A Single Amino Acid Substitution in a Segment of the CA Protein within Gag That Has Similarity to Human Immunodeficiency Virus Type 1 Blocks Infectivity of a Human Endogenous Retrovirus K Provirus in the Human Genome

Abstract

Human endogenous retrovirus K (HERV-K) is the most intact retrovirus in the human genome. However, no single HERV-K provirus in the human genome today appears to be infectious. Since the Gag protein is the central component for the production of retrovirus particles, we investigated the abilities of Gag from two HERV-K proviruses to support production of virus-like particles and viral infectivity. HERV-K113 has full-length open reading frames for all viral proteins, while HERV-K101 has a full-length gag open reading frame and is expressed in human male germ cell tumors. The Gag of HERV-K101 allowed production of viral particles and infectivity, although at lower levels than observed with a consensus sequence Gag. Thus, including HERV-K109, at least two HERV-K proviruses in human genome today have functional Gag proteins. In contrast, HERV-K113 Gag supported only very low levels of particle production, and no infectivity was detectable due to a single amino acid substitution (I516M) near the extreme C terminus of the CA protein within Gag. The sequence of this portion of HERV-K CA showed similarities to that of human immunodeficiency virus type 1 and other primate immunodeficiency viruses. The extreme C terminus of CA may be a general determinant of retrovirus particle production. In addition, precise mapping of the defects in HERV-K proviruses as was done here identifies the key polymorphisms that need to be analyzed to assess the possible existence of infectious HERV-K alleles within the human population.

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

“…Within primate lentiviruses, which share substantial homology in their CA sequences, coassembly was observed for HIV-1 and HIV-2 and for HIV-1 and SIVmac (10,11). In this regard, it is intriguing that even though HIV-1 and HERV-K belong to different genera, the CA C-terminal domain (CTD) of HERV-K is more similar to the CA CTD of HIV-1 Gag than to the CA CTDs of other betaretroviruses such as MMTV (26 (26). Such high sequence similarity is not present between the HIV-1 CA CTD and the MLV CA CTD.…”

“…The functions of these domains are less well understood for HERV-K Gag than in HIV-1 Gag, although some functional motifs are shared by both Gag proteins (21,32). Notably, a single postinsertion mutation, which was corrected in both HERV-K CON and oricoHERV-K113, is responsible for the defect in assembly of HERV-K113 (26). Thus, these restored HERV-K113 sequences serve as good models for studying assembly of HERV-K Gag.…”

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

“…This provirus is one of the most studied elements. It has preserved open reading frames for all viral proteins but has been inactivated by postinsertional mutations affecting, among other things, the expression and function of Env (5,29).…”

“…Although the method used to identify postinsertional changes was based, like those previously used to generate consensus sequences (21,39), on sequence alignments, it also allows likely mutations and shared polymorphisms in a minority of elements to be discriminated (29). The young age of the 11 proviruses used for alignment makes it very unlikely that the shared polymorphisms are the result of matching postinsertional mutations in more than two individual proviruses or of ectopic recombination events (30) between the env genes of HERV-K113 and two or more of the other aligned elements, although these cannot be completely ruled out.…”

“…Due to its low prevalence of less than 20%, it escaped recognition by the Human Genome Project, and the provirus was identified in a human bacterial artificial chromosome library on chromosome 19p13.11 of an unknown DNA donor (54). As previously reported by us and by others, despite a functional long terminal repeat promoter and open reading frames for all proteins, a few substitutions in the reverse transcriptase and one critical substitution in the gag gene of the provirus contribute to its lack of replication (5,29,54). In addition, the envelope protein of the cloned HERV-K113 provirus is not incorporated into viral particles, suggesting further postinsertional damage by mutation(s) (5,20).…”