THE VERTEBRATE coagulation system is a complicated cascade of enzymes, yet it evolved by the gradual addition of enzymes. It is thought that this complex system evolved by the repeated duplication and divergence of two ancestral genes. We are most familiar with the prothrombin activators as essential clotting cascade elements, yet some snakes have weaponized these enzymes.
Australian brown snakes produce group D prothrombin activators that are secreted in their venom. These proteins are similar to FXa, a key factor in the conversion of prothrombin to thrombin. In the brown snake Tropidechis carinatus Reza and coworkers found that the FX gene was more similar to that species’ venom prothrombin activator gene, trocharin D, than any other vertebrate FX gene. This is not surprising, since it is consistent with the idea that the trocharin D gene evolved by duplication of FX and divergence of the roles of the gene duplicates, with one continuing in its ancestral role in blood coagulation, and the other evolving a role as a venom component. This is consistent with the model for gene evolution of innovation, amplification, and divergence.
The authors also studied the brown snake Pseudonaja textilis. This snake produces a group C venom prothrombin activator called pseutarin C. This is a protein complex containing a catalytic subunit called PCCS, also similar to FXa. Since group C and group D prothrombin activators are thought to have evolved independently, the authors attempted to sequence the FX gene expressed in the liver in order to explore the similarity of PCCS to this gene. However, they found two FX isoforms in the liver, something that has not before been reported in any organism.
A comparison of the sequences of the two FX isoforms, PFX1 and PFX2, and venom component PCCS showed that PFX2 has a sequence intermediate between the two. It appears that PFX1 and PFX2 are the product of the duplication of an ancestral PFX gene (aPFX), and following this duplication PFX2 underwent a second duplication to product PFX2 and PCCS. PFX2 and PCCS share an insertion and a deletion that are not present in PFX1, and PCCS has two additional insertions that are not present in PFX2. Combining this information provides the evolutionary scheme shown to the right.
PFX2 is not expressed in the liver, and if not expressed in other tissues it may now be inactive. If so, it is probably a matter of time before this gene is lost to mutation. In its current form, it is a genetic “transitional fossil” that preserves the evolutionary history of the PCCS gene. This gene provides a rare opportunity to study gene duplication and divergence, since such intermediates are not often preserved.
REZA, M.A., MINH LE, T.N., SWARUP, S., MANJUNATHA KINI, R. (2006). Molecular evolution caught in action: gene duplication and evolution of molecular isoforms of prothrombin activators in Pseudonaja textilis (brown snake). Journal of Thrombosis and Haemostasis, 4(6), 1346-1353. DOI: 10.1111/j.1538-7836.2006.01969.x