The role of beta gamma and alpha gamma complexes in the assembly of human fibrinogen

The role of alpha gamma and beta gamma dimers as intermediates in the assembly of fibrinogen was examined in cell fusion experiments using stably transfected baby hamster kidney cell lines expressing one or combinations of fibrinogen chains. Fibrinogen was readily formed and secreted into the culture media when cells co-expressing beta and gamma chains and generating beta gamma complexes were fused with cells expressing only the alpha chain. Likewise, when cells co-expressing alpha and gamma chains and generating alpha gamma complexes were fused with cells expressing only the beta chain, fibrinogen was also formed and secreted. The relative amounts of alpha gamma or beta gamma intermediates observed during fibrinogen biosynthesis were determined by the levels of the component chains; i.e. when the beta chain was limiting the alpha gamma dimer was the predominant intermediate; likewise, when the alpha chain was limiting, the beta gamma complex was the predominant intermediate. The incorporation of preformed alpha gamma and beta gamma complexes into secreted fibrinogen did not require concurrent protein synthesis, as shown by experiments employing cycloheximide. These data strongly support the role of alpha gamma and beta gamma complexes as functional intermediates in the assembly of fibrinogen.