Bacterial toxin colicin N-T domain structure changes to ordered state upon binding C-terminal domain of TolA

Abstract

IntroductionColicins are plasmid-encoded bactericidal proteins produced by immune Escherichia coli that are active against sensitive E. coli and its closely related cells. Their toxic activities are of various types: some colicins form ion channels in the cytoplasmic membrane of sensitive cells, while others act as nucleases that degrade DNA or 16S RNA in the cytoplasm. One colicin, colicin M, inhibits the biosynthesis of murein (Lakey et al., 1994). Their toxic activities against target cells are known to occur in 3 distinct stages (Figure 1). First is receptor recognition and binding, where colicins bind to a specific receptor at the cell surface. Second is the translocation step, where colicins cross both the outer membrane and the periplasm to reach their cellular target (Lakey et al., 1994;Lazdunski, 1995). The final stage is the killing action, where colicins exert their lethal effects by forming a pore in the cytoplasmic membrane (Lazdunski, 1995), by DNAse or RNAse activity, or by inhibiting murein biosynthesis in the cytoplasm (Lakey et al., 1994).Colicins have 3 linearly organized functional domains, each domain implicated in a specific stage of the colicin activity. As shown in Figure 1, the central domain (R-domain) is responsible for the receptor-binding activity. The N-terminal domain (T-domain) is involved in translocation. The C-terminal domain (P-domain) carries out the lethal activity; this domain either forms a voltagegated pore in the cytoplasmic membrane or digests nucleic acids in the cytoplasm (Raggett et al., 1998;Vetter et al., 1998;Papadokus et al., 2011).Colicin N is a group A (Tol-dependent) pore-forming colicin whose target receptor is the E. coli outer membrane protein OmpF (Pugsley, 1987). It is composed of a largely unstructured T-domain linked by a glycine-rich sequence to a central R-domain containing a 6-stranded β-sheet structure. The R-domain is connected to the P-domain (a 10 α-helical structure) by means of a 65 Å α-helix (El-Kouhen et al., 1993;Gokce et al., 2000). While receptor binding and pore formation have been extensively studied, much remains unknown about the translocation step. Colicin N translocation requires 3 members of the Tol locus: TolA, TolQ, and TolR (Webster, 1991). TolQ and TolR are integral membrane proteins of the E. coli inner membrane and there is no evidence that they reach across Abstract: Colicin N is a bacterial toxin that kills Escherichia coli and related cells. Its mode of action is of interest in protein import and toxicology. Colicin N translocates across the E. coli outer membrane and periplasm by interacting with several receptors. The translocation process involves the interaction of the colicin N with the outer membrane porin OmpF and subsequently with the integral membrane protein TolA. The N-terminal domain of colicin N is involved in the import process. TolA consists of 3 domains. The N-terminal domain of colicin N interacts with the C-terminal domain of TolA at later stages of the translocation process. Our aim was to produce a large ...

Investigation of cytotoxic activity of full-length and truncated colicin n on human cancer cells

“…Antibacterial activity of colicin N started at R domain interact with OmpF for recognition and transport across the outer membrane and the T domain interacts with TolA in the periplasm through to porin pores. After that, the P-domain inserts into the inner membrane and their forming voltage-gated channels which result in cell death (26) and (27) as shown in Figure 4.…”