Brief Definitive Report BIOGENESIS OF MEMBRANE-BOUND AND SECRETED IMMUNOGLOBULINS I. Two Distinct Translation Products of Human/x-Chain, with Identical N-Termini and Different C-Termini* By J M. McCUNE, V. R. LINGAPPA, S. M FU,:~ G. BLOBEL, AND H G KUNKEL From the Departments of Cell Bwlogy and Immunology, The Rockefeller Unwerszty, New York 10021 Two forms of IgM, functionally and topologically distinct, are produced during the course of B cell differentiation. On the plasma membrane of resting B cells, an 8S monomer serves as an antigen receptor, conveying the signal of antigen recognition across the lipid bilayer. As B cells mature to IgM plasma cells, IgM is, instead, secreted as a soluble, 19S pentameric structure. Notably, both forms derived from a single B cell clone have been shown to share similar V regions by idiotypic analysis (1, 2). Differences in function and in topology between membrane-bound and secreted IgM have been attributed in the past to structural differences in their respective heavy chains, ~m and/~, as evidenced by the following observations: #,1 exhibits hydrophobic properties not associated with/~s (3-5); ~m appears to be glycosylated to a different degree than/~ (6-8), /~m and ~ts generate similar, yet unique, peptide maps (8-10): and/~m migrates more slowly than/t~ on sodium dodecyl sulfate (SDS) polyacrylamide gel electrophoresis (PAGE) (7, 11), even after biosynthesis in the presence of tunica- mycin (4). The results of amino acid sequence analysis have been ambiguous. Several groups have reported identical C-terminal sequences for the two (7, 8, 12), whereas one group has found that/Xm, from the cell line Daudi, differs in C-terminal sequence from/~, secreted by another cell line, Ram. (9). The above results are all consistent with the notion that/~m and/x~ differ in primary structure. However, because all of the data were generated in systems that analyze mature proteins, the relative contribution of co- and posttranslational modifications to the observed differences cannot be assessed. Differential glycosylation (e.g., of O- linked carbohydrates in the presence of tunicamycin) and proteolysis of otherwise identical primary translation products are alternative possibilities that have not been ruled out. In the present studies, we have looked at the primary translation products of the ~-chain, with RNA extracted from human lymphoblastoid cell lines positive for both surface and secretory IgM. /~m and /~ are encoded as distinct polypeptide chains, * Supported m part by U S Pubhc Health grants RR-102, CA-24338, AI-10811, and CA-27155 :~ Scholar of the Leukemia Society of America, Inc J Exr MEn © The Rockefeller Umversity Press • 0022-1007/80/08/0463/06 $1 00 463 Volume 152 August 1980 463-468 Downloaded from https://rupress.org/jem/article-pdf/152/2/463/475668/463.pdf by guest on 02 June 2020