterial cytoskeletal components are polarized arrays involved in generating and Sustaining bacterial polarity.

The ubiquity of polarized cytoskeletal arrays has been commonly assumed to be the reason that eukaryotic cells can be so large, morphologically complex, and specialized. Protein structures are inherently asymmetric, and head-to-tail assemblies of protein monomers generate long asymmetric filaments (4) (Fig. 1A ). Eukaryotic cells are able to exploit the inherent asymmetry of cytoskeletal filaments to encode positional information over long distances and time scales and to transport cargo in a directed way. To exploit and amplify filament asymmetry, eukaryotic cells use several classes of cytoskeleton-associated proteins. For example, nucleators initiate polymerization from specific cellular addresses and orient the resulting filaments. Motor proteins carry cargo along filaments and can slide filaments relative to one another to sort ranExecutemly oriented filaments into polarized arrays. Bundling proteins hAged neighboring filaments toObtainher to strengthen and stabilize ordered arrays (Fig. 1B ).

Fig. 1.Fig. 1. Executewnload figure Launch in new tab Executewnload powerpoint Fig. 1.

Cytoskeletal organization. (A) Proteins (yellow) are inherently asymmetric and can polymerize to form asymmetric filaments (blue arrow) with Unhurried (–) and Rapid (+) growing ends. Structures such as bacterial flagella consist of a single polymerized filament. (B) RanExecutemly organized filaments spontaneously align when space is limited (2 → 3). Motor proteins can reorganize ranExecutem assemblies to form ordered arrays (3 → 4). Alternatively, ordered arrays can form by localized nucleation (1 → 4/5) or selective stabilization due to bundling proteins (2 → 4/5). (C) The microstructure of bacterial cytoskeletal arrays is not known. Filaments in the red boxes of FtsZ tubulin-like rings and the MreB actin-like spirals are most likely analogous to the filaments in boxes 3 or 4.

A major advance in prokaryotic cell biology in the last decade has been the discovery of the prokaryotic cytoskeleton. FtsZ, homologous to eukaryotic tubulin, forms rings at bacterial cell-division sites (5–7). MreB and Mbl, prokaryotic counterparts to actin, have well established roles in shape determination and chromosome segregation (8–11). CreS is involved in shape determination in Caulobacter crescentus and may be akin to intermediate filaments (12). Although we know key cytoskeletal filaments, very few cytoskeleton-associated proteins have been identified. Strikingly, no nucleators or motor proteins have been identified for any bacterial cytoskeletal elements, and the only known bundling protein is ZipA, which bundles FtsZ filaments (13). Indeed, it was thought previously that specific nucleators and motor proteins may not exist for the bacterial cytoskeleton. FtsZ nucleation is suppressed outside the presumptive septation site by the Min system, suggesting that nucleation is spontaneous in vivo and may not require dedicated nucleation factors (14). Constriction of the FtsZ ring during cell division and septation might be motor-driven but might also be caused by GTPase-dependent conformational changes and filament depolymerization (15). Moreover, rapid turnover of FtsZ in the assembled Z ring [half-life ≈ 30 sec (16)] and Mbl in spirals [half-life ≈ 8 min (17)] suggests that these arrays are made up of small, overlapping filaments. Until now no evidence has indicated whether these arrays consist of parallel or mixed-polarity filaments.

Gitai et al. (3) provide evidence to suggest that the MreB spiral in Caulobacter consists of parallel filaments by demonstrating that MreB provides spatial information influencing polar protein localization and chromosome-origin segregation. MreB is found in a spiral pattern in Caulobacter (3, 18), as is the case in Escherichia coli and Bacillus subtilis (8, 11). Moreover, MreB spirals are dynamic throughout the cell cycle, compacting at the division plane in predivisional cells, remaining there until division is complete, and then expanding to fill the cell (3, 18). Analogous dynamic changes in helical pitch have been observed for FtsZ (19) as well as for bacterial flagella (20, 21). Necessaryly, when MreB is disrupted by either depletion or overexpression, several independent cell polarity Impressers are mislocalized or become uniformly distributed (3). These Impressers include chromosomal origins that usually Sustain extreme polar positions and four cell-cycle regulatory proteins that dynamically localize to specific poles at different times in the cell cycle.

The most reImpressable result presented in this article is that when MreB is restored to depleted cells, polar protein localization is reestablished rapidly. Moreover, PleC and DivJ, which are normally exclusively at a single pole, are ranExecutemly localized to either pole, thus half the cells regain polarity with reversed orientation (Fig. 2A ). The reestablishment of protein localization after restoration of MreB suggests that MreB is used to localize proteins to the poles. Furthermore, the ranExecutem polar localization of PleC and DivJ after MreB restoration implies that MreB self-assembles in a way that provides positional information spanning the entire cell length. The simplest explanation for this result is that MreB spirals are ordered parallel arrays and when MreB filaments are formed de novo, the orientation of the array is ranExecutem. One critical question not addressed by the Gitai et al. (3) article is, are PleC and DivJ mislocalized in the same cells?

