SUMMARY: Mimiviruses are giant viruses that co-evolved with eukaryotic hosts and virophages. Virophages are tiny viruses that hijack the viral factories giant viruses set up inside the host. Just as the eukaryotic host and virophages have evolved a sort of symbiotic relationship to defend against giant virus attacks, giant viruses have also evolved a defense mechanism against virophages. This viral “adaptive immune system”, called the MIMIVIRE system functions like the CRISPR-Cas9 system of bacteria. In this article, researchers examined one of the enzymes, R354. From its function, active site, and structure, the authors concluded that this enzyme is a functional homolog to one of the enzymes in the CRISPR-Cas9 system. R354 has both endo- and exonuclease activity and structurally, it is a dimer (compared to the toroidal oligomers of CRISPR). Why is it important to understand how MIMIVIRE systems work? Because knowing more about these systems can help us identify other similar systems in other microorganisms.
LESSON COMMENTS: While this article can be used in an evolution lesson, it can also be used for classes covering protein synthesis, protein structure, DNA repair, genetic engineering tools, and the immune system. The concept of functional homology can also be tied into a class on reproduction and embryology. It would be interesting for students to look at the similarities between how scientists compare embryos and how they compare enzymes. What conclusions can we draw from genetic comparisons versus structural comparisons? Can two organisms or proteins be genetically different but structurally similar and vice versa?
Dou, C., Yu, M., Gu, Y., Wang, J., Yin, K., Nie, C., Zhu, X., Qi, S., Wei, Y., & Cheng, W. (2018). Structural and Mechanistic Analyses Reveal a Unique Cas4-like Protein in the Mimivirus Virophage Resistance Element System. iScience, 3, 1–10. https://doi.org/10.1016/j.isci.2018.04.001