A High Production Rate of Translatable IgG mRNA Accounts for the Amplified Synthesis of IgG in Myeloma Cells

The aim of the present work was to determine whether the accumulation of Ig mRNA in myeloma cells is due to a high rate of production or to a high stability of these molecules. Specific mRNAs for the light and heavy polypeptide chains of IgG were isolated from the murine MPC‐11 myeloma tumor cells by immune precipitation of polysomes which synthesize these chains. It was found that the immune‐precipitated polysomes were enriched 10 –30‐fold in the γ and χ mRNA sequences respectively. In the wheal germ cell‐free system the χ mRNA preparation was translated mainly into three polypeptides of M r 25000, 18000, and 15000. The method of immune precipitation of polysomes was also used to characterize three variant clones of MPC‐11 myeloma. It was found that little if any γ‐chain polysomes are present in the L‐chain producer and non‐producer clones, while a substantial amount of χ‐chain polysomes was present in the non‐producer clone. This may be due to the presence in the non‐producer cells of the constant region χ‐chain fragment. In order to determine the relative synthesis rate of χ and γ mRNAs, pulse‐labeled polysomes were immune precipitated using antibodies to χ and γ chains. It was found that χ and γ mRNA molecules are produced at a very high relative rate each accounting for 10–15% of the total labeled mRNA after 1 h of labeling. These values are higher than the steady‐state pool size of χ and γ mRNA, which was 5–6%, and indicates that the half‐life of these molecules is not unusually high. It is concluded that the amplified synthesis of immunoglobulin chains in myeloma cells is mainly due to a high rate of production of translatable χ and γ mRNAs.