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Biosynthesis and assembly of IgM. Addition of J chain to intracellular pools of 8S and 19S IgM

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The addition of J chain to intracellular and secreted IgM has been studied in mitogen-stimulated mouse spleen cells and two mouse myelomas (MOPC 104E and Y5781). The cells of Y5781 contain a large pool of intracellular 19S IgM and it is shown that this IgM is assembled by two pathways, one via the addition of J chain to the 8S IgM subunit and the second by direct addition of J chain to 19S IgM. The absence of J chain on the intracellular IgMs of MOPC 104E and mitogen-stimulated spleen cells suggests that IgM may be assembled in these cells predominantly by the second pathway. The detection in Y5781 cells of a population of 19S IgM molecules without J chain, to which J chain is added shortly before or during secretion, demonstrates that J chain is not necessary for the polymerisation of IgM in the intact cell.

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