THE N-TERMINAL CYCLOPHILIN-HOMOLOGOUS DOMAIN OF A 150-KILODALTON TUMOR RECOGNITION MOLECULE EXHIBITS BOTH PEPTIDYLPROLYL CIS-TRANS-ISOMERASE AND CHAPERONE ACTIVITIES

A cyclophilin-related protein has recently been found to be involved i n tumor recognition by natural killer cells. The N-terminal end of thi s 150-kDa surface molecule (NK-TR) is homologous to cyclophilin/peptid ylprolyl 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 show n to accelerate the isomerization of the prolyl peptide bond of the su bstrate N-succinyl-Ala-Ala-Pro-Phe-p-nitroanilide with a k(cat)/K-M va lue of 7.4 X 10(5) M(-1) s(-1). The isomerase activity of the NK-TR cy clophilin homolog has been determined to be relatively insensitive to inhibition by the immunosuppressive drug cyclosporin A, with an IC50 v alue 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 p rotein folding process as a chaperone by preventing the aggregation of early folding intermediates of carbonic anhydrase. The implications o f the finding of both peptidylprolyl cis-trans-isomerase and chaperone activities within the N-terminal domain of a large, cell type-restric ted, surface molecule are discussed.