In vivo proteolytic cleavage of colicins requires specific receptor binding

Abstract

COLICINS are plasmid-specified antibiotic proteins produced by some strains of coliform bacteria¹. They are distinguished from other bacterial antibiotics such as aminoglycosides and tetracyclines by their large molecular weights (40–80,000) and their narrow host range. Colicins only kill bacteria having the appropriate cell-surface receptors^1,2, and even then the ability of a cell to produce a given colicin usually confers immunity to killing by that colicin. We are interested in the comparative properties of colicins and their similarities to bacterial toxins that are active against mammalian cells, some of which are also encoded by extra-chromosomal elements. A fundamental question concerning the action of both colicins and toxins is how these large, essentially hydrophilic proteins traverse the mainly hydrophobic cell surface to reach the lesion sites. For colicins these include DNA for colicin E2 (ref. 3), RNA (colicin E3 and cloacin DF13)^4,5, and presumably the inner cell membrane, where membrane potential and energy metabolism are affected (colicins E1, K and Ia (refs 2, 6, 7)). Here, we show that binding of colicin to its specific receptor can result in a specific proteolytic cleavage to generate two peptide fragments. The smaller of these, derived from the C-terminal region of the colicin, has a greater specific biological killing activity in vitro than the parent molecule. We propose that in vivo the proteolytically derived C-terminal fragment is transported across the cell surface to the lesion site. This process seems analogous to the apparent cleavage of diphtheria toxin during its entry into mammalian cells⁸.