We’ve been talking about protein structure and folding in my Biol 105 class. Proteins are made of chains of amino acids and the sequence of amino acids, or primary structure, dictates the way the protein will fold into its final 3D or tertiary structure. We may assume that two proteins with similar sequences would have a similar structure, and that two proteins with very different sequences would have different structures. However, this is not true. Proteins with very different sequences can end up with similar 3D structures.
A great example of this is the structure of capsid proteins from three very different viruses. Adenoviruses infect animals (eukaryotes), and is one of many viruses that cause colds. PRD1 is a bacteriphage, a virus that infects bacteria. STIV (Sulfolobus turreted icosahedral virus) infects Sulfolobus, an archaea that lives in geothermal hotsprings in Yellowstone National Park. STIV and its host love the 80 degree celsius, pH 3 environment of the hotsprings. The fact that there are viruses that infect archaea in those extreme environments is cool enough. But it turns out that the capsid proteins of these three viruses are actually quite similar. Their sequence differs significantly, but their tertiary structures are highly similar, meaning these very different polypeptides fold into essentially the same shape.
What is the basis of this similarity? Do all theses viruses share a common ancestor, which would have existed before the three domains of cellular life (eukarya, bacteria, archaea) diverged over 3 billion years ago? Is it convergent evolution? Was there a horizontal gene transfer event in which a gene moved among all three domains? The authors of the paper argue for a common ancestor but the other possibilities have not been formally excluded. We still don’t really know, and it raises interesting questions about the origin of viruses.