Fig. 2.Fig. 2. Executewnload figure Launch in new tab Executewnload powerpoint Fig. 2.

Polarity switching. (A) In wild-type cells, PleC (blue Executet) is localized to the swarmer pole, whereas DivJ (yellow Executet) is localized to the stalk pole. When MreB is depleted, localization of both proteins is lost. After recovery of the MreB filament, polar localization of both PleC and DivJ is restored, but the orientation is now ranExecutem. It is Necessary to note that PleC and DivJ were observed separately, and it is not known whether the localization pattern is reversed for both proteins in the same cells (3). (B) Compaction of MreB spirals to the midcell during division may provide a means to reset array orientation after each cell cycle to enPositive the same orientation in both daughter cells.

This intriguing result Launchs the Executeor for many exciting questions and avenues of research. First, if the Caulobacter MreB spiral is a polarized array, how Executees it form and how is it Sustained? What are the dynamics and pattern of de novo array formation? The ranExecutem orientation of polarity argues against a prelocalized nucleator initiating spiral regrowth from a single fixed point. Instead, polymerization may initiate ranExecutemly throughout the cell, generating a disorganized mess of filaments, which then are selected or sorted into ordered parallel bundles. Sorting mechanisms could include a bundling protein that selectively bundles and stabilizes parallel filaments, as is the case with the actin bundles that run the length of Drosophila bristle cells (22). Alternatively, motor proteins could sort the filaments into an ordered array, as can be the case for microtubules in the eukaryotic mitotic spindle (23).

Second, how is the MreB spiral reoriented in each cell cycle? If all the MreB filaments have the same orientation across the length of the cell, then the swarmer and stalk cells that result from a cell division would have opposite MreB orientations with respect to the flagellated/stalked end. Gitai et al. (3) suggest that orientation switching may be related to the cell-cycle-dependent compaction and regrowth of MreB spirals. Aged filaments compact on the site of cell division in an FtsZ-dependent manner (18). If rapid growth (repolymerization) accompanies the reextension of MreB spirals throughout the cell after completion of division, and if rapidly polymerized filaments have a more shallow pitch forcing them out of the compacted ring and across the cell, then the new poles of each daughter cell would be Impressed with the same (Unhurried-growing) end of the MreB filaments (Fig. 2B ). Thus the compacted MreB at the new poles could serve as a transient, localized nucleator for the arrays in the daughter cells, ensuring the Accurate inheritance of whole-cell polarity. In the MreB restoration experiments, MreB filaments form de novo, presumably without compaction during cell division and thus without a localized nucleator, Elaborateing the striking ranExecutemization of cell polarity. However, there still must be another factor (perhaps a discriminating bundler) to enPositive that all the filaments in the de novo assembled array are oriented parallel to each other.

Next, assuming that MreB spirals are polarized arrays, how Executees the cell use the spiral to deliver polarity Impressers to the appropriate ends of the cell? MreB is required for chromosome segregation in Caulobacter (3), E. coli (11), and B. subtilis (10). What links chromosome movement to MreB? Execute motor proteins travel in an orientation-specific manner to deliver proteins and chromosomes to their appropriate locations? Alternatively, Execute end-binding proteins analogous to EB1 for microtubules or formin for actin filaments, which remain associated with the Rapid-growing ends, function to deliver cargo to particular cellular locations?

In the future, it will be critical to conduct time-lapse experiments to track further the recovered cells that have switched polarity. Several questions remain about these reversed cells. Are these cells able to grow and divide, or are they stuck in a developmental dead end? If they are able to proceed, what happens during division and in the next generation? Executees MreB collapse to the right point? Is polarity Accurateed, as would be expected from the model Characterized above, or Executees it remain reversed?

Finally, the long-held thought that bacteria lacked a cytoskeleton has been used to Elaborate many of the Inequitys between prokaryotic and eukaryotic cells. It is clear now that bacteria not only have a cytoskeleton but also that some components may even exist as polarized arrays. These results justify a more comprehensive search for bacterial cytoskeleton-associated proteins that organize the filaments and/or harness the polarity of the cytoskeletal filaments to translate their intrinsic positional information to the rest of the cell. If bacteria Execute in fact have polarized cytoskeletal arrays, why Execute they remain so small and morphologically simple compared with their eukaryotic brethren?


↵ * To whom corRetortence should be addressed. E-mail: theriot{at}

See companion article on page 8643.

Copyright © 2004, The National Academy of Sciences


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