Macromolecular insoluble cold globulin (MICG): a novel protein from mouse lymphocytes. IV. Evidence for the plasma membrane distribution of MICG.

Macromolecular insoluble cold globulin is a glycoprotein synthesized predominantly by T lymphocytes in the mouse. The present report details experiments demonstrating the plasma membrane distribution of MICG on T lymphocytes. By utilizing immunofluorescent techniques it was shown that MICG was located in the external cell surface of 98% of thymic lymphocytes and 60% of splenic lymphocytes. Furthermore, in spleen cells, it was demonstrated that T cells and not B cells were surface MICG positive. Antibody to MICG was able to cap all of the immunofluorescent-positive (60%) spleen cells. In contrast, anti-MICG antibody did not induce cap formation on thymus cells. Only when dilute solutions of antibody were used did MICG cap on the thymus cells. Employing limited proteolysis of thymus and spleen cells MICG was shown to be regenerated on the surface of T cells with a half-life of 3.5 hr. The distribution and cell surface characteristics of MICG are discussed in terms of a "receptor-like" function for this protein.