Synthesis of a carboxyl-terminal (constant region) fragment of the immunoglobin light chain by a mouse myeloma cell line

The MPC11 mouse myeloma cell line synthesizes not only heavy chains and light chains but also an 11,600 molecular weight light chain fragment. The fragment comprises 1% of the newly synthesized protein, compared to 8% for the complete light chain. Similar amounts of fragment are produced by a number of heavy plus light chain producing subclones, 18 independently generated light chain producing variant clones, and five independent non-producing variant clones. For both the heavy plus light chain producing and the non-producing cell types, less than 20% of the fragment appears to be secreted, while the remainder is metabolized with a half-life of 30 minutes. Radiochemical peptide analyses and radiochemical amino -terminal sequence analyses are consistent with the fragment containing most of the peptide sequences present in the carboxyl-terminal half (constant region) of the parent kappa light chain, but none of the variable region peptides. The fragments produced by a heavy plus light chain producing clone and a non-producing variant clone were identical by radiochemical peptide analysis. The results suggest that the constant region fragment may be a primary gene product, and in addition, they raise the possibility that the fragment may be specified by a gene discrete from the gene specifying a light chain.