β2-Microglobulin occurs in three size classes in normal mouse serum. Apart from β2-microglobulin in free form, it occurs associated with H-2 antigen fragments as well as with a high-molecular-weight protein complex. The latter material has been isolated in highly purified form. During the isolation procedure, the high-molecular-weight β2-microglobulin-containing serum material dissociated to yield a component of considerably smaller size. The highly purified material was comprised of molecules composed of two subunits. The small subunit displayed size, charge, and antigenic characteristics which made it indistinguishable from β2-microglobulin derived from regular H-2K and D antigens. The large subunit had an apparent molecular weight of about 42 000 and was, thus, slightly larger than the water-soluble H-2 antigen heavy-chain fragment produced by limited proteolysis. The serum-component heavy chain had a lower isoelectric point than the corresponding H-2 antigen chain and the serum material exhibited less charge heterogeneity than the H-2 antigens. Whereas papain-solubilized H-2 antigens were efficient in inhibiting alloantiserum-induced cytotoxicity, the serum component had no such effect even at 1000-fold higher concentrations. With use of similar techniques, the serum component was shown to be devoid of easily recognizable alloantigenic determinants shared with la or TL antigens. Antiserum against the H-2 region-controlled Ss protein did not react with the serum material. Rabbit antisera raised against highly purified serum component and H-2 antigen preparations recognized preferentially the immunogens but cross-reacted reciprocally. With use of the rabbit antiserum against the isolated serum component, attempts were made to find out the polypeptide chain composition of the intact high-molecular-weight fc-microglobulin-containing serum material. Indirect immunoprecipitation of the 125I-labeIed, partially purified high-molecular-weight material followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed the coprecipitation of two polypeptide chains with apparent molecular weights of 65 000 and 75 000 in addition to the 12 000 and 42 000 molecular weight chains. Thus, all data taken together strongly suggest that normal mouse serum contains a high-molecular-weight multisubunit protein complex which contains one polypeptide chain which is similar, but distinct, from the alloantigenic H-2 antigen subunit. © 1978, American Chemical Society. All rights reserved.
