The natural occurrence of human fibrinogen variants disrupting inter-chain disulfide bonds (AaCys36Gly, AaCys36Arg and AaCys45Tyr) confirms the role of N-terminal Aa disulfide bonds in protein assembly and secretion

Fibrinogen is a 340-kDa dimeric protein, both halves being composed of three polypeptide chains (Aa, Bb, γ) encoded by a cluster of three genes. The combination leads to a molecule organized with a central E domain connected by stranded coiled-coils to two distal D domains. The coiled-coils are in the N-terminal portion of each Aa, Bb and γ chains. The chain assembly starts with either an Aa−γ or a Bb−γ association, with further addition of the third chain leading to an Aa−Bb−γ half molecule, and finally to a (Aa−Bb−γ)2 dimer. Analyses of deletions and substitutions expressed in several recombinant models have shown that these coiled-coil portions are essential for chain-chain interactions, together with 29 interand intra-chain disulfide bonds. These bonds include AaCys45-γCys23 and AaCys36-BbCys65, the disruption of which can prevent formation of the (Aa−Bb−γ)2 dimer. However, only one naturally occurring variant has been reported so far that would confirm these data. Three mutations of the critical AaCys36 and AaCys45 residues associated with hypofibrinogenemia are reported here together with their clinical and biological consequences. These novel variants occurring in four different families reinforce the likely role attributed to these amino acids in maintaining molecular integrity.