Analysis of surface mu-chain expression in human lymphoblastoid cell lines that do not produce light chains

It has been suggested that light chains (LC) are necessary for the surface expression of mu heavy chains. Fluorescent antibody screening of 42 human lymphoblastoid cell lines transformed in our laboratory, however, disclosed four lines that expressed surface mu-chains without LC. The biosynthesis, glycosylation, and turnover of mu-chains in these cell lines was compared to mu-chain production in cell lines synthesizing both heavy chains and LC. LC production could not be detected in the mu +LC- cell lines by either surface or biosynthetic labeling. The mu-chains expressed on the surface of the LC- cells appeared as disulfide-linked dimers and migrated slightly faster on SDS-polyacrylamide gels (70 Kd) than did mu-chains from IgM monomers (H2 L2) (78 Kd) after reduction. Biosynthetic labeling in the presence of tunicamycin demonstrated that the smaller size of free mu heavy chains was due to incomplete glycosylation of these molecules and not to amino acid deletions. The mu-chains produced by mu + LC- cells lines were degraded faster than mu-chains from LC+ cell lines, but their rate of transit to the cell surface was identical in both cell types. Thus, although LC are necessary for the formation of an intact antigen-binding site, they are not involved in the synthesis or expression of membrane mu-chains.