Colicin K decreases the density of intramembrane particles (IMP) in the cell membrane of Escherichia coli.

Toxic effects of both main colicin types, i.e. of porin and nuclease types, involve the direct contact of their molecules with the plasma membrane of sensitive cells. In the present study, it was tested whether this contact provokes a lateral or vertical movement of intramembrane protein particles (IMP) or a direct cleavage of the proteins. IMP were visualized by freeze-fracturing and electron microscopy on the protoplasmic fracture face (PF) of colicin-treated cells of Escherichia coli. Possible changes in distribution and in density of IMP due to treatment with colicins E1-E7 and K were followed. As a control, the bacteria were equilibrated at 0 degrees C before quenching, which caused a reversible formation of smooth areas and a decrease in the mean density of IMP on the PF. Colicins E1-E7 had no clear-cut effect on the disposition of IMP. Only colicin K decreased the IMP density, by 10% in E. coli strain 58-161 and by 17% in strain C6; the distribution of IMP remained homogeneous. Trypsin reactivation of colicin-K-inactivated bacteria was not reflected by restoration of the original density of IMP; on the contrary, it led to a further decrease, of 1-13%, in IMP density, presumably by proteolytic cleavage. Varying densities of IMP in different strains of the same bacterial species (under standard conditions) were confirmed.