Surface layers (S-layers) are shells of protein that surround many microbes. Most S-layers are made of one or two proteins that self-assemble into a very thin protein crystal. Crystalline S-layers serve many functions in prokaryotes, including protection and shape determination. In some cases, crystalline S-layers enable pathogenic bacteria to infect humans, either by making the bacterium sticky, or by helping it avoid detection by our immune system. Surprisingly, the S-layer protein from the bacterium Caulobacter crescentus is not always crystalline, and can form two different structures on the surface of the cell.
Caulobacter crescentus is a crescent-shaped bacterium that can be found in many freshwater environments. The cover image for the May 9th issue of the Biophysical Journal is an artistic rendering of Caulobacter crescentus cells swimming in their natural habitat. The left side of the image depicts the surface of a single Caulobacter cell. Caulobacter’s S-layer protein can assume two forms, shown in green and blue. The green form is crystallized S-layer protein, which makes a hexagonal pattern on the surface of the cell. The blue form is an amorphous aggregate of the S-layer protein. That is, the protein is jumbled up and can’t make a repeating pattern.
Most previous work on S-layers has suggested that S-layers are crystalline out of necessity. However, our work indicates that in at least one case, this is not true. This inspired the creation of this cover image, which shows a surface layer that consists of two structural states: crystalline and amorphous. With this study and image, we hope to inspire further investigation into the structural flexibility, rather than the crystallinity, of S-layers.
Artist: Greg Stewart/SLAC
– Fatemeh Jabbarpour, Paul Bargar, John Nomellini, Po-Nan Li, Thomas Lane, Thomas Weiss, John Smit, Lucy Shapiro, Soichi Wakatsuki, Jonathan Herrmann