Interferon: evidence for subunit structure.

During the purification of mouse and human interferons, multiple active components have been detected. Mouse interferon was purified over 500-fold by differential precipitation, centrifugation, gel chromatography, and isoelectric focusing. On electrofocusing, two molecular forms (A and B) were noted. Form B (isoelectric point 7.35) had a molecular weight of about 38,000 and Form A (isoelectric point 7.15), which was equally active, a molecular weight of 19,000. Purified Form B was dissociable into Form A, but the reverse reaction occurred to a much lesser extent. Human interferon, purified about 1550-fold, is also composed of multiple molecular forms. Form B (pI 5.60) had a molecular weight of about 24,000 and Form A (pI 5.35), which may contain up to 85% of the total activity, a molecular weight of 12,000. Both forms appear to be equally active. The dissociation of both human and mouse interferons into subunits appears to take place during dialysis versus low salt (0.01 M Tris pH 7.4). The data are consistent with the idea that the native molecule exists as a dimer of similar or identical subunits. Dimer formation, which probably occurs within the cells, does not seem to lead to a measurable cooperative effect between the subunits.