Cyclophilin represents a novel class of protein kinases

Cyclophilin (CyP, Mr 17,737, pI 9.6), a highly specific cytosolic receptor for cyclosporin A (CsA) has ser/thr protein kinase activity. Incorporation of 32P into bovine histone H3 (BH3) was catalyzed by major and minor CyP isozymes at the same rate. Salt effects were biphasic with optimal kinase activity between 50-100 mM Na+ or K+. Kinase activity was maximal at 370C, stable for 5 min at 450, labile at 560, optimal between pH 6.8 and 8.0 and had an apparent Km of 20 uM ATP with both isozymes. The specific activity of CyP was 1.0 nmole P/mg protein/min with chicken histone H1 (CH1), 0.2 nmoles P/mg prot/min with BH3 and less than 0.01 nmoles P/mg prot/min with synapsin, casein, phosvitin, and ribosomal protein S6. Cofactors including Mn++, Zn++, Ca++, phosphatidyl serine, diolein and phorbol ester, cAMP, cGMP and Ca++ did not affect basal CyP kinase activity. CsA (<200 ng/ml) inhibited phosphorylation of CH3 by 50% but did not inhibit phosphorylation of other histones; 2ug CsA/ml was required to cause 50% inhibition of cAMP and Ca++/CaM dependent kinases. A non-immunosuppressive analog (Me-leu-11-CsA) that does not bind to CyP did not inhibit CH3 phosphorylation. Thus, CyP is a novel protein kinase that mediates immunosuppression by CsA