Reassessment of the putative chaperone function of prolyl‐cis/trans‐isomerases

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Abstract
The folding of proteins can be assisted by two unrelated groups of helper molecules. Chaperones suppress non-productive side reactions by stoichiometric binding to folding intermediates, and folding enzymes catalyze slow rate-limiting steps of folding. We reinvestigated, whether peptidyl-prolyl-cis/trans-isomerases of the cyclophilin type act simultaneously as chaperones and as folding catalysts in the reactivation of human carbonic anhydrase II, as reported recently [Freskgård, P.-O. et al. (1992) Science 258, 466-468; Rinfret, A. et al. (1994) Biochemistry 33, 1668-1673]. No increase in the yield of native carbonic anhydrase-II could be detected in the presence of three different prolyl isomerases, when reactivation was followed by a sensitive assay for an extended time of 4 h. We conclude that the role of prolyl isomerases in the refolding of carbonic anhydrase can be explained solely by their isomerase activity. There is no need to invoke simultaneous functions as chaperones for these folding catalysts.