The evolution of vertebrate fibrinogen

The origins of the vertebrate fibrinogen molecule and its constituents chains have been considered, first by examining certain features of the molecule as it exists in a primitive vertebrate (the lamprey), and then by comparing the amino acid sequences of certain portions of the three nonidentical chains as they exist in the human molecule. Although the lamprey protein is distinctly homologous to mammalian fibrinogens, at the present stage of characterization its three nonidentical chains appear to be even more different one from another than are the three mammalian chain types. On the other hand, certain sequence resemblances in the three human chains clearly indicate a common ancestry for the alpha-, beta-, and gamma-chains. It is concluded that the ancient fibrinogen molecule was composed of all identical chains and that its differentiation into three chain types occurred long before the divergence of lampreys and higher vertebrates.