Presence of a vertebrate fibrinogen-like sequence in an echinoderm

Sequence comparisons of the three homologous polypeptide chains that compose vertebrate fibrinogens imply that the molecule evolved before the divergence of vertebrates and invertebrates, but, to our knowledge, no protein resembling vertebrate fibrinogen has even been reported from an invertebrate. We used primers based on sequences conserved between lamprey and human fibrinogens and applied the polymerase chain reaction (PCR) to cDNA preparations from various invertebrates. A fibrinogen-like sequence was identified in cDNA prepared from the soft tissues of a sea cucumber, parastichopus parvimensis. The PCR-prepared material was then used to clone two closely related mRNA sequences from a sea cucumber soft tissue cDNA library. The putative fibrinogen-related proteins, FReP-A and FReP-B, correspond to the carboxyl-terminal two-thirds of vertebrate fibrinogen beta and gamma chains. Computer comparisons of various fibrinogen-related sequences indicate that the sea cucumber proteins diverged before the beta-gamma gene duplication.