Assembly and secretion of recombinant chains of human inter-alpha-trypsin inhibitor in COS-7 cells

The inter-alpha-trypsin inhibitor (ITI) family is a group of structurally r elated plasma serine protease inhibitors. The In family members consist of combinations of mature heavy chains named HC1, HC2, HC3 linked to bikunin ( a Kunitz-type protease inhibitor) by a covalent interchain protein-glycosam inoglycan-protein cross-link. The biosynthesis of the ITI family members ta kes place in the liver. In this report we examine the biosynthesis of these proteins using transient transfected COS-7 cells expressing one or more co mbinations of human ITI chains. The processing and secretion of alpha(1)-mi croglobulin and bikunin does not require the ITI heavy chains. A small prop ortion of the H3 chain seems to be processed into the HC3 form in the absen ce of the other ITI chains. In contrast, the processing of H2 into HC2 need s the presence of the L chain. The COS-7 cells are able to link the HC2 and HC3 heavy chains with bikunin by means of a chondroitin sulfate bridge, an d thus to generate 260-kDa ITI-like proteins as well as pre-alpha-trypsin i nhibitor (P alpha I). However, the maturation of the H1 chain into HC1 and the assembly of HC1 inside multichain proteins may take place according to a mechanism which differs from that of the H2 and H3 chains. These results indicate that the assembly of the constituent chains of the ITI-like protei ns and P alpha I is not dependent on the liver machinery.