Structure and significance of beta2-microglobulin.

beta2-Microglobulin shares many structural features with the homology regions of the immunoglobulins. Particularly significant is the fact that its amino acid sequence is homologous to the sequences of the constant regions of both classes of light chains (kappa and lambda) and to the constant homology regions of at least three classes (gamma, mu and epsilon) of heavy chains, especially the carboxyl-terminal regions Cgamma3 Cmu4 and Cepsilon4. Molecules similar to human beta2-microglobulin have been found in other vertebrate species. The properties of beta2-microglobulin suggest that the gene for this protein may have evolved from a precursor gene that by duplication gave rise to immunoglobulin light and heavy chains. Furthermore, the observation that beta2-microglobulin is synthesized by and appears on the surfaces of a variety of cell types, including nonlymphoid cells, suggests that the concepts derived from analysis of the immune system may be applicable to other areas of cell biology. In particular, the close association of this immunoglobulin-like molecule with the histocompatibility antigens has a number of implications for the origin, structure, and function of these as well as other cell surface glycoproteins.