The Penicillin‐Binding Proteins in Streptococcus faecalis ATCC 9790

Abstract

Streptococcus faecalis ATCC 9790 possesses seven membrane-bound penicillin-binding proteins. They have been characterized with respect to their apparent molecular weights, relative abundance, specificity profiles for 15 different β-lactam antibiotics and stability under various conditions. In water and at 37°C, all the native penicillin-binding proteins have half-lives longer than 20 h except protein 3b (half-life of about 600 min) and protein 4 (half-life of about 175 min). The short-lived 80000-M_r protein 4 is spontaneously converted into a 73000-M_r water-soluble, penicillin-binding protein 4^*. Similarly, the short-lived 82000-M_r protein 3b seems to be the protein from which the 72000-M_r water-soluble protein X spontaneously originates during incubation of the membranes. Release of both proteins 4^* and X from the membrane is maximal under alkaline conditions; it is not inhibited by various protease inhibitors. After exposure to trypsin, the 43000-M_r membrane-bound penicillin binding protein 6 (a DD-carboxypeptidase) gives rise to a 30000-M_r water-soluble protein 6^*. Like the parent protein, protein 6^* exhibits both DD-carboxypeptidase activity and penicillin-binding ability. With proteins 6 and 6^*, low dose levels of p-chloromercuribenzoate prevent both enzyme activity and combination with penicillin, thus strongly suggesting that a thiol group is involved in the enzyme active center. We have shown previously [Coyette et al. in Eur. J. Biochem. 88, 297–305 (1978) and 75, 231–239 (1977)] that the DD-carboxypeptidase protein 6 fragments the benzylpenicillin molecule with formation of phenylacetylglycine. Breakdown of the complex formed between [¹⁴C]benzylpenicillin and the 140000-M_r membrane-bound protein 1 is also ‘enzyme-catalysed’. Most likely, however, the released product is penicilloate. With all the other penicillin-binding proteins whose molecular weights are intermediate between those of proteins 1 and 6, breakdown of the complexes formed with [¹⁴C]benzylpenicillin results from proteolysis and is not due to the release of the bound metabolite. None of the penicillin-binding proteins behaves, by itself, as a lethal target for β-lactam antibiotic action on the living cells.