Architecture of Bacterial Cell Division Protein FtsZ Polymers

BPJ_113_8.c1.inddFtsZ is a self-assembling protein that forms the contractile ring guiding the cell division machinery in most bacteria. FtsZ is structurally homologous to tubulin, the subunit of eukaryotic microtubules. FtsZ monomers associate head-to-tail forming  single-stranded filaments that hydrolyze GTP, in a partially understood process. However, how FtsZ filaments organize in the dynamic division ring is still a challenging problem. Rather than forming a well-defined structure, such as band or tubule, FtsZ filaments laterally associate among them in a relatively disordered fashion.  FtsZ filaments bind partner and regulatory proteins, including those tethering them to the inner face of the plasma membrane.

The cover image for the October 17 issue of Biophysical Journal is an artistic representation of the organization that we propose for FtsZ assemblies.  FtsZ filaments made of FtsZ monomers laterally associate through the disordered C-terminal tails, forming loose bundles. Small-angle X-ray solution scattering results (exemplified by the graph on the left) indicated a characteristic 7 nm center-to-center lateral spacing between FtsZ filaments. By modeling comprehensive building and scattering calculations we saw that multiple associated filaments of variable curvature and length were required to reproduce the X-ray scattering features. These calculations also showed a 2-nm gap was left between core filament structures. We hypothesized that the gap would be bridged by the FtsZ intrinsically disordered C-terminal linker region, as in the model bundle in the center of the image. Cryo-electron microscopy provided views of unstained individual assemblies in vitrified solutions (blue background in the bottom half). Analyzing polymers assembled from FtsZ protein constructs with diverse C-termini supported the model.

Combining several biophysical approaches has provided insight into the self-organizing properties of FtsZ that we think underlie the assembly of the bacterial division ring. It should be noted that bacterial division is still a clinically unexplored target for the discovery of new antibacterials needed to counter the spread of antibiotic resistant pathogens.

-Sonia Huecas, Erney Ramirez-Aportela, Albert Vergoñós, Rafael Nuñez-Ramirez, Oscar Llorca, David Juan-Rodriguez, María A. Oliva, Patricia Castellen, and José M. Andreu

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A Unique Look at a Translucent Actin Filament

Image

We are very excited for the opportunity to illustrate our work as cover art for Biophysical Journal.  We recognize and value the importance of visual representation in scientific communication and designed the image with a perspective that captures the functional implications of our research findings.  The image shows a translucent actin filament viewed down the long filament axis. The green and blue spheres represent filament-associated cations–one of each per filament subunit– that promote filament assembly and enhance mechanical stiffness, respectively. Because the actin is translucent, the radial position of the cations along the filament axis can be readily visualized, revealing its rotational symmetry and organization.

–Enrique De La Cruz, Yale University