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. IMPORTANCEApproximately 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. E ndogenous retroviruses (ERVs) comprise approximately 8% of the human genome (1). Such ERVs provide a physical record of ancient infections by once exogenous retroviruses; however, the degraded states of most sequences largely obscure their biological properties. Consequently, relatively little is known about the earliest events of endogenization, including how the viruses initially entered the germ line to become vertically transmitted elements. The process of endogenization begins with cellular attachment and viral entry, which is mediated by the envel...

Contributions of the four essential entry glycoproteins to HSV-1 tropism and the selection of entry routes

“…As a control, sucrose also prevented the endocytic uptake of Alexa Fluor 488-labeled transferrin into both C10 and CHO-HVEM cells (Fig 5I). As expected, sucrose did not inhibit entry of VSVΔG-PIV5 into either cell line because PIV5 enters by fusion at the plasma membrane (31), thus serving as a negative control (Fig 5D, H). Entry of VSV occurs by an endocytic route (37–39), and, accordingly, sucrose blocked entry of VSVΔG-G into C10 cells (Fig 5C).…”

“…VSVΔG-G is VSVΔG pseudotyped in trans with native VSV glycoprotein G and is known to enter susceptible cells by a clathrin-mediated endocytosis (CME) (30). VSVΔG-PIV5 is VSVΔG pseudotyped with entry glycoproteins HN and F from parainfluenza virus 5 (PIV5), which enters susceptible cells by fusion at the plasma membrane (31) just as PIV5 itself. Both VSVΔG-G and VSVΔG-PIV5 pseudotypes infected all 9 tested cell lines, with varying efficiency (Fig 2C, D), which suggested that limited tropism of VSVΔG-BHLD pseudotype cannot be explained by VSV particle morphology or some other aspect of VSV biology.…”

Characterization of Vesicular Stomatitis Virus Pseudotypes Bearing Essential Entry Glycoproteins gB, gD, gH, and gL of Herpes Simplex Virus 1

“…1A). Robust incorporation was achieved without substituting the native 109-amino-acid cytoplasmic domain of gB for the 29-amino-acid cytoplasmic tail of VSV-G, a strategy that is often used to improve incorporation of challenging glycoproteins (29,30). The presence of HSV-1 gB in VSV⌬G-gB preparations was verified by Western blotting with anti-gB PAb R68 (Fig.…”

“…Prepared pseudotyped virions were free from contamination with residual VSV-G from helper virus as judged by SDS-PAGE, Western blotting, and the inability of the neutralizing anti-VSV-G antibody to block VSV⌬G-BHLD infectivity. Successful incorporation of heterologous glycoproteins into VSV particles often requires that their cytoplasmic tail be replaced with that of VSV-G (29,30). But the incorporation of gB into the pseudotyped virions was robust and comparable to that of native VSV-G despite the presence of a relatively large cytoplasmic domain.…”

viGEN: An open source pipeline for the detection and quantification of viral RNA in human tumors

“…These variants map to frameshift and missense mutations in the putative envelope protein of this virus (Q779_gp1, also called 'env'). Studies have shown that this envelope protein mediates infections of cells [67]. HERV K113 is a provirus and is capable of producing intact viral particles [68].…”