The assembly of human fibrinogen - The role of the amino-terminal and coiled-coil regions of the three chains in the formation of the alpha gamma and beta gamma heterodimers and alpha beta gamma half-molecules

Fibrinogen is a plasma protein consisting of six polypeptide chains which are linked by disulfide bonds. During protein synthesis, assembly of the molecule proceeds through the formation of alpha gamma and beta gamma heterodimers followed by the generation of alpha beta gamma half-molecules and dimerizing to generate the mature six-chain molecule. In the present study, sequences required for the formation of the alpha gamma and beta gamma heterodimers were examined in stably transfected baby hamster kidney cells expressing combinations of normal as web as modified polypeptide chains. Deletion of the amino terminus and the proximal first half of the coiled-coil region of the three fibrinogen chains had little or no effect on heterodimer and half-molecule formation. These deletions, however, did prevent half-molecules from forming the six-chain molecule. Deletion of the distal second half of the coiled-coil region of each chain completely prevented the assembly process. Point mutations ill the second half of the coiled-coil region also indicated that hydrophilic residues that form ion pahs between interacting chains were not critical in the formation of the heterodimeric complexes. These results suggest that the initial formation of the alpha gamma and beta gamma complexes depends primarily on hydrophobic interactions of amino acids located in the second half of the coiled-coil region of the molecule. These interactions occur in the rough endoplasmic reticulum in the presence of various chaperones such as BiP.