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

## TL;DR

The paper studies the structure and function of PpiC, a protein in E. faecium that could be a target for vaccines and drugs against antibiotic-resistant infections.

## Contribution

The paper reports the crystal structure of PpiC and reveals its dimeric organization and functional interplay between its domains.

## Key findings

- PpiC forms a dimeric structure with two parvulin-type PPIase domains.
- N- and C-terminal regions are crucial for dimerization and folding activity.
- Protein swapping in dimerization links foldase and PPIase functions.

## Abstract

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.

PpiC is an enterococcal lipoprotein endowed with both Peptidyl‐prolyl cis‐trans isomerase (PPIase) and foldase activities. It is a potential target for novel antimicrobials and a promising vaccine antigen. We determined the crystal structure of the enzyme, showing that PpiC adopts a dimeric organisation embedding two catalytic parvulin‐type PPIase domains joined by a large crevice. Biophysical studies suggested the importance of protein swapping in the PpiC dimerisation and folding.