Identification of B @ 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 BB may be a rate-limiting factor in the assembly of human fibrinogen. To determine the domains of BO which interact with the other two component chains of fibrinogen, deletion mutants of BB were transiently co-expressed, together with Aa and y chains, in COS cells, and fibrinogen assembly and secretion were measured. Deletion of the COOH-terminal half of the BB chain (amino acids 208-461) did not affect assembly and secretion. Assembly of Aa, y, and Be also occurred when the first NHz-terminal 72 amino acids of BB were deleted, but not when 93 amino acids were deleted. This indicates that the BB domain between amino acids 73 and 93 is necessary for the assembly of the three fibrinogen chains. This domain marks the start of the a-helical “coiled-coil” region of fibrinogen.