Binding of Bacitracin to Cells and Protoplasts of Micrococcus lysodeikticus

Bacitracin was tritiated by the Wilzbach technique and purified by carboxymethylcellulose chromatography. Binding studies between [3H]bacitracin and Micrococcus lysodeikticus cells and protoplasts indicated that the cells or protoplasts were saturated at 2 x 105 molecules per cell. This value correlates well with the number of C55-isoprenyl pyrophosphate molecules found in the membrane and adds further support to the hypothesis that the antibiotic activity of this peptide is due to complex formation with this functional lipid. The Km for binding of bacitracin to cells or protoplasts was 3.7 x 10-6 molar which is consistent with the in vitro binding constant of 1 x 106 m-1 observed for the interaction of bacitracin A with the purified C55-isoprenyl pyrophosphate. In vitro binding studies between bacitracin A and purified bacterial phospholipids indicated that the antibiotic does not interact strongly with these lipids. It is concluded from these studies that binding of bacitracin to the bacterial cells can be accounted for quantitatively by virtue of a specific interaction with the membrane C55-isoprenyl pyrophosphate. Furthermore, the protoplast permeability changes induced by bacitracin, under certain conditions, may arise from interaction between bacitracin and the C55-isoprenyl pyrophosphate molecules in the membrane.