Peptidyl-prolyl cis-trans isomerase of Bacillus subtilis: Identification of residues involved in cyclosporin a affinity and catalytic efficiency

The 17-kDa peptidyl-prolyl cis-trans isomerase from Bacillus subtilis (PPiB) is a member of the cyclophilin family and shows strong homology to PPIases of eukaryotic origin (40%) and less identity to PPIase sequences of Gram-negative bacteria (27-32%). Although the majority of residues that form the PPIase active site are highly conserved, three residues, V52, H90, and H109 in the sequence of the B. subtilis PPIase, were found to differ from the sequences found in human (hCyP) and Escherichia coli (eCyP). Also, the binding affinity of cyclosporin A (CsA) to the different PPIases varies in IC50 values from 6 nM for human PPIase hCyPA and 84 nM for the human hCyPB to over 120 nM for B. subtilis and 3000 nM for E. coli. In addition, a variety of k(cat)/K-m values, ranging from 1.1 mM(-1) s(-1) for the B, subtilis PPIase to over 10 mM(-1) s(-1) for human and 13 mM(-1) s(-1) for E. coli, were detected using the common substrate suc-Ala-Ala-Pro-Phe-pNA. Through site-specific mutagenesis we demonstrate that the differences in the three mentioned residues are mainly responsible for the variations in IC50 and k(cat)/K-m values. Replacement of H90 to N90, or H109 to W109, resembling the amino acid sequence of human hCyPA, resulted in more efficient CsA binding (IC50 value for H90N, 60 nM, and for H109W, 95 nM), whereas replacement of H90 to R90, or H109 to F109, resembling the amino acid sequence of E. coli eCyP, resulted in less efficient CsA binding (IC50 value for H90R, 2000 nM, and for H109F, 5000 nM). In addition to lower CsA affinity, mutant protein H109F shows a k(cat)/K-m value of 10.5 mM(-1) s(-1) comparably high to that of the wild-type E. coli protein. In contrast, other mutants like C57F, H90N, H90R, and H109W do not differ significantly in k(cat)/K-m values from wild-type PPiB. Replacement of V52 to M52, which is conserved in E. coli and all known eukaryotic PPIases, does not show any effect in CsA binding affinity (IC50 value for V52M, 120 nM), but it raises the catalytic efficiency by 12-fold to k(cat)/K-m of 14 mM(-1) s(-1). In conclusion, our studies suggest that the unique histidine residues H90 and H109 in B. subtilis PPIase are, at least in part, responsible for its intermediate CsA affinity and that the V52 residue confers the low conversion rate.