Catalysis of protein folding by cyclophilins from different species

Cyclophilins are a class of ubiquitous proteins with yet unknown function. They were originally discovered as the major binding proteins for the immunosuppressant cyclosporin A. The only known catalytic function of these proteins in vitro is the cis/trans isomerization of Xaa-Pro bonds in oligopeptides. This became clear after the discovery that bovine cyclophilin is identical with porcine prolyl isomerase. This enzyme accelerates slow, proline-limited steps in the refolding of several proteins. Here we demonstrate that the cyclophilins from man, pig, Neurospora crassa, Saccharomyces cerevisiae, and Escherichia coli are all active as prolyl isomerases and as catalysts of protein folding. This evolutionary conservation suggests that catalysis of prolyl peptide bond isomerization may be an important function of the cyclophilins. It could be related with de novo protein folding or be involved in regulatory processes. Catalysis of folding is very efficient in the presence of the high cellular concentrations of prolyl isomerase.