Bacterial Armor Holds Clues for Self-Assembling Nanostructures

Many bacteria and archaea encase themselves within a self-assembling protective shell of S-layer proteins, like chainmail armor. The process is a model for the self-assembly of 2D and 3D organic and inorganic nanostructures.

Berkeley Lab Study Reveals Key Details in Formation of S-layer Nanosheets

Imagine thousands of copies of a single protein organizing into a coat of chainmail armor that protects the wearer from harsh and ever-changing environmental conditions. That is the case for many microorganisms. In a new study, researchers with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) have uncovered key details in this natural process that can be used for the self-assembly of nanomaterials into complex two- and three-dimensional structures.

Caroline Ajo-Franklin, a chemist and synthetic biologist at Berkeley Lab’s Molecular Foundry, led this study in which high-throughput light scattering measurements were used to investigate the self-assembly of 2D nanosheets from a common bacterial surface layer (S-layer) protein. This protein, called “SbpA,” forms the protective armor for Lysinibacillus sphaericus, a soil bacterium used as a toxin to control mosquitoes. Their investigation revealed that calcium ions play a key role in how this armor assembles. Two key roles actually.

“Calcium ions not only trigger the folding of the protein into the correct shape for nanosheet formation, but also serve to bind the nanosheets together,” Ajo-Franklin says. “By establishing and using light scattering as a proxy for SbpA nanosheet formation, we were able to determine how varying the concentrations of calcium ions and SbpA affects the size and shape of the S-layer armor.”

Details on this study have been published in the journal ACS Nano in a paper titled “Ion-Specific Control of the Self-Assembly Dynamics of a Nanostructured Protein Lattice.” Ajo-Franklin is the corresponding author. Co-authors are Behzad Rad, Thomas Haxton, Albert Shon, Seong-Ho Shin and Stephen Whitelam.

In the microbial world of bacteria and archaea, external threats abound. Their surrounding environment can transition from extreme heat to extreme cold, or from highly acidic to highly basic. Predators are everywhere. To protect themselves, many bacteria and archaea encase themselves within a shell of S-layer proteins. While scientists have known about this protective coating for many years, how it forms has been a mystery.

Ajo-Franklin and her colleagues have been exploring self-assembling proteins as a potential means of creating nanostructures with complex structure and function.

“At the Molecular Foundry, we’ve gotten really good at making nanomaterials into different shapes but we are still learning how to assemble these materials into organized structures,” she says. “S-layer proteins are abundant biological proteins known to self-assemble into 2D crystalline nanosheets with lattice symmetries and pore sizes that are about the same dimensions as quantum dots and nanotubes. This makes them a compelling model system for the creation of nanostructured arrays of organic and inorganic materials in a bottom-up fashion.”

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