Disulfide-bonded Homodimer Research Articles

Interleukin 2 activation of natural killer cells rapidly induces the expression and phosphorylation of the Leu-23 activation antigen.

IL-2 potentiates both growth and cytotoxic function of T lymphocytes and NK cells. Resting peripheral blood NK cells can respond directly to rIL-2, without requirement for accessory cells or cofactors, and enhanced cytotoxicity can be measured within a few hours after exposure to this lymphokine. In this study, we describe an activation antigen, Leu-23, that is rapidly induced and phosphorylated after IL-2 stimulation of NK cells and a subset of low buoyant density T lymphocytes. Previously, it has been uncertain whether all NK cells or only a subset are responsive to IL-2. Since within 18 h after exposure to IL-2, essentially all NK cells express Leu-23, these findings indicate that all peripheral blood NK cells are responsive to stimulation by IL-2. The Leu-23 antigen is a disulfide-bonded homodimer, composed of 24-kD protein subunits with two N-linked oligosaccharides. Appearance of this glycoprotein on NK cells is IL-2 dependent and closely parallels IL-2-induced cytotoxicity against NK-resistant solid tumor cell targets.

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

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 two-dimensional (intact, reduced) NaDodSO4/polyacrylamide gel electrophoresis. [Hynes, R. O. & Destree, A. (1977) Proc. Natl. Acad. Sci. USA 74, 2855-2859]. Few lymphocyte surface proteins were found to consist 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 immunoglobulin 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 Mr 35,000 (alpha 2) noncovalently associated with a second pair of homodimers of Mr 30,000 (beta 2). Peptide mapping showed these polypeptides to be homologous. A third disulfide-bonded homodimer, which was heterogeneous in apparent Mr, appeared to be part of the Ly-2/3 complex. All cultured T- and B-lymphoma lines examined were found to possess a major surface protein that appeared to be a disulfide-bonded homodimer of a polypeptide of Mr 95,000. This protein was identified as the receptor for transferrin. It is suggested that the presence of two or more subunits in cell surface receptors renders their ligand functionally bivalent, making ligand-induced receptor aggregation possible.