A synthetic material inspired by the natural protective shell of bacteria, could someday yield a plethora of new technologies with applications in biofuel production, medicine and the industrial sector.

Researchers from the lab of Cheryl Kerfield at Michigan State University have designed a genetically engineered shell based on natural structures and the principles of protein evolution in bacteria.

Bacteria is comprised of nanometer-sized factories that have a number of different jobs, including making nutrients and isolating toxic materials that can cause harm, but all include a common exterior shell made of protein tiles.

Bacterial microcompartments are subcellular compartments found in many prokaryotes, consisting of a protein shell that encapsulates enzymes to perform a variety of functions. The shell is crucial in protecting the cell from potentially toxic intermediates and colocalizes enzymes for higher efficiency.

This structure could be of particular value to biotechnological applications if it can be artificially duplicated in the lab.

Previously, the researchers structurally characterized an intact 40-nanometer shell comprised of three different types of proteins, including BMC-H, which forms a cyclic hexamer and is the most abundant protein found in bacteria shells.

The three proteins merged together at some point in their evolutionary history to form BMC-T in a hexagonal shape.

“The two halves of a BMC-T protein can evolve separately while staying next to each other, because they are fused together,” Bryan Ferlez, a postdoc in the Kerfeld lab, said in a statement. “This evolution allows for diversity in the structures and functions of BMC-T shell proteins, something that we want to recreate by design in the lab.”

In the new study, the research team engineered a synthetic version of the protein that consists of a tandem duplication of BMC-H that is connected by a short linker.

The artificial protein, dubbed BMC-H², was made by fusing two BMC-H protein sequences together.

“To our surprise, BMC-H² proteins form shells on their own,” Sean McGuire a former undergraduate research student and technician in the Kerfeld lab, said in a statement. “They look like wiffle balls, with gaps in the shell.” 

The researchers then capped the gaps in the shell with BMC-P—a third type of shell protein that forms pentamers.

The synthetic protein forms cyclic trimers that self-assemble to form a smaller 25 nanometer icosahedral shell with gaps at the pentamer positions.

When coexpressed in vivo with the pentamer fused to an affinity tag, the researchers purified the complete icosahedral shells, which constitutes a minimal shell system, to study permeability.     

 “The result is a shell, about 25 nanometers wide, made up of only two protein types: the new BMC-H² and BMC-P,” Ferlez said. “It is around half the size of the structure built with all three protein types.”

The researchers now plan to fit the shell with custom enzymes and refine it to enhance the chemical reactions occurring within.

The study was published in ACS Synthetic Biology.