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

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 quantity of colicin N T-domains for spectroscopic and crystallization studies. These both require a correctly folded and stable protein. Here we present an expression of the complex between the N-terminal domain of colicin N and the C-terminal domain of TolA obtained by fusing these 2 domains with a flexible linker. Circular dichroism spectroscopy studies indicated that unstructured bacterial toxin colicin N T-domains changed to an ordered state upon binding to the C-terminal domain of TolA; this fusion protein has a secondary and tertiary structure.