Cyps (cyclophilins) are ubiquitous proteins of the immunophilin superfamily with 
proposed functions in protein folding, protein degradation, stress response and 
signal transduction. Conserved cysteine residues further suggest a role in redox 
regulation. In order to get insight into the conformational change mechanism and 
functional properties of the chloroplast-located CYP20-3, site-directed 
mutagenized cysteine-->serine variants were generated and analysed for enzymatic 
and conformational properties under reducing and oxidizing conditions. Compared 
with the wild-type form, elimination of three out of the four cysteine residues 
decreased the catalytic efficiency of PPI (peptidyl-prolyl cis-trans isomerase) 
activity of the reduced CYP20-3, indicating a regulatory role of 
dithiol-disulfide transitions in protein function. Oxidation was accompanied by 
conformational changes with a predominant role in the structural rearrangement 
of the disulfide bridge formed between Cys(54) and Cys(171). The rather negative 
E(m) (midpoint redox potential) of -319 mV places CYP20-3 into the redox 
hierarchy of the chloroplast, suggesting the activation of CYP20-3 in the light 
under conditions of limited acceptor availability for photosynthesis as realized 
under environmental stress. Chloroplast Prx (peroxiredoxins) were identified as 
interacting partners of CYP20-3 in a DNA-protection assay. A catalytic role in 
the reduction of 2-Cys PrxA and 2-Cys PrxB was assigned to Cys(129) and 
Cys(171). In addition, it was shown that the isomerization and 
disulfide-reduction activities are two independent functions of CYP20-3 that 
both are regulated by the redox state of its active centre.