Unfolding of Vibrio cholerae hemolysin induces oligomerization of the toxin monomer

Vibrio cholerae hemolysin (HlyA) is a pore-forming toxin that exists in two stable forms: a hemolytically active water-soluble monomer with a native molecular weight of 65,000 and a hemolytically inactive SDS-stable heptamer with the configuration of a transmembrane diffusion channel. Transformation of the monomer into the oligomer is spontaneous but very slow in the absence of interaction with specific membrane components like cholesterol and sphingolipids. In this report, we show that mild disruption of the native tertiary structure of HlyA by 1.75 M urea triggered rapid and quantitative conversion of the monomer to an oligomer. Furthermore, the HlyA monomer when unfolded in 8 M urea refolded and reconstituted on renaturation into the oligomer biochemically and functionally similar to the heptamer formed in target lipid bilayer, suggesting that the HlyA polypeptide had a strong propensity to adopt the oligomer as the stable native state in preference to the monomer. On the basis of our results, we propose that (a) the hemolytically active HlyA monomer represents a quasi-stable conformation corresponding to a local free energy minimum and the transmembrane heptameric pore represents a stable conformation corresponding to an absolute free energy minimum and (b) any perturbation of the native tertiary structure of the HlyA monomer causing relaxation of conformational constraints tends to promote self-assembly to the oligomer with membrane components playing at most an accessory role.