Characterization of calreticulin as a protein interacting with protein kinase C

A protein kinase C (PKC)-binding protein was purified to homogeneity from t he Triton-insoluble fraction from rat hepatocytes homogenates. The protein was identified as the mature calreticulin chain by N-terminal amino acid se quencing and by its immunoreactivity with anti-calreticulin antibody raised against the C-terminal KDEL (single-letter code) sequence. The calculated molecular mass was 46.6 kDa but the protein migrates in SDS/PAGE as a doubl et with apparent molecular masses of 60 and 55 kDa. Studies in vitro with p urified calreticulin with the use of an overlay assay approach demonstrated that it binds to activated PKC isoenzymes expressed in rat hepatocytes. Ph osphorylation of purified calreticulin with a PKC isoenzyme-specific immune complex kinase assay showed that it is also a very good substrate for all PKC isoforms in vitro. The treatment of intact cells with phorbol ester or with adrenaline (epinephrine) plus propranolol increased calreticulin phosp horylation, which was blocked by the pretreatment of cells with the PKC-spe cific inhibitor Ro 31-8220. The analysis of calreticulin immunoprecipitates from control or treated cells indicated that PKC alpha, PKC beta, PKC thet a, PKC zeta and PKC mu, but not PKC delta or PKC epsilon, coimmunoprecipita ted with calreticulin. Taken together, our results indicate that PKC intera cts in vivo with calreticulin and suggest that they can operate in common s ignalling pathways.