Crystallographic studies of bovine beta2-microglobulin.

Abstract

Crystals of the bovine milk protein lactollin yield x-ray diffraction data extending to a resolution of 2.8 A.Lactollin is a bovine analogue of #2-microglobulin, a protein that is homologous in amino acid sequence to the constant domains of immunoglobulins and is the light chain of the human and murine major histocompatability antigens. The protein crystallizes in the orthorhombic space group P212121 with a = 77.4, b = 47.9, and c = 34.3 A. The #2-Microglobulin is a protein distinguished by its similarity to certain portions of immunoglobulins (1-3) and by its association on cell surfaces with the major histocompatability antigens of man (HLA) and mouse (H-2) (4-7). Since its discovery in human urine in 1968, the overall properties, synthesis, and amino acid sequence of this protein have been characterized (2,(8)(9)(10)(11). Similar proteins have been detected in a variety of species, and all appear as a single polypeptide [molecular weight (Mr) = 11,600], with few differences in amino acid sequence among the proteins from different species (2,3,(12)(13)(14).Comparison of the amino acid sequence and other features of human #32-microglobulin with immunoglobulins (1, 15) has indicated that it closely resembles the constant homology regions (16) of immunoglobulins G, M, and E. In intact immunoglobulin molecules, each homology region is closely associated with the corresponding homology region of another immunoglobulin chain in pairs of domains (16, 17), whereas ,62-microglobulin is found associated with the heavy chain of HLA or H-2 antigens or the closely related thymus leukemia antigen (4-7, 18, 19), suggesting that such paired domains may be structural features of these cell surface molecules. In solution,purified Or-microglobulin, however, is generally unpaired; i.e., it appears as a free monomer (8) The elucidation of the three-dimensional structure of ,32-microglobulin would be of particular interest because it would provide an opportunity to compare this molecule in detail with immunoglobulin domains, and thus contribute to an understanding of its interaction with the heavy chains of H-2 and HLA antigens. It has recently been found that lactollin, a crystalline protein isolated from bovine milk and colostrum, has an amino-terminal amino acid sequence sufficiently similar to those of 6B2-microglobulins from several species to indicate that it is probably a bovine homologue of the protein (M. L. Groves, private communication; refs. 20 and 21). Although the reported molecular weight of lactollin is 43,000 (22), it is composed of subunits of 12,000 daltons. We have confirmed the molecular weight of the subunit and the similarity in amino acid sequence to 62-microglobulin. We report here x-ray diffraction studies ofThe costs of publication of this article were defrayed in part by the payment of page charges from funds made available to support the research which is the subject of the article. This article must therefore be hereby marked "advertisement" in accordance with 18 U. S. Amino acid sequence ana...

Localization of the sites of iodination of human .beta.2-microglobulin; quaternary structure implications for histocompatibility antigens

“…This is consistent with the circular dichroism data, which indicates most of /32m is in /3 sheet (Isenman et al, 1975). Moreover, two models that predict secondary structure, based primarily on homology to immunoglobulin (Becker et al, 1977) or primary sequence (Cohen et al, 1980), propose this particular /3-strand alignment, when a sequence correction that removes a serine between Y-66 and Y-67 is taken into account (Suggs et al, 1981;Parker & Strominger, 1982). There are two possible explanations for the low level of iodination of Y-63 of /32m in the HLA-B7 complex.…”

“…On the basis of these ideas, two independent models that predict a particular strand alignment for the /3-pleated sheets of j32m have been proposed (Becker et al, 1977;Cohen et al, 1980). However, no chemical evidence has yet been obtained that identifies the strands that comprise the putative interdomain interface.…”

Three-dimensional structure of beta 2-microglobulin.

“…The preparation and crystallization of 832m from bovine milk and colostrum have been reported previously (32,33). The molecule crystallizes in the orthorhombic space group P212121 with a = 77.27, b = 47.99, and c = 34.42 A.…”

Cell-free synthesis and segregation of beta 2-microglobulin.

“…The molecule is also found free in various biological fluids including urine, serum, amniotic fluid, milk, and seminal fluid (10). Initially discovered in man, f2-microglobulin is probably present in nearly all vertebrate species (10,11), and one such molecule from cow's milk has recently been crystallized (12). A distinguishing feature of this protein is that it resembles the constant region domains (CL, CHI, CH2, and CH3) of immunoglobulins in overall structure (13), in amino acid sequence (14)(15)(16), and in the arrangement of its disulfide bond (16).…”