Becker J W, Reeke G N
Proc Natl Acad Sci U S A. 1985 Jun;82(12):4225-9. doi: 10.1073/pnas.82.12.4225.
The three-dimensional structure of beta 2-microglobulin, the light chain of the major histocompatibility complex class I antigens, has been determined by x-ray crystallography. An electron density map of the bovine protein was calculated at a nominal resolution of 2.9 A by using the methods of multiple isomorphous replacement and electron density modification refinement. The molecule is approximately 45 X 25 X 20 A in size. Almost half of the amino acid residues participate in two large beta structures, one of four strands and the other of three, linked by a central disulfide bond. The molecule thus strongly resembles Ig constant domains in polypeptide chain folding and overall tertiary structure. Amino acid residues that are the same in the sequences of beta 2-microglobulin and Ig constant domains are predominantly in the interior of the molecule, whereas residues conserved among beta 2-microglobulins from different species are both in the interior and on the molecular surface. In the crystals studied, the molecule is clearly monomeric, consistent with the observation that beta 2-microglobulin, unlike Ig constant domains, apparently does not form dimers in vivo but associates with the heavy chains of major histocompatibility complex antigens. Our results demonstrate that, at the level of detailed three-dimensional structure, the light chain of the major histocompatibility class I antigens belongs to a superfamily of structures related to the Ig constant domains.
主要组织相容性复合体I类抗原的轻链β2-微球蛋白的三维结构已通过X射线晶体学确定。通过多重同晶置换和电子密度修正精修方法，计算出了牛β2-微球蛋白的电子密度图，标称分辨率为2.9埃。该分子大小约为45×25×20埃。几乎一半的氨基酸残基参与形成两个大的β结构，一个由四条链组成，另一个由三条链组成，通过一个中心二硫键相连。因此，该分子在多肽链折叠和整体三级结构上与免疫球蛋白恒定区极为相似。β2-微球蛋白和免疫球蛋白恒定区序列中相同的氨基酸残基主要位于分子内部，而不同物种β2-微球蛋白之间保守的残基既位于分子内部，也位于分子表面。在所研究的晶体中，该分子显然是单体，这与β2-微球蛋白与免疫球蛋白恒定区不同，在体内显然不形成二聚体，而是与主要组织相容性复合体抗原的重链结合的观察结果一致。我们的结果表明，在详细的三维结构水平上，主要组织相容性I类抗原的轻链属于与免疫球蛋白恒定区相关的结构超家族。
