Isopeptide bonds in bacterial pili and their characterization by X-ray crystallography and mass spectrometry

Pili are long, filamentous protein assemblies which extend from the surfaces of many bacteria, and mediate their adhesion to host cells and other matrices. For pathogenic bacteria they are critical to colonization and infection. Whereas the pili of gram-negative bacteria are formed by noncovalent association of their pilin subunits, those of gram-positive bacteria are assembled with the aid of sortase enzymes that mediate the formation of covalent isopeptide bonds between successive pilin subunits. Sequence comparisons, mutagenesis and crystallography have implicated specific lysine residues in the formation of these intermolecular bonds and mass spectral analyses of native and modified pili have now provided definitive proof of these linkages. Crystallographic studies of pilin subunits have also led to the unexpected discovery of internal isopeptide crosslinks formed between lysine and asparagine residues. These, too, have been confirmed by mass spectrometry.