Immunoglobulin subunits of murine B lymphocytes: structure and associations with other membrane proteins.

The Ig subunit structure of murine B lymphocytes was studied by employing different radiolabelling techniques in combination with chemical cross-linking. The main membrane structure of IgM was a half molecule that was disulphide-linked to proteins with MW 30,000, 45,000 and 55,000, respectively. Small amounts of mu 2L2, microL disulphide-linked to a protein with MW 50,000, and free microL were also detected. The main IgD structures were half molecules disulphide-linked to two proteins with MW 14,000 and two proteins with MW 16,000. Furthermore, IgD half molecules disulphide-linked to a protein with MW 16,000 and free half molecules could be demonstrated. Labelling with hydrophobic reagents showed that all Ig molecules and the protein with MW 50,000, linked to microL, penetrated the lipid bilayer, whereas the other IgM- and IgD-linked proteins probably did not. Additional proteins which were associated exclusively with IgM were detected by chemical cross-linking. These findings offer new possibilities for the investigation of the function(s) of antigen receptors on B cells.