Subunit structure of cell surface proteins: disulfide bonding in antigen receptors, Ly-2/3 antigens, and transferrin receptors of murine T and B lymphocytes.

Abstract

The surface proteins of lymphocytes from spleen and thymus and several cultured lymphoid tumor lines were radioiodinated in situ, solubilized with Triton X-100, and examined for the presence of disulfide-bonded subunits by 2-dimensional (intact, reduced) NaDodSO4/polyacrylamide gel electrophoresis. Few lymphocyte surface proteins consisted of disulfide-bonded subunits, and the most prominent of these could be identified. In normal B lymphocytes and B-lymphoma cells, IgD or IgM (or both) were the major disulfide-bonded proteins, and these were easily detectable, even without immunoprecipitation. In contrast, analysis of thymocytes and T-lymphoma cells did not reveal any protein resembling Ig in its chain structure. The major labeled thymocyte membrane protein consisting of disulfide-bonded subunits was identified as the Ly-2/3 antigen. It appeared to contain disulfide-bonded homodimers of MW 35,000 (.alpha.2) noncovalently associated with a 2nd pair of homodimers of MW 30,000 (.beta.2). Peptide mapping showed these polypeptides to be homologous. A 3rd disulfide-bonded homodimer, which was heterogeneous in apparent MW, appeared to be part of the Ly-2/3 complex. All cultured T- and B-lymphoma lines examined possessed a major surface protein that appeared to be a disulfide-bonded homodimer of a polypeptide of 95,000 MW. This protein was identified as the receptor for transferrin. The presence of 2 or more subunits in cell surface receptors may render their ligand functionally bivalent, making ligand-induced receptor aggregation possible.