Complete amino acid sequence of human plasma beta 2-glycoprotein I.

We have determined the complete amino acid sequence of beta 2-glycoprotein I (Mr, congruent to 50,000), a human plasma protein that is associated with lipids and binds to platelets but whose function is not yet known. The protein consists of 326 amino acids and has five attached glucosamine-containing oligosaccharides. The protein is rich in cysteine and proline, and the sequence is notable for the frequent occurrence of Cys-Pro linkages at regular intervals. Computerized analysis of the sequence reveals five consecutive homologous segments in which cysteine, proline, and tryptophan appear to be highly conserved. This suggests that beta 2-glycoprotein I may have evolved by repeated duplications of a gene coding for a 60-amino acid segment of protein.