title: Infectious Entry Pathway Mediated by the Human Endogenous Retrovirus K Envelope Protein.
abstract: Endogenous retroviruses (ERVs), the majority of which exist as degraded remnants of ancient viruses, comprise approximately 8% of the human genome. The youngest human ERVs (HERVs) belong to the HERV-K(HML-2) subgroup and were endogenized within the past 1 million years. The viral envelope protein (ENV) facilitates the earliest events of endogenization (cellular attachment and entry), and here, we characterize the requirements for HERV-K ENV to mediate infectious cell entry. Cell-cell fusion assays indicate that a minimum of two events are required for fusion, proteolytic processing by furin-like proteases and exposure to acidic pH. We generated an infectious autonomously replicating recombinant vesicular stomatitis virus (VSV) in which the glycoprotein was replaced by HERV-K ENV. HERV-K ENV imparts an endocytic entry pathway that requires dynamin-mediated membrane scission and endosomal acidification but is distinct from clathrin-dependent or macropinocytic uptake pathways. The lack of impediments to the replication of the VSV core in eukaryotic cells allowed us to broadly survey the HERV-K ENV-dictated tropism. Unlike extant betaretroviral envelopes, which impart a narrow species tropism, we found that HERV-K ENV mediates broad tropism encompassing cells from multiple mammalian and nonmammalian species. We conclude that HERV-K ENV dictates an evolutionarily conserved entry pathway and that the restriction of HERV-K to primate genomes reflects downstream stages of the viral replication cycle.
  
  
  OBJECTIVE
  Approximately 8% of the human genome is of retroviral origin. While many of those viral genomes have become inactivated, some copies of the most recently endogenized human retrovirus, HERV-K, can encode individual functional proteins. Here, we characterize the envelope protein (ENV) of the virus to define how it mediates infection of cells. We demonstrate that HERV-K ENV undergoes a proteolytic processing step and triggers membrane fusion in response to acidic pH--a strategy common to many viral fusogens. Our data suggest that the infectious entry pathway mediated by this ENV requires endosomal acidification and the GTPase dynamin but does not require clathrin-dependent uptake. In marked contrast to other betaretroviruses, HERV-K ENV imparts broad species tropism in cultured cells. This work provides new insights into the entry pathway of an extinct human virus and provides a powerful tool to further probe the endocytic route by which HERV-K infects cells.
authors.affiliation: Program in Virology, Harvard Medical School, Boston, Massachusetts, USA Department of Microbiology and Immunobiology, Harvard Medical School, Boston, Massachusetts, USA.
authors.forename: Lindsey R
authors.initials: LR
authors.lastname: Robinson
authors.affiliation: Program in Virology, Harvard Medical School, Boston, Massachusetts, USA Department of Microbiology and Immunobiology, Harvard Medical School, Boston, Massachusetts, USA swhelan@hms.harvard.edu.
authors.forename: Sean P J
authors.initials: SP
authors.lastname: Whelan
mesh_terms: D000818:Animals; D002459:Cell Fusion; D020077:Endogenous Retroviruses / Q000235:genetics / Q000502:physiology*; D045683:Furin / Q000378:metabolism; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008322:Mammals; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism; D018116:Vesiculovirus / Q000235:genetics / Q000502:physiology; D014759:Viral Envelope Proteins / Q000235:genetics / Q000378:metabolism*; D056189:Viral Tropism; D053586:Virus Internalization
chemical_list: D011994:Recombinant Proteins; D014759:Viral Envelope Proteins; D045683:Furin