Abstract
Endogenous retroviral sequences provide a molecular fossil record of ancient infections whose analysis might illuminate mechanisms of viral extinction. A close relative of gammaretroviruses, HERV-T, circulated in primates for ~25 million years (MY) before apparent extinction within the past ~8 MY. Construction of a near-complete catalog of HERV-T fossils in primate genomes allowed us to estimate a ~32 MY old ancestral sequence and reconstruct a functional envelope protein (ancHTenv) that could support infection of a pseudotyped modern gammaretrovirus. Using ancHTenv, we identify monocarboxylate transporter-1 (MCT-1) as a receptor used by HERV-T for attachment and infection. A single HERV-T provirus in hominid genomes includes an env gene (hsaHTenv) that has been uniquely preserved. This apparently exapted HERV-T env could not support virion infection but could block ancHTenv mediated infection, by causing MCT-1 depletion from cell surfaces. Thus, hsaHTenv may have contributed to HERV-T extinction, and could also potentially regulate cellular metabolism.
DOI:http://dx.doi.org/10.7554/eLife.22519.001
Over millions of years, viruses and the animals that they infect have been locked in a battle for survival, where each has needed to evolve ways to counteract the effects of the other. While the evolution of ancient animals can be studied by looking at the fossilized remains of their extinct relatives, studying how ancient viruses have evolved is more difficult as they usually do not leave behind physical traces of their existence. However, a family of viruses called retroviruses is a notable exception to this rule.
Retroviruses have a step in their life cycle in which their genetic material is integrated into the genome (the name for an organism’s complete set of genetic material) of the cell that they have infected. In rare cases, when that cell is a precursor of a sperm or egg cell, then the viral genes may then be passed on to the animal’s offspring, ultimately leaving genetic traces that can be studied in modern animals. This acts as a genetic ‘fossil record’ of extinct viruses.
HERV-T was a retrovirus that spread among our primate ancestors for about 25 million years before its extinction roughly 11 million years ago. Blanco-Melo et al. have now analyzed the genetic remains left by HERV-T in the genomes of humans and related primates, and were able to use this information to recreate a protein that made up the outer envelope that surrounded the virus. Further experiments showed that this viral protein helped HERV-T to infect human cells by interacting with a protein called MCT1 on the cell surface.
Blanco-Melo et al. also found a particular HERV-T gene that was unexpectedly well preserved in the human genome. The gene retained its ability to produce an envelope protein for about 13 to 19 million years. It is likely that ancient primates ‘hijacked’ the viral gene and used the protein it produced to remove the MCT1 protein from the surface of their own cells. Without MCT1 on the surface, HERV-T was unable to infect the cells. Thus, these findings present an example of how viruses themselves can provide the genetic material that animals use to combat them, potentially leading to their extinction.
DOI:http://dx.doi.org/10.7554/eLife.22519.002
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