Human endogenous retrovirus HERV-K113 is capable of producing intact viral particles

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...

Section: Importancementioning

confidence: 99%

Infectious Entry Pathway Mediated by the Human Endogenous Retrovirus K Envelope Protein

Robinson

Whelan

2016

“…In many cases, homologous recombination has left only a single long terminal repeat at the integration site (31). None of the more complete proviruses appear to be replication competent, although some of them have retained the capacity to form particles (8,11). Recently, infectious HERV-K(HML-2) viruses have been produced by generating consensus sequences based on humanspecific elements and, in an alternative approach, by assembling the functional regions of three authentic proviruses into a single element (21,39).…”

mentioning

confidence: 99%

Reconstitution of the Ancestral Glycoprotein of Human Endogenous Retrovirus K and Modulation of Its Functional Activity by Truncation of the Cytoplasmic Domain

Hanke

Krämer

Seeher

et al. 2009

Self Cite

Endogenous retroviruses present in the human genome provide a rich record of ancient infections. All presently recognized elements, including the youngest and most intact proviruses of the human endogenous retrovirus K(HML-2) [HERV-K(HML-2)] family, have suffered postinsertional mutations during their time of chromosomal residence, and genes encoding the envelope glycoprotein (Env) have not been spared these mutations. In this study, we have, for the first time, reconstituted an authentic Env of a HERV-K(HML-2) provirus by back mutation of putative postinsertional amino acid changes of the protein encoded by HERV-K113. Aided by codon-optimized expression, we demonstrate that the reconstituted Env regained its ability to be incorporated into retroviral particles and to mediate entry. The original ancient HERV-K113 Env was synthesized as a moderately glycosylated gp95 precursor protein cleaved into surface and transmembrane (TM) subunits. Of the nine N-linked oligosaccharides, four are part of the TM subunit, contributing 15 kDa to its apparent molecular mass of 41 kDa. The carbohydrates, as well as the cytoplasmic tail, are critical for efficient intracellular trafficking, processing, stability, and particle incorporation. Whereas deletions of the carboxy-terminal 6 residues completely abrogated cleavage and virion association, more extensive truncations slightly enhanced incorporation but dramatically increased the ability to mediate entry of pseudotyped lentiviruses. Although the first HERV-K(HML-2) elements infected human ancestors about 30 million years ago, our findings indicate that their glycoproteins are in most respects remarkably similar to those of classical contemporary retroviruses and can still mediate efficient entry into mammalian cells.

“…Although almost all HERV genomes appear to lack intact open reading frames (ORFs) and are therefore likely defective in replication, some of them, such as HERV-K113, have complete ORFs for all viral proteins (52). Nonetheless, HERV-K113 is poorly expressed and is not capable of replication (6,9,37). Recently, using bioinformatics approaches, two groups reconstructed infectious HERV-K sequences (18,37).…”

mentioning

confidence: 99%

Human Endogenous Retrovirus K Gag Coassembles with HIV-1 Gag and Reduces the Release Efficiency and Infectivity of HIV-1

Monde

Contreras-Galindo

Kaplan

et al. 2012

Human endogenous retroviruses (HERVs), which are remnants of ancestral retroviruses integrated into the human genome, are defective in viral replication. Because activation of HERV-K and coexpression of this virus with HIV-1 have been observed during HIV-1 infection, it is conceivable that HERV-K could affect HIV-1 replication, either by competition or by cooperation, in cells expressing both viruses. In this study, we found that the release efficiency of HIV-1 Gag was 3-fold reduced upon overexpression of HERV-K CON Gag. In addition, we observed that in cells expressing Gag proteins of both viruses, HERV-K CON Gag colocalized with HIV-1 Gag at the plasma membrane. Furthermore, HERV-K CON Gag was found to coassemble with HIV-1 Gag, as demonstrated by (i) processing of HERV-K CON Gag by HIV-1 protease in virions, (ii) coimmunoprecipitation of virion-associated HERV-K CON Gag with HIV-1 Gag, and (iii) rescue of a late-domain-defective HERV-K CON Gag by wild-type (WT) HIV-1 Gag. Myristylation-deficient HERV-K CON Gag localized to nuclei, suggesting cryptic nuclear trafficking of HERV-K Gag. Notably, unlike WT HERV-K CON Gag, HIV-1 Gag failed to rescue myristylation-deficient HERV-K CON Gag to the plasma membrane. Efficient colocalization and coassembly of HIV-1 Gag and HERV-K Gag also required nucleocapsid (NC). These results provide evidence that HIV-1 Gag heteromultimerizes with HERV-K Gag at the plasma membrane, presumably through NC-RNA interaction. Intriguingly, HERV-K Gag overexpression reduced not only HIV-1 release efficiency but also HIV-1 infectivity in a myristylation- and NC-dependent manner. Altogether, these results indicate that Gag proteins of endogenous retroviruses can coassemble with HIV-1 Gag and modulate the late phase of HIV-1 replication.