Expression and Characterization of Ancient Retrovirus Envelope Genes

Abstract

Endogenous retroviruses (ERVs) make up a significant portion of vertebrate genomes, and serve as a fossil record of past retroviral infections. Although most ERV genes acquire inactivating mutations over time, some loci retain open reading frames (ORFs) across one or more of the viral genes. The ERV-Fc family, for example, endogenized in multiple mammalian hosts 10 to 30 million years ago, yet many copies maintain intact ORFs corresponding to the env gene, including loci in humans (HERV-Fc1-env) and baboons (babERV-Fc2-env). We previously identified intact ERV-Fc-related env sequences in eight additional mammalian species: chimpanzee, bonobo, aardvark, grey mouse lemur, squirrel monkey, marmoset, dog, and panda. Here we present the results of our assays of expression of these full-length Env proteins. We found that most of the precursors were not cleaved to form the functional surface (SU) and transmembrane (TM) subunits, even when a canonical furin cleavage site was still intact. An exception was babERV-Fc2, in which reconstruction of the cleavage site led to cleavage into SU and TM subunits. Furthermore, removal of 22 residues from the C-terminus of the cytoplasmic tail of babERV-Fc2 enhanced syncytia formation and the ability of babERV-Fc2 pseudotyped virions to infect 293T cells, suggesting the presence of an R-peptide cleavage mechanism. A survey of a small panel of cells revealed that only human cell lines were infectable by babERV-Fc2 pseudotyped murine leukemia virus (MLV) particles, whereas cells of old world monkey, canine, feline and chicken origin were not susceptible to infection. Ectopic expression of native Env codon optimized babERV-Fc2 Env can also inhibit infection by reconstructed babERV-Fc2 pseudotyped virus, raising the possibility that the endogenous glycoprotein encoded in the baboon genome may function as a viral entry inhibitor. Our results suggest that exaptation of ERV Env proteins as antiviral defense genes involves a combination of selective pressures: selection to preserve the receptor-binding and receptor interference functions of Env, but also selection to eliminate the membrane fusion related functions.