A cyclophilin-related protein has recently been found to be involved in tumor recognition by natural killer cells. The N-terminal end of this 150-kDa surface molecule (NK-TR) is homologous to cyclophilin/peptidylprolyl cis-trans-isomerase. We have constructed a soluble bacterial fusion protein between the cyclophilin-homologous domain of the NK-TR molecule and glutathione S-transferase (GST) to test for the presence of peptidylprolyl cis-trans-isomerase and chaperone activities and for cyclosporin A binding. The purified NK-cyp-GST fusion protein is shown to accelerate the isomerization of the prolyl peptide bond of the substrate N-succinyl-Ala-Ala-Pro-Phe-p-nitroanilide with a kcat/KM value of 7.4 x 10(5) M-1 s-1. The isomerase activity of the NK-TR cyclophilin homolog has been determined to be relatively insensitive to inhibition by the immunosuppressive drug cyclosporin A, with an IC50 value of 770 nM as compared to 19 nM for human cyclophilin. Furthermore, the NK-cyp-GST fusion protein has been found to participate in the protein folding process as a chaperone by preventing the aggregation of early folding intermediates of carbonic anhydrase. The implications of the finding of both peptidylprolyl cis-trans-isomerase and chaperone activities within the N-terminal domain of a large, cell type-restricted, surface molecule are discussed.