Methionine aminopeptidase II: A molecular chaperone for sarcoplasmic reticulum calcium ATPase.

The monoclonal antibody to the beta-subunit of H(+)/K(+)-ATPase (mAbHKbeta) cross-reacts with a protein that acts as a molecular chaperone for the structural maturation of sarcoplasmic reticulum (SR) Ca(2+)-ATPase. We partially purified a mAbHKbeta-reactive 65-kDa protein from Xenopus ovary. After in-gel digestion and peptide sequencing, the 65-kDa protein was identified as methionine aminopeptidase II (MetAP2). The effects of MetAP2 on SR Ca(2+)-ATPase expression were examined by injecting the cRNA for MetAP2 into Xenopus oocytes. Immunoprecipitation and pulse-chase experiments showed that MetAP2 was transiently associated with the nascent SR Ca(2+)-ATPase. Synthesis of functional SR Ca(2+)-ATPase was facilitated by MetAP2 and prevented by injecting an antibody specific for MetAP2. These results suggest that MetAP2 acts as a molecular chaperone for SR Ca(2+)-ATPase synthesis.