Family: Metaviridae

Genus: Errantivirus

Distinguishing features

Most errantiviruses carry a third env-like gene and are considered to be true or potential endogenous viruses, although infectivity has been empirically demonstrated only for Drosophila melanogaster Gypsy virus (DmeGypV).

Virion

Morphology

Transcription of the DmeGypV genome results in production of irregular virus-like particles (VLPs) of approximately 100 nm in diameter and also much smaller non-enveloped particles. 

Physicochemical and physical properties

The env gene of DmeGypV encodes a protein of 54 kDa. The actual protein has an apparent mass of 66 kDa and is N-glycosylated as indicated by susceptibility to endoglycosidase F. More rapidly migrating molecules of 54 kDa and 28 kDa are also observed and are inferred to result from proteolytic processing by host enzymes, as is the case for Env proteins encoded by members of the family Retroviridae. 

Genome organization and replication

DmeGypV is the prototypical member of the genus Errantivirus. The genome of DmeGypV is 7469 bp including two long terminal repeates (LTRs) each of 482 bp (Figure 2. Metaviridae). This element differs from most retroviruses and LTR retrotransposons in that the termini are composed of 5′-AG…TT-3′, rather than 5′-TG…CA-3′. The primer binding site (PBS) is found immediately adjacent to the 5´-LTR and, depending on the errantivirus, may be complementarity to different tRNA molecules. In DmeGypV, the DNA flanking the insertion includes 4 bp repeats, and the insertion site preference is for 5′-YRYRYR-3′ sequence (where Y=purine and R=pyrimidine). The genomic RNA contains one ORF encoding the major structural protein, a second ORF overlapping in the −1 frame, encoding the protease (PR), reverse transcriptase-ribonuclease H (RT-RH) and integrase (INT) domains, and a third ORF, env, encoding a 54 kDa Env protein, which is translated from a spliced 2.1 kb mRNA. The Env protein of DmeGypV is analogous (but not homologous in terms of sequence similarity) to the Env protein of retroviruses by virtue of the hydrophobic putative membrane spanning domains, localization to the viral membrane, similarity of processing sites for cleavage into trans-membrane and surface domains, and glycosylation. Due to the sequence similarity between errantivirus Env proteins and proteins encoded by certain baculoviruses, it has been suggested that errantiviruses have acquired their env gene from a baculovirus (Malik et al., 2000). 

Biology

The host distribution of errantiviruses in eukaryotes is restricted to insects. In Drosophila, the expression of DmeGypV (and other transposable elements) is modulated intracellularly by the chromatin Gypsy-insulator, which is an RNA interference (RNAi) system constituted by flamenco, a locus that is the source of antisense small RNAs interacting with Piwi to repress the expression and infective properties of DmeGypV (Sarot et al., 2004). As uncontrolled expression of DmeGypV may result in random insertions of this provirus with negative consequences for the host, flamenco is usually considered to be part of the adaptive immunity of Drosophila to keep DmeGypV under control (Bergman et al., 2006). A similar path of activity has also been suggested for Drosophila melanogaster Zam virus (DmeZamV) and Drosophila melanogaster Idefix virus (DmeIdeV). Infection has been demonstrated to occur when susceptible strains were raised in the presence of DmeGypV particles mixed into their food. 

Antigenicity

The Env protein of errantiviruses presents antigenic sites that probably elicit a host immune response to infection. However, the intercellular mechanisms modulating such response are yet to be studied in flies. One member of this group, DmeGypV, has been shown to be infectious (Pelisson et al., 2002). Incubation with antibodies against the errantivirus Env protein decreases the level of infection. 

Species demarcation criteria

Members of the genus Errantivirus split into two phylogenetic clades (Figure 3. Metaviridae). The Gypsy clade includes DmeGypV and its close relatives, while the Zam clade includes DmeZamV and relatives. Consistent with the phylogenetic analysis, the PBS of viruses in the Gypsy cladeis usually complementary to tRNA^Lys, while the PBS of viruses in the Zam clade normally match tRNA^Ser. Members of individual species in the genus Errantivirus have less than 50% identity in their Gag protein sequences compared to those from other species while the Gag sequences of the two clade representatives - DmeZamV and DmeGypV - are only 35% identical. 

Related, unclassified viruses

Virus names and virus abbreviations are not official ICTV designations.