Amino acid sequence of human plasma alpha 1B-glycoprotein: homology to the immunoglobulin supergene family.

The complete amino acid sequence has been determined for alpha 1B-glycoprotein (alpha 1B), a protein of unknown function present in human plasma. This protein (Mr approximately equal to 63,000) consists of a single polypeptide chain N-linked to four glucosamine oligosaccharides. The polypeptide has five intrachain disulfide bonds and contains 474 amino acid residues. Analysis of the amino acid sequence by several computer programs shows that alpha 1B exhibits internal duplication and consists of five repeating structural domains, each containing about 95 amino acids and one disulfide bond. alpha 1B has a unique amino acid sequence. However, several domains of alpha 1B, especially the third, show statistically significant homology to variable regions of certain immunoglobulin light and heavy chains. alpha 1B also exhibits sequence similarity to other members of the immunoglobulin supergene family such as the receptor for transepithelial transport of IgA and IgM and the secretory component of human IgA. Because of its internal duplication and its sequence homology to immunoglobulin-like proteins, alpha 1B appears to have evolved from an ancestral gene similar to that of the immunoglobulin supergene family.