Removal of an atypical region from a staphylococcal cyclophilin affects its structure, function, stability, and shape

SaCyp, a cyclophilin having 197 amino acid residues, acts both as a protein-folding catalyst and a virulence factor in Staphylococcus aureus. Interestingly, a region, homologous to the SaCyp region carrying 121-148 amino acid residues, is present in many putative cyclophilins but absent in well-studied cyclophilins. To determine the exact roles of this unusual region in SaCyp and related proteins, we have investigated a deletion mutant (rCypΔ) of a recombinant SaCyp (rCyp) using various probes. The data reveal that rCypΔ has significantly less catalytic activity and possesses altered structure and hydrophobic surface compared to rCyp. Conversely, the deletion substantially increased inhibitor binding affinity and altered the shape of rCyp. However, both proteins were unfolded by a non-two-state mechanism in the presence of urea. Additionally, the stability of rCyp was significantly reduced due to the deletion of the residues 121-148. Our MD simulation study also indicated the considerable alteration in structure, shape, and fluctuations of SaCyp due to the removal of the region carrying 121-148 residues. Hence, the atypical region located in SaCyp might be vital for maintaining its unique structure, function, stability, and shape.