Multiple Common Properties of Human β ₂ -Microglobulin and the Common Portion Fragment Derived from HL-A Antigen Molecules

Abstract

ABSTRACT11,000-Dalton common portion fragments derived from HL-A antigen molecules were isolated and found to have a significantly high homology to p32-microglobulin in amino-acid composition. Common portion fragments are also very similar to 132-microglobulin with respect to molecular size, charge, and distribution in tissues. Moreover, both are found in the spent culture media of human cell lines and in human plasma and urine.Thus it appears that j62-microglobulin may well be the same substance as the common portion fragment of HL-A antigen molecules.We recently reported the isolation of a small fragment ofHIA antigens which appears to be a characteristic invariant portion of the structure of HLA antigens. It was isolated from papain-solubilized HL-A antigen molecules of various HL-A allotypes by mild degradation procedures (1, 2). The molecular size is about 11,000 daltons. The small fragment does carry an HI-A common antigenic activity which is a characteristic antigenic marker of HL-A antigens (2, 3) although the fragment is devoid of HL-A alloantigenic activity. This small fragment has been designated HLA common portion fragment. Cresswell et al. (4) recently reported the isolation of a similar I1,000-dalton fragment from papainsolubilized HL-A antigen. Similar 11,000-dalton fragments are present in human blood plasma (5) and urine and in spent culture media from cultured human cell lines (1). We recently purified the small fragments from spent culture media of cultured human lymphoid cell lines and found them to be identical to the HL-A common portion fragment that was obtained by acid cleavage of papain-solubilized HLA antigen molecules with respect to molecular size, electrophoretic mobility, and isoelectric point (unpublished). No antigenic differences were observed.It now appears to us that the above fragment is very closely related or identical to the fl2-microglobulin that was originally described by Berggard and Bearn (6) and that has a structural similarity to the constant parts of human immunoglobulin G (7,8).We have determined the amino-acid composition of the HL-A common portion fragment isolated from spent culture media of a human lymphoid cell line and compared it with the composition of the ,B2-microglobulin reported by Berggfrd and Bearn (6). Assessment of compositional relatedness between proteins, by the method described by Metzger et al.(9), indicated that ,32-microglobulin has a significantly high homology to HLA common portion fragment.The fragment subjected to amino-acid analysis was purified from spent culture media of RPMI 1788 cells by isolation 2863 procedures that involved differential ultrafiltration, gel filtration, and column electrophoresis. The HL-A common antigenic activity was followed by radioimmunoassay (3). The culture of RPMI 1788 cells with RPMI 1640 medium supplemented with 10% fetal-bovine serum and with penicillin and streptomycin was harvested at day 3. The spent culture medium was separated by centrifugation and filtered with an HA Millipore filter (Millipore Co...

Evolutionary relationship between immunoglobulins and transplantation antigens.

“…The molecular-weight data are fully consistent with a tetrameric structure constituted by two types of polypeptide chains. In agreement with previous findings, the present observations demonstrate that the small subunit, ,32-microglobulin, is bound to the large, alloantigenic polypeptide chain by noncovalent interactions only (3)(4)(5)(6)(7)(8)(9)(10)(11). In contrast, the heavy polypeptide chains are held together by disulfide bridge(s).…”

“…The large subunit carries the alloantigenic determinants, whereas the small polypeptide chain is invariant. The small subunit has been identified as #2-microglobulin both in man (5,(9)(10)(11) and mouse (7,8). This finding is of particular interest, since previous studies of the amino-acid sequence of 2-microglobulin have shown that this protein is highly homologous to the various IgG heavy and light chain homology regions (12)(13)(14).…”

Use of synthetic oligonucleotides as hybridization probes: isolation of cloned cDNA sequences for human beta 2-microglobulin.

“…Am is a small protein (molecular weight 11,800) that was isolated from urine (7). Subsequently, A3~m was found associated with cell surfice antigens of the major histocompatibility locus (8,9). The exact function of A2m is unclear, although recent evidence suggests that the molecule may stabilize the tertiary structure of associated proteins (10).…”

Assignment of the gene for beta 2-microglobulin (B2m) to mouse chromosome 2.

“…It is found on the surface ofmost nucleated mammalian cells as a component of the major histocompatibility antigens but does not itself associate with the cell membrane (2)(3)(4)(5). The precise function of B2M is unknown but it is thought to play an important role in those immunological phenomena, such as immune responsiveness, immune suppression, T-B cell interaction, and restriction of T-cell specificity, that depend on the expression of histocompatibility antigens (6).…”

“…The chromosomal location of the gene coding for B2M in other species is not known. Recently, allelic forms of B2M in the mouse have been described, and preliminary studies have suggested linkage between B2m and genes coding for the Ly-4 and H-3 antigens, which have been assigned to mouse chromosome 2 (11,6,12).…”