Labelling and cross-linking of Escherichia coli penicillin-binding proteins with bis-beta-lactam antibiotics

We have synthesized a number of radioactively labelled bis-beta-lactams, which are nearly symmetrical dimers of well-known beta-lactam antibiotics and act as bifunctional specific cross-linking reagents for the penicillin-binding proteins of Escherichia coli envelopes. We have observed that some of our bis-beta-lactam antibiotics cross-link two molecules of penicillin-binding protein 1b or 1c or 3. Furthermore our bis-beta-lactam antibiotics bind to E. coli proteins with higher affinities than the beta-lactams from which they were derived. Therefore, a number of monomeric protein bands are consistently labelled with our bis-beta-lactam antibiotics (190, 170, 145 and 124 kDa) as well as the previously described penicillin-binding proteins 1a, 1b, 1c, 2, 3, 4, 5, 6, 7 and 8. We do not know yet the possible enzymic activities of the penicillin-binding proteins of 190 kDa, 170 kDa, 145 kDa and 125 kDa that were not described previously. We have also detected some protein bands moving very slowly, which appear to be dimers or cross-linking species of these new high-molecular-mass penicillin-binding proteins described above.