The bacterial cytoskeleton.

Abstract

One of the pleasures of teaching introductory biology courses is learning new things about old, familiar subjects … such as the differences between eukaryotes and prokaryotes. For a eukaryotic cell biologist, such learning usually entails examining how bacteria function, in ways other than how they replicate and transcribe DNA and how they synthesize protein. I find it interesting, for example, to understand how bacteria maintain their distinctive spherical, rod-like or spiral shapes; or how they make an external cell wall, if they have one; or how they segregate the products of DNA replication faithfully into daughter cells. New answers to these questions are especially interesting because, in my mistaken eukaryote-centric view, bacteria lack cytoskeletons and cytoskeletal proteins, which might be involved in maintaining cell shape, regulating cell wall synthesis, and erecting something like a mitotic apparatus. Which brings me to a second and equally delightful pleasure derived from teaching introductory biology: debunking worn-out notions. Bacteria do possess cytoskeletons made of proteins which resemble the actin and tubulin familiar to eukaryotic cell biologists. Here I review several, recently published videos that characterize the in vitro behaviors of the actin-like protein, ParM (also known as StbA), and the tubulin-like protein, FtsZ and its in situ localization during cell division. For sake of completeness, I also briefly mention some recent work on the protein crescentin (CreS), an intermediate filament-like molecule, in the absence of published videos. By way of background material, readers may find the recent review by Michie and Lowe (2006) on the dynamics of bacterial cytoskeletal proteins helpful, including the authors' provocative list of “Future Issues to be Resolved.” Also, students and their teachers may wish to compare the videos reviewed below with those involving tubulin and actin (Watters, 2002 , 2004 , 2005 ), which could generate some interesting discussions about the differences and similarities of prokaryotes and eukaryotes. In such discussions, for example, students may raise questions concerning the evolution of “the cytoskeleton”; in which case, they may also find an earlier review on the subject (van den Ent et al., 2001 ) and commentary (Erickson, 2001 ) very helpful, for both an overview and a relevant bibliography. Students and their teachers will also want to discuss critically whether the similarities exhibited by the bacterial and eukaryotic cytoskeletal proteins reflect phylogenetic homologies or, rather, represent good examples of convergent evolution. At this point, readers more familiar with eukaryotic cell biology should be advised the videos being reviewed (as well as the review figures below) were, with one exception, obtained using 100× objectives: that is, at the limits of light microscopy resolution. Thus, the fields of view are small and the images do not seem as large or as sharp as seen in lower-magnification images of eukaryotic cells. Moreover, all but one of the video images were obtained by fluorescence microscopy at low light intensities, which required longer time exposures and time-lapse digital imaging (with accompanying loss of intervening visual detail). In contrast, the set of images portraying FtsZ behavior in vitro was obtained using an atomic force microscope (AFM), which is not a microscope in the usual sense of the term. An AFM lacks lenses and forms an image by means of a probe that traverses the object in a systematic manner, one line at a time. The image formed by the kind of AFM most commonly used for molecular studies is topographical in nature, and image details receive contrast from their size (changes in the z-axis). These images are created in a raster-like manner by the movement of a very fine, whisker-like projection across the surface of an object. AFM resolution, consequently, reflects the size of the probe tip, relative to the detail being probed, and not the diffraction of electromagnetic or electron radiation as seen in more familiar micrographs. With a very small tip, spatial resolution in an AFM image can be very high. Temporal resolution, however, is limited, because of the time usually required to achieve raster-like movements across the field. More information about AFM may be obtained at http://stm2.nrl.navy.mil/how-afm/how-afm.html#General%20concept.