Synthesis and processing of the alpha heavy chains of secreted and membrane-bound IgA.

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Abstract
The synthesis and processing of Ig .alpha. chains in 2 murine cell lines, a B cell lymphoma that expresses membrane-bound IgA and a hybridoma that secretes IgA, were compared. Results of biosynthetic labeling experiments demonstrated that membrane-bound and secreted .alpha. chains have 2 distinct intracellular precursors of different MW and isoelectric points. RNA from both of these cell lines direct the synthesis in vitro of 2 .alpha. polypeptides of MW 59,000 and 62,000, the larger one being the precursor for membrane-bound .alpha. chain and the smaller one being the precursor for secreted .alpha. chain. These cell lines each contain 3 RNA, 1.7, 2.1 and 3.1 kilobases in length, which hybridize with c[complementary]DNA for the .alpha. constant region and which are present in different concentrations. The smallest RNA probably encodes the secreted .alpha. chain; 1 or both of the larger RNA probably encode(s) the membrane-bound .alpha. chain.