Biochemical nature and evolution

02M consists of a single, nonglycosylated, 99 amino acid polypeptide chain. Homologs of (32M have been described and sequenced for a number of mammalian species. Comparison of amino acid sequences indicates a high degree of evolutionary conservation of this protein. The overall sequence identity among the various mammalian (i2Ms is approximately 70%, except for mouse and rat P2M, which, as might be expected, show greater identity (86%). The high degree of structural conservation of (i2M among species is further indicated by the observation that (32M of different species can functionally substitute for each other. Since ยก32M is located entirely outside the cell with no direct attachment to the cell membrane, and since the association between |32M and the class I heavy chain is noncovalent, (32M on the cell surface can exchange with free (32M present in serum used to grow cultured cells. Mammalian p3M

contains two cysteine residues at positions 25 and 80, and these are involved in a disulfide bond. Because of sequence similarity of (32M to Ig molecules and because of the presence of a disulfide loop with a predicted tertiary structure similar to that of Ig domains, (32M has been considered a member of the Ig gene superfamily. The primary structural identity between (32M and the Ig constant region domain is about 30%. The similarity between them is more significant considering matches of specific key amino acids. The placement of p2M in the Ig gene superfamily has been confirmed by crystal structure studies discussed below. The heavy chain of class I MHC proteins is also a member of Ig super-family.

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