Subunits of IgM and IgD on the surface of murine B-lymphocytes

Abstract

Immunoglobulin isolated from 125I-labelled cell surface proteins of murine B-lymphocytes was analyzed by a sensitive two-dimensional polyacrylamide gel electrophoresis technique (2D-SDS-PAGE). Uncleaved Ig molecules were electrophoresed in the first dimension in an SDS-polyacrylamide gel, and after subsequent reduction of the disulphide bonds by mercaptoethanol, the cleaved polypeptides were separated in the second dimension using again an SDS-polyacrylamide gel. This technique enables the identification of unreduced Ig molecules and their corresponding subunit components. In the case of IgM as well as IgD the four chain structure (H 2L 2), half molecules (HL), and disulphide-linked heavy chains (HH) could be identified. Since all Ig subunits were isolated by an anti-IgG antiserum by virtue of its L-chain specificity we conclude that L-chains are noncovalently associated with the disulphide-linked heavy chains (HH). Free noncovalently bound L-chains could actually be identified by 2D-SDS-PAGE. However, this technique does not determine whether half Ig molecules (HL) are noncovalently associated with each other. In addition to free L-chains noncovalently linked μ- and δ-chains were found. Control experiments showed that the identified Ig subunits are not artefacts of the isolation procedure or reduction moieties of H 2L 2 molecules (e.g. incubation of isolated μ 2L 2 and δ 2L 2 with detergent extracts of spleen cells does not result in the formation of subunits). On the basis of 125I-radioactivity incorporated into the Ig subunits it was estimated that besides μ 2L 2 and δ 2L 2, δL (40–50% of total IgD) is the main Ig structure on B-lymphocytes. The other Ig subunits (μL, δ 2, μ 2) contribute only 10% or less to the total amount of IgM and IgD. However, the numbers of such structures ( ca. 10 4 molecules/B-cell) are sufficient to act as antigen recognition structures and to be involved in B-lymphocyte triggering and tolerance induction.