Are there temperature-dependent structural transitions in the "intrinsically unstructured" protein prothymosin alpha?

Prothymosin alpha, a typical member of the class of the so-called "intrinsically unstructured" proteins, adopts a random-chain conformation under physiological environmental conditions. An apparent formation of ordered secondary structure and a moderate compaction are observed upon the change from neutral to acid pH at room temperature. We have addressed the question of whether there are temperature-dependent changes of the conformational state of prothymosin alpha at low pH using circular dichroism spectroscopy and static and dynamic light scattering. In contrast to previous investigations, we did not observe a heat-induced conformational transition. For comparison, we have also carried out the same experimental procedures with acid-unfolded phosphoglycerate kinase from yeast. In this case we observed a weak compaction and a slight apparent increase in ordered secondary structure with increasing temperature, probably caused by the higher average hydrophobicity as compared to prothymosin alpha. In the absence of a clear structural transition, we deduce the observed effects result mainly from a progressive redistribution in the population of phi-psi angles of the polypeptide backbone when the temperature is increased. Furthermore, the paper should demonstrate the difficulties in distinguishing between such a progressive change amongst a continuum of states within the ensemble of unfolded conformations from the formation of authentic stable secondary structures in highly unfolded proteins. This problem is not solved presently and convincing evidence can only be supplied by the combination of various experimental techniques.