The IgG2b-producing MPC 11 mouse myeloma cell line has yielded a number of variants which synthesize heavy chains characteristic of a different immunoglobulin subclass, IgG2a, as shown initially by serology, peptide maps, and assembly profiles. Primary structural analysis of the immunoglobulin synthesized by one variant, ICR 188.8.131.52, showed that the Fc fragment was most probably identical with that of MOPC 173, an IgG2a protein of known sequence, and different from the parental 72b Fc fragment. We report here our studies on the protein synthesized by a second 72a-producing variant, ICR 11.19.3. The Fc fragment of ICR 11.19.3 differed from that of ICR 184.108.40.206 by comparative peptide mapping and was shown by partial sequence determination to contain a long C-terminal stretch of γ2a sequence and a short stretch of γ2b sequence at the amino terminus. Iden tification of additional residue positions which discriminate between the two subclasses have localized the junction of γ2b and γ2a sequences in ICR 11.19.3 between residues N-308 and N-331, some 8-32 residues N terminal to the CH2/CH3 domain boundary. This junction comprises 24 amino acids which, with one possible exception, are identical between γ2b and γ2a subclasses. Our isolation and characterization of CNBr fragments from the N-terminal region of the parental MPC 11 heavy chain allowed us to describe an additional γ2b constant region fragment. The ICR 11.19.3 variant protein contained a homologous fragment which seemed to be identical, thus confirming the presence of 72b sequence in this region. From these studies, we conclude that ICR 11.19.3 synthesizes a γ2b~γ2a hybrid immunoglobulin heavy chain.
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