The C-terminal half of the anti-sigma factor, FlgM, becomes structured when bound to its target, sigma(28)

The interaction between the flagellum specific sigma factor, sigma(28), and its inhibitor, FlgM, was examined using multidimensional heteronuclear NMR. Here we observe that free FlgM is mostly unfolded, but about 50% of the residues become structured when bound to sigma(28). Our analysis suggests that the sigma(28) binding domain of FlgM is contained within the last 57 amino acids of the protein while the first 40 amino acids are unstructured in both the free and bound states. Genetic analysis of flgM mutants that fail to inhibit sigma(28) activity reveal amino acid changes that are also isolated to the C-terminal 57 residues of FlgM. We postulate that the lack of structure in free and bound FlgM is important to its role as an exported protein.