FKBP-12 is not an inhibitor of protein kinase C

It was recently noted that the amino acid sequence of FK506 binding protein (FKBP-12) is nearly identical to that of an endogenous inhibitor of protein kinase C, PKCI-2. To follow up on this observation, we have tested the hypothesis that FKBP-12 is an inhibitor of PKC. The kinase activity of rat brain protein kinase C (PKC) was not inhibited by the presence of up to 700 micrograms recombinant human FKBP-12 per ml, in either the presence or absence of FK506. FKBP-12 also did not affect PMA-induced phosphorylation of an endogenous PKC substrate, an 80 kDa protein, in permeabilized cells. To test whether FKBP-12 could account for endogenous PKC inhibitory activity in cells, Jurkat cell lysate was chromatographed on an anion exchange column. A peak of PKC inhibitory activity was eluted at approximately 200 mM NaCl. As shown by both Western blots and FK506 binding activity, FKBP-12 was eluted only in the flow-through and wash fractions. These results demonstrate that FKBP-12 is clearly distinct from endogenous PKC inhibitory activity.