Two out of the three kinds of subunits of inter-alpha-trypsin inhibitor are structurally related.

Inter-alpha-trypsin inhibitor is a 240-kDa plasma-protein complex of three different types of glycoproteins. Their stoichiometric relation in the complex is not yet known. One subunit results from proteolytic processing of a precursor protein composed of alpha 1-microglobulin and a double-headed Kunitz-type proteinase inhibitor protein. From this, only the inhibitor protein becomes part of the inter-alpha-trypsin inhibitor complex. Another subunit whose function is not yet understood is structurally unrelated to the first one as well as to other proteins of various data collections. Now we have obtained a cDNA clone coding for 837 amino acid residues of a precursor protein of the third subunit. Its primary structure is 40% identical to that of the completely known second-subunit precursor. Peptide sequences obtained from isolated inter-alpha-trypsin inhibitor represent a distinct part of only about two thirds of the predicted polypeptide precursor, suggesting that its maturation is very similar to that of the second subunit. Therefore, we conclude that the deduced primary structure covers about 98% of the mature third subunit.