Three-dimensional structure of beta 2-microglobulin.

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Abstract
The 3-dimensional structure of .beta.2-microglobulin, the L 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 .ANG. by using the methods of multiple isomorphous replacement and electron density modification refinement. The molecule is .apprx. 45 .times. 25 .times. 20 .ANG. in size. Almost half of the amino acid residues participate in 2 large .beta. structures, one of 4 strands and the other of 3, 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 molecule 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 H chains of major histocompatibility complex antigens. These results demonstrate that, at the level of detailed 3-dimensional structure, the L chain of the major histocompatibility class I antigens belongs to a superfamily of structures related to the Ig constant domains.