Some bacterial enzymes have interesting character arcs. DNA polymerase, courtesy of Thermus aquaticus, soaks up the limelight as an enzyme pulled out of bacteria and catapulted into molecular biology stardom. One enzyme that is certainly less-known is FbaB, which helps Streptococcus pyogenes adhere to and invade host cells. In the last few years, the protein has been plucked from its role as an orchestrator of streptococcal infections and revamped into a valuable molecular tool.
FbaB is a fibronectin-binding protein that facilitates host cell infection and plays a key role in the pathogenesis of Gram-positive bacteria. During host cell infection, Gram-positive pili mediate a cascade of steps that includes FbaB-involved adhesion. Despite their delicate spaghetti-strand appearance, these pili are remarkably sturdy, possessing mechanical strength thanks to isopeptide bonds within each pilus and within FbaB. Like steel scaffolding, these lysine-asparagine and lysine-aspartate bonds confer resistance to mechanical stress, proteolysis, and high temperatures, enabling species like Streptococcus to colonize host cell surfaces despite shear forces from swallowing, chewing, coughing, sneezing - the body’s works. Some Gram-positive pili can bear hundreds of piconewtons of tensile force, which is a mechanical stress greater than that experienced by a person dangling a pick-up truck in midair by one arm! The enzymatic architect behind the isopeptide bonds is the CnaB-type domains of these proteins.
This remarkable ability of FbaB was co-opted in SpyTag/SpyCatcher technology, a system useful for making covalent fusion proteins first developed in 2012. In Streptococcus pyogenes, the CnaB2 domain of FbaB contains a lysine residue and an aspartate residue whose side chains react to form the irreversible isopeptide bond. In SpyTag/SpyCatcher, this domain has been chopped into two pieces: the piece dubbed "SpyTag" contains the aspartate, and the "SpyCatcher" piece contains the lysine (plus the catalytic glutamate). These reactive ends are like dabs of superglue applied onto two surfaces to be joined: by genetically encoding these pieces into desired polypeptides, one can cyclize enzymes to confer extra stability (a "SpyRing"), create modular assemblies of proteins for hydrogels and tissue engineering, add decorations to nanoparticles for vaccine development, characterize RNA-protein interactions ("SpyCLIP"). The list goes on.
Some hardware updates from the Howarth lab have come since the 2012 design, including versions of the system in which the CnaB2 domain is split into three instead of two peptides, and another system based on the pilus adhesin RrgA called SnoopTag/SnoopCatcher, which was – in a shout-out to scientist humor – followed by SnoopTagJr/DogTag. The Gram-positive pilus and co. are evidently a cache of proteins that brim with biotechnological potential. And so the pilus-derived Spy gadget storyline continues.