Similarity of the conformation of diphtheria toxin at high temperature to that in the membrane-penetrating low-pH state.

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Abstract
At high temperature, nicked free monomers of diphtheria toxin undergo a transition to a thermally denatured state, with a midpoint of 45-50 degrees C. In this report, the high-temperature (60 degrees C) conformation has been compared to the native (neutral pH) and low-pH (pH less than 5) conformations. The low-pH and high-temperature conformations are similar although not identical. As in the conformation at low pH, and unlike the toxin in its native conformation, the protein in its conformation at high temperature is hydrophobic, has low fluorescence intensity, and has increased exposure of tryptophan to aqueous solution. As at low pH, at high temperature the circular dichroism spectrum shows at most only partial unfolding of secondary structure. In contrast, the conformation of the toxin in guanidinium chloride is much closer to a random coil. The effects of high temperature and low pH interact in the sense that sensitivity of the native conformation to one is increased by the other. That is, the transition temperature between native and thermally denatured states is decreased as pH is decreased, and the transition pH between neutral-pH and low-pH states is increased as temperature is increased. This implies that there is some region on the protein where high temperature and low pH can disrupt conformation in a similar manner. Taken together, these results indicate that the low-pH and high-temperature conformations can both be defined as partially denatured states, even though unfolding may not be extensive at low pH. Similar behavior may occur in other proteins that undergo functionally important conformational disruption at low pH.