Macromolecular insoluble cold globulin (MICG): A novel protein from mouse lymphocytes—I

Murine thymus and spleen cells synthesize a 225,000 dalton macromolecule (MICG). This protein is insoluble in the cold in non-ionic detergents and migrates as a β-globulin on electrophoresis. MICG can be isolated by a two-step procedure consisting of cold precipitation of cellular lysates in NP-40, followed by gel chromatography in detergent solution. Thymocytes synthesized three times more MICG than splenocytes. Immunological analysis demonstrated that anti-MICG antiserum reacted with the cold insoluble protein. Trypsin digestion of radiolabeled, isolated MICG illustrated the incorporation of¹⁴C-leucine into a number of peptides. MICG is a glycoprotein which accounts for 3.5% of the protein synthesized in thymus cells and is not secreted from thymocytes or spleen cells.