Identification of 69-kd and 100-kd forms of 2-5A synthetase in interferon-treated human cells by specific monoclonal antibodies.

Recently, the existence of 40-kd and 46-kd 2-5A synthetases in interferon-treated cells has been confirmed by cloning and characterization of cDNA corresponding to these small size enzymes. By the use of specific monoclonal antibodies, we describe here two forms of high mol. wt 2-5A synthetases of 69 and 100 kd in human cells. The monoclonal antibodies immunoprecipitate either a 69- or a 100-kd 2-5A synthetase. These purified 2-5A synthetases in immune complex preparations are active, i.e. addition of poly(I).poly(C) and ATP results in the synthesis of 2-5A. Both 2-5A synthetases are composed of several subspecies with similar isoelectric points in the range of 7-8 but have different subcellular localizations: 100-kd synthetase is recovered from the microsomal pellet whereas 69-kd synthetase is found to be associated with cell membranes as well as with the microsomal pellet. Different types of interferon-treated human cells express both or either forms of these enzymes. The 69- and 100-kd 2-5A synthetases were also identified by electrophoretic transfer immunoblot analysis using rabbit polyclonal antibodies against a synthetic peptide common on both 46- and 40-kd 2-5A synthetases. These results indicate that small and large size isozymes share a common peptide sequence.