Antigen receptor on B lymphocytes: structure and association with other membrane proteins

IgM and IgD function as receptors for antigens on B lymphocytes: The mechanism(s) and structures by which these Igs transmit the signals induced by antigen binding are unknown. Thus, it is relevant to propose plasma membrane molecules anchored with the Igs which are responsible for the transformation of signals through to the interior of the cells. The aim of this study, therefore, was to identify plasma membrane structures associated with or covalently linked to Ig molecules. Ig molecules were isolated from the lysates of surface radioiodinated B lymphocytes and analyzed by two-dimensional SDS polyacrylamide gel electrophoreses (1st. dimension: unreduced; 2nd dimension: reduced). It was shown that various polypeptides (mol. wts: 56, 50, 46, 42, 35 and 30 kdaltons) could be identified as being covalently linked (by S-S bridges) to IgM and IgD half molecules. Employing the chemical crosslinking of associated surface proteins on intact B cells, we could demonstrate that two proteins (mol. wts: 56 and 46 kdaltons) were non-covalently linked to IgM half molecules. In order to study the biological significance of molecules anchored with Ig, we analyzed their behaviour on B lymphocytes activated by anti-Ig antibodies. Within 10 min. of B lymphocyte activation by anti-mu antibodies IgM half molecules and their accompanying polypeptides, both covalently and non-covalently bound, were removed from cell's surface.