Structural characterisation of the natively unfolded enterocin EJ97

Bacteriocins belong to the wide variety of antimicrobial ribosomal peptides synthesised by bacteria. Enterococci are Gram-positive, catalase-negative bacteria that produce lactic acid as the major end product of glucose fermentation. Many enterococcal strains produce bacteriocins, named enterocins. We describe in this work, the structural characterisation of the 44 residues-long enterocin EJ97, produced by Enterococcus faecalis EJ97. To this end, we have used a combined theoretical and experimental approach. First, we have characterised experimentally the conformational properties of EJ97 in solution under different conditions by using a number of spectroscopic techniques, namely fluorescence, CD, FTIR and NMR. Then, we have used several bioinformatic tools as an aid to complement the experimental information about the conformational properties of EJ97. We have shown that EJ97 is monomeric in aqueous solution and that it appears to be chiefly unfolded, save some flickering helical-or turn-like structures, probably stabilised by hydrophobic clustering. Accordingly, EJ97 does not show a cooperative sigmoidal transition when heated or upon addition of GdmCl. These conformational features are essentially pH-independent, as shown by NMR assignments at pHs 5.9 and 7.0. The computational results were puzzling, since some algorithms revealed the natively unfolded character of EJ97 (FoldIndex, the mean scaled hydropathy), whereas some others suggested the presence of ordered structure in its central region (PONDR, RONN and IUPRED). A future challenge is to produce much more experimental results to aid the development of accurate software tools for predicting disorder in proteins.