The ToxR protein of Vibrio cholerae forms homodimers and heterodimers.

The ToxR protein of Vibrio cholerae regulates the expression of several virulence factors that play important roles in the pathogenesis of cholera. Previous experiments with ToxR-alkaline phosphatase (ToxR-PhoA) fusion proteins suggested a model for gene regulation in which the inactive form of ToxR was a monomer and the active form of ToxR was a dimer (V. L. Miller, R. K. Taylor, and J. J. Mekalanos, Cell 48:271-279, 1987). In order to examine whether ToxR exists in a dimeric form in vivo, biochemical cross-linking analyses were carried out. Different dimeric cross-linked species were detected depending on the expression level of ToxR: when overexpressed, ToxR+ToxR homodimers and ToxR+ToxS heterodimers were detected, and when ToxR was expressed at normal levels, exclusively ToxR+ToxS heterodimers were detected. The amount of overexpression was quantitated by using ToxR-PhoA fusion proteins and was found to correspond to 2.7-fold the normal level of ToxR. The formation of both homodimeric ToxR species and heterodimeric ToxR+ToxS species is consistent with previously reported genetic data that suggested that both types of ToxR oligomeric interactions occur. However, variation in the amount of either the homodimeric or heterodimeric form detectable by this cross-linking analysis was not observed to correlate with laboratory culture conditions known to modulate ToxR activity. Thus, genetic and biochemical data indicate that ToxR is able to interact with both itself and ToxS but that these interactions may not explain mechanistically the observed changes in ToxR activity that occur in response to environmental conditions.