Evidence for hydrophobic region within heavy chains of mouse B lymphocyte membrane-bound IgM

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Abstract
The gel filtration behavior, in the presence of detergents, of membrane-bound Ig[immunoglobulin]M from normal mouse spleen B [bone marrow-derived] lymphocytes was compared to that of secretory IgM from mouse plasma cells. The proteins were labeled by surface radioiodination or biosynthetically with radioactive amino acids. Cell lysates were fractionated on calibrated Sepharose 6B columns in the presence of the detergents Nonidet P-40 or deoxycholate. Eluted fractions were immunoprecipitated and the reduced or unreduced precipitates were analyzed by sodium dodecyl sulfate gel electrophoresis followed by radioautography. Surface 125I-labeled 8S IgM exhibited a gel filtration pattern in Nonidet P-40 corresponding to much higher apparent MW than that of secretory 8S IgM, a difference that almost disappeared when gel filtration was performed in the presence of deoxycholate, which forms much smaller micelles than does Nonidet P-40. Biosynthetically labeled lymphocytes contain 2 types of IgM molecules differing in their gel filtration behavior and fate: 1 identical to secretory 8S IgM of plasma cells and secreted in the medium during chase periods, and the other identical to surface 125I-labeled IgM and remaining cell-associated. Because the surface-bound 8S IgM was not associated with other labeled molecules, the detergent-binding behavior of surface IgM is probably due to a hydrophobic segment carried by these Ig molecules. That lymphocytes synthesize 2 types of .mu. chains was also shown by the use of tunicamycin, an inhibitor of glycosylation. In its presence, 2 unglycosylated .mu. chains were observed: 1 identical in size to that made by tunicamycin-treated plasma cells, and the 2nd slightly larger. Gel filtration in Nonidet P-40 of the cell lysates of tunicamycin-treated lymphocytes showed that the nonsecretory 8S IgM contains this 2nd type of .mu. chains, whereas the IgM molecules of the secretory type contain plasma cell-like .mu. chains. Membrane IgM .mu. chains apparently contain a hydrophobic segment which is responsible for its association to the membrane.