Partial amino acid sequence of the precursors of blocked heavy and light chains from C 3 H mouse myeloma 5563.

A messenger RNA fraction from the C 3 H mouse myeloma 5563 was used to direct the synthesis of heavy (gamma 2a) and light (chi) chain precursors. The synthetic products were radiolabeled by inclusion of [3H] or [35S] amino acids in wheat-germ cell-free translation system. Precursor peptides for both H and L chains were indicated by comparison by polyacrylamide gel analysis of apparent molecular weights of chains synthesized in vitro vs. in vivo. The nonglycosylated H chain synthesized in vivo in the presence of tunicamycin was used for comparison. This approach should be generally applicable for the demonstration of precursor forms of N-glycosylated polypeptides. Following immunoprecipitation and preparative polyacrylamide gel electrophoresis, the isolated chains were subjected to automated Edman degradation. The amino acid sequence data obtained for these precursors were significantly different from those reported for BALB/c immunoglobulins. The 5563 H chain precursor bears little homology to the previously reported H chain precursor from MOPC 315 (IgA) (Jilka, J. A. and Pestka, S., Proc. Nat. Acad. Sci. USA 1977. 74:5692). The 5563 H chain can be assigned to a heretofore undescribed H chain variable region subgroup on the basis of partial sequence data. The amino terminal sequence of the 5563 L chain precursor has maximum homology (25-50%) to the ssequence o the precursor peptide of MOPC 41 chi chain.