A protein shapes bacteria.
Bacteria exist in numerous shapes, which influence their survival in different environments. Despite extensive studies, the specific factors that dictate the shapes of bacterial cells are still largely unclear. A research team led by Martin Thanbichler has uncovered the mechanism responsible for the spiral form of Rhodospirillum, providing fresh insights into the relationship between cell shape and survival.
Bacteria display an astonishing array of shapes. In addition to the well-known rod-shaped bacterium E. coli, many species exhibit curves and spirals. The curvature is vital for bacteria to attach to surfaces and navigate thick environments, which can also lead to diseases like those caused by Vibrio cholerae or Helicobacter pylori. Researchers around the world are investigating the molecular details of bacterial curvature, aiming to manipulate it to develop treatments against pathogens.
An international team of researchers, led by Max Planck Fellow Martin Thanbichler, who is a Professor at the University of Marburg in Germany, has shed light on the structure of the photosynthetic bacterium Rhodospirillum rubrum. This species is commonly found in nature and has potential in biotechnology due to its ability to use carbon monoxide, fix nitrogen, and generate hydrogen as well as components for bioplastics.
The team made an unexpected discovery that in Rhodospirillum, two porins—channel proteins previously thought to only facilitate nutrient exchange across the outer membrane—are arranged in a helical pattern along the outer curvature of the cell. These structures connect closely with the cell wall through another protein known as PapS. When PapS was absent or unable to bind to the porins, the cells straightened completely.
Porins with dual roles
So, what makes PapS crucial for maintaining cell curvature? “The porins appear to have developed an additional function beyond nutrient exchange,” explains Martin Thanbichler. “Along with PapS, they regulate a molecular machine that rotates around the cell body. This machine adds new material to the cell wall, promoting elongation. In rod-shaped bacteria like E. coli, this machine moves evenly across the cell, keeping it straight. However, in R. rubrum, the helical Porin-PapS arrangement forms a type of molecular cage. Its tight configuration encircles the machinery responsible for extending the cell length, partially fixing it along the cell’s outer curve. This localized elongation at the Porin-PapS site causes the cell body to twist into a spiral form.”
This study, a collaboration between the Marburg team and researchers from Kiel, Freiburg, England, and Australia, has revealed a new method that bacteria use to determine their shape, directly influenced by outer membrane proteins on how cell growth is controlled spatially. The discoveries are likely applicable to all curved relatives of Rhodospirillum, and it will be intriguing to learn if this mechanism is utilized by other bacteria with more complex shapes.
“We now have the chance to alter the cell shape of R. rubrum, enabling us to investigate the advantages of the helical shape for these bacteria in their environment,” states Sebastian Pöhl, the study’s lead author. This may lead to valuable insights into how cell shape influences the occupation of ecological niches, the formation of symbiotic relationships with plants, or disease development.
