Crystal structure and biophysical characterisation of the enterococcal foldase PpiC, a cross-opsonic antigen against gram-positive nosocomial pathogens.

Enterococcus faecium have high rates of antibiotic resistances, with vancomycin-resistant E. faecium acknowledged as the most important in the clinical setting and declared by WHO to be a threat to humankind, for which rapid actions are needed. PpiC is a membrane-bound lipoprotein of E. faecium endowed with both a peptidyl-prolyl isomerase and a foldase activity, and plays a key role in assisting the folding of many secreted enterococcal proteins. It is located at the membrane-wall interface, therefore easily accessible to inhibitors and to the immune system and an ideal target for drug and vaccine development. Despite their potential, enterococcal peptidyl-prolyl isomerases have been understudied. We previously identified PpiC as an important cross-protective vaccine antigen. To gain a better understanding of the PpiC biological role in E. faecium survival, we determined the crystal structure of PpiC and investigated its biophysical properties. Consistent with PpiC's folding activity, the biological assembly of PpiC is a bowl-shaped structure containing two parvulin-type peptidyl-prolyl cis/trans isomerase domains. We also dissected the role of N- and C-terminal regions of the molecule in its dimerisation, an event which is predicted to play an important role in the folding of client proteins. Our data point to a functional cross-talk between the foldase and peptidyl-prolyl isomerase activities of PpiC, through the protein-swapping involved in dimerisation. Also, our work provides key structural data for the design of antimicrobials and cross-protective vaccine antigens against nosocomial infections.