The Sortase A enzyme that attaches proteins to the cell wall of Bacillus anthracis contains an unusual active site architecture

Making and breaking peptide bonds: protein engineering using sortase

Sortases are a class of bacterial enzymes that possess transpeptidase activity. It is their ability to site-specifically break a peptide bond and then reform a new bond with an incoming nucleophile that makes sortase an attractive tool for protein engineering. This technique has been adopted for a range of applications, from chemistry-based to cell biology and technology. In this Minireview we provide a brief overview of the biology of sortase enzymes and current applications in protein engineering. We identify areas that lend themselves to further innovation and that suggest new applications.

Architects at the bacterial surface - sortases and the assembly of pili with isopeptide bonds

The cell wall envelope of Gram-positive bacteria can be thought of as a surface organelle for the assembly of macromolecular structures that enable the unique lifestyle of each microorganism. Sortases - enzymes that cleave the sorting signals of secreted proteins to form isopeptide (amide) bonds between the secreted proteins and peptidoglycan or polypeptides - function as the principal architects of the bacterial surface. Acting alone or with other sortase enzymes, sortase construction leads to the anchoring of surface proteins at specific sites in the envelope or to the assembly of pili, which are fibrous structures formed from many protein subunits. The catalysis of intermolecular isopeptide bonds between pilin subunits is intertwined with the assembly of intramolecular isopeptide bonds within pilin subunits. Together, these isopeptide bonds endow these sortase products with adhesive properties and resistance to host proteases.

Crystal structure of Spy0129, a Streptococcus pyogenes class B sortase involved in pilus assembly

Sortase enzymes are cysteine transpeptidases that mediate the covalent attachment of substrate proteins to the cell walls of Gram-positive bacteria, and thereby play a crucial role in virulence, infection and colonisation by pathogens. Many cell-surface proteins are anchored by the housekeeping sortase SrtA but other more specialised sortases exist that attach sub-sets of proteins or function in pilus assembly. The sortase Spy0129, or SrtC1, from the M1 SF370 strain of Streptococcus pyogenes is responsible for generating the covalent linkages between the pilin subunits in the pili of this organism. The crystal structure of Spy0129 has been determined at 2.3 Å resolution (R = 20.4%, Rfree  = 26.0%). The structure shows that Spy0129 is a class B sortase, in contrast to other characterised pilin polymerases, which belong to class C. Spy0129 lacks a flap believed to function in substrate recognition in class C enzymes and instead has an elaborated β6/β7 loop. The two independent Spy0129 molecules in the crystal show differences in the positions and orientations of the catalytic Cys and His residues, Cys221 and His126, correlated with movements of the β7/β8 and β4/β5 loops that respectively follow these residues. Bound zinc ions stabilise these alternative conformations in the crystal. This conformational variability is likely to be important for function although there is no evidence that zinc is involved in vivo.