On the binding mechanism of the peptide receptor of the oligopeptide transport system of Lactococcus lactis

Lactococcus lactis degrades exogenous proteins such as p-casein to peptides of 4-30 amino acids, and uses these as nitrogen sources. The binding prote in or receptor (OppA(Ll)) of the oligopeptide transport system (Opp) of L.l actis has the unique capacity to bind peptides from five up to at least 20 residues. To study the binding mechanism of OppA(Ll), nonameric peptides we re used in which the cysteine at position 1, 3, 4, 5, 6, 7 or 9 was selecti vely labeled with either bulky and non-fluorescent or bulky and fluorescent groups. Also, nonameric peptides with a non-natural residue, azatryptophan , at positions 3 or 7 were used. The fluorescence of azatryptophan reports on the polarity of the environment. The studies indicate that the binding p rotein encloses the first six amino acids of the peptide, whereas the remai ning residues stick out and interact with the surface of the binding protei n. The peptide binding mechanism of OppA(Ll) is discussed in relation to kn own three-dimensional structures of members of this class of proteins, and an adaptation of the general binding mechanism is proposed.