| Popis: |
Fibrinogen is a dimer with each half-molecule composed of three different chains (A alpha, B beta, gamma). Previous studies showed that amino-terminal disulfide bonds, as well as the disulfide rings that flank the "coiled-coil" region, are necessary for chain assembly and secretion (Zhang, J.Z., and Redman, C.M. (1994) J. Biol. Chem. 269, 652-658). We now determine whether other amino-terminal domains are involved in linking the half-molecules. Fibrinogen chains, with deletions at the amino terminus, were co-expressed in COS cells together with normal fibrinogen chains. Elimination of the first 8 amino acids of the B beta chain did not affect dimer assembly, but deletion of amino acid residues 9-72 had a small inhibitory effect on dimer formation. Deletion of the first 72 amino acids of the B beta chain further inhibited dimer formation and resulted in nearly equal amounts of half-molecule and dimeric fibrinogen being formed and secreted. Deletion of the first 80 residues, which includes the cysteine residues that form the amino-terminal disulfide ring, completely eliminated dimer formation, and only half-molecules were secreted. By contrast deletion of the fist 41 amino acid residues of the A alpha chain or the first 15 residues of the gamma chain, which correspond to B beta delta 1-72, did not affect chain assembly and secretion. However, co-expression of both A alpha delta 1-41 and gamma delta 1-15 with normal B beta, inhibited dimer formation. Taken together, these results indicate that in addition to disulfide bonds, noncovalent interactions of other amino-terminal amino acid residues in the three fibrinogen chains also participate in dimer formation. |
