We have identified an 8.4 kDa minor immunophilin from calf thymus and Jurka
t T cells as ubiquitin. It binds tacrolimus and sirolimus with K-d's of 0.8
and 0.08 nM, respectively, The binding of this protein to cyclosporin A is
negligible. Binding of tacrolimus to two commercial sources of ubiquitin,
a bovine product, and a recombinant human ubiquitin, was also demonstrated
after HPLC purification of the purchased preparations. (C) 2000 Academic Pr
ess.