Multiple isoforms of a protein kinase C inhibitor (KCIP‐1/14‐3‐3) from sheep brain

A potent inhibitor of protein kinase C (PKC), inhibitor protein‐1 (KCIP‐1), isolated form sheep brain has been shown to consist of eight isoforms by reverse‐phase HPLC. Direct protein sequence analysis has revealed these to be the same as those of 14‐3‐3 protein, described as an activator of tyrosine and tryptophan hydroxylases involved in neurotransmitter biosynthesis. The N‐termini of KCIP‐1 isoforms were shown to be acetylated, and secondary structure predictions revealed a high degree of α‐helix with an amphipathic nature. KCIP‐1 showed no inhibitory activity towards protein kinase M (the catalytic fragment of PKC) and had no effect on the activities of three other protein kinases, cAMP‐dependent protein kinase, Ca2+/calmodulin‐dependent protein kinase II and casein kinase 2. Four forms of KCIP‐1 were shown to be substrates for PKC in vitro, but none were phosphorylated by the other protein kinases mentioned above.