Identification of B beta chain domains involved in human fibrinogen assembly

Fibrinogen chains are assembled in a stepwise manner in the rough endoplasmic reticulum prior to secretion of the final six-chain dimeric molecule. Previous studies indicated that the synthesis of B beta may be a rate-limiting factor in the assembly of human fibrinogen. To determine the domains of B beta which interact with the other two component chains of fibrinogen, deletion mutants of B beta were transiently co-expressed, together with A alpha and gamma chains, in COS cells, and fibrinogen assembly and secretion were measured. Deletion of the COOH-terminal half of the B beta chain (amino acids 208-461) did not affect assembly and secretion. Assembly of A alpha, gamma, and B beta also occurred when the first NH2-terminal 72 amino acids of B beta were deleted, but not when 93 amino acids were deleted. This indicates that the B beta domain between amino acids 73 and 93 is necessary for the assembly of the three fibrinogen chains. This domain marks the start of the alpha-helical "coiled-coil" region of fibrinogen.