Synthesis and secretion of mouse immunoglobulin chains from Xenopus oocytes

Abstract

The secretion of immunoglobulins from mouse plasmacytoma cell lines is a valuable model for the study of mechanisms of protein secretion from eukaryotic cells. Some cells do not synthesize heavy chain immunoglobulin while retaining the ability to synthesize though not to secrete the complementary light chain^1–3. In the MOPC 21 variant tine, NS1, it is absence of heavy chain that prevents secretion^4–6. A reciprocal dependence, that is, a requirement of light chain for heavy chain secretion, has not been tested directly because no variants producing normal heavy chain alone have been detected^4,7–9. In view of the known insolubility characteristics of isolated heavy chains in vitro^10–12, this failure has led to the idea that free heavy chains would be toxic to the cell^7,9. Recently, Xenopus laevis oocytes have been shown to provide a surrogate system for studying protein secretion after microinjection of the encoding mRNA^13,14. Using this technique we demonstrate here that tetrameric mouse immunoglobulin is assembled and secreted by the oocyte but that free light chains are not secreted. Moreover, using purified heavy chain mRNA we show that heavy chain dimers accumulate in the oocyte and are not secreted unless light chain mRNA is also injected.