Rhabdoviridae

Rhabdoviridae (from Greek rhabdos = rod) are a large and ecologically diverse family of enveloped viruses with a characteristic projectile-like structure. The helically arranged genome consists of a (-)stranded RNA packaged with the capsid protein. The family Rhabdoviridae includes terrestrial and aquatic vertebrate, invertebrate, and plant infecting pathogens of public health, agricultural, and fisheries importance (Dietzgen et al. 2011). Most rhabdoviruses are transmitted from arthropods to vertebrate or plant hosts, but lyssaviruses (e.g., rabies virus) and novirhabdoviruses (e.g., infectious hematopoietic necrosis virus) have evolved to circulate among vertebrates without a biological vector. Sigma viruses (e.g., Drosophila melanogaster sigma virus) are congenitally transmitted in fruit flies. High-throughput sequencing of host genomes has demonstrated that rhabdovirus-like elements are incorporated into the genomes of some arthropods and plants Rhabdoviridae (from Greek rhabdos = rod) are a large and ecologically diverse family of enveloped viruses with a characteristic projectile-like structure. The helically arranged genome consists of a (-)stranded RNA packaged with the capsid protein. The family Rhabdoviridae includes terrestrial and aquatic vertebrate, invertebrate and plant infecting pathogens of public health, agricultural and fisheries importance (Dietzgen and Kuzmin, 2011). Most rhabdoviruses are transmitted from arthropods to vertebrate or plant hosts, but lyssaviruses (e.g., rabies virus) and novirhabdoviruses (e.g., infectious hematopoietic necrosis virus) have evolved to circulate among vertebrates without a biological vector, and sigma viruses (e.g., Drosophila melanogaster sigma virus) are congenitally transmitted in fruit flies. High-throughput sequencing of host genomes provided evidence for the integration of rhabdovirus-like elements into the genomes of some arthropods and plants (Ballinger et al. 2012; Fort et al. 2012). This indicates an ancient evolutionary origin and a long-standing association of rhabdoviruses with their hosts. (Ballinger et al. 2012; Fort et al. 2012).

Rhabdoviruses are taxonomically classified into thirteen genera in the family Rhabdoviridae, order Mononegavirales, which also includes the families Bornaviridae, Filoviridae, Paramyxoviridae, and the Nyamiviridae (Dietzgen et al. 2011).

Classical rhabdoviruses, typified by vesicular stomatitis virus (VSV) and Sonchus yellow net virus (SYNV), form characteristic spherical or conical enveloped virions containing unsegmented, negative-sense, single-stranded (ss) RNA genomes 11-16 kb in length.

Virions range in size from 100 to 430 × 45 to 100 nm. The basic genome organization common to all rhabdoviruses includes five canonical genes encoding (from 3′ to 5′) the nucleoprotein (or nucleocapsid protein, N), phosphoprotein (P), matrix protein (M), glycoprotein (G), and large protein (L, RNA-dependent RNA polymerase) (Fig. 2). This group of structural protein genes is flanked by regulatory 3′-leader and 5′-trailer sequences that exhibit terminal complementarity and contain promoter sequences to initiate replication. The individual genes are flanked by conserved transcriptional stop and start signals separated by short, non-transcribed intergenic sequences (Dietzgen et al. 2012). The infectious nucleocapsid core [a ribonucleoprotein (RNP) complex], which is active in transcription and replication, consists of the genomic RNA, which is always closely associated with the N protein, and the P and L proteins. The M protein is responsible for condensation of the RNP complex during virion assembly at the host plasma membrane, and the transmembrane spike protein G likely plays an important role in assembly, budding, and entry into the host cell (Dietzgen et al. 2012).

The replication mechanism of rhabdoviruses is nearly universal throughout the family. It follows the universal pathway of the cytoplasmic replication cycle (i) cell entry facilitated by clathrin-mediated or receptor-binding endocytosis (or vector-mediated penetration of the plant cell wall); (ii) uncoating; (iii) transcription and translation; (iv) genome replication and encapsulation; and (v) assembly and release (budding). Fusion of endocytosed virus to endosomes and subsequent lysis releases the RNP complex into the cytoplasm, allowing initiation of early transcription and replication events. A critical step in this process is the dissociation of the M protein from the nucleocapsid (Mire et al. 2010), which is required for the initiation of viral transcription, also called primary transcription. Transcription of the negative-stranded genome is facilitated by a transcriptase complex and occurs progressively on a decreasing molar gradient based on gene distance from the genomic 3′-end (e.g., N→P→M→G→L).

The production and accumulation of virally encoded proteins signals a switch in polymerase function, from viral mRNA transcription to genome replication, in which N plays a critical role. An essential step in viral replication of the nascent positive-sense genome (antigenome) relies on its encapsulation, a process facilitated by cis-acting conserved sequences at the 3′-ends of the viral genome and antigenome. In addition, the N and P proteins are critical for promoting genome replication, as the N/P complex provides the structural and chaperone support for nascent RNA to bind to the N protein via sugar-phosphate interactions (Albertini et al. 2006). The bound antigenome then acts as a template for the synthesis of encapsulated negative-sense genomes that are assembled into progeny virions.

Virion assembly is a staggered process in which the various components [nucleocapsid core (RNP), G and M proteins] are sequestered in different cellular compartments and converge in the final steps of the process. The nucleocapsid is assembled in the cytoplasm during RNA replication, as observed in members of the genera vesiculovirus, lyssavirus, ephemerovirus, and novirhabdovirus. The viral G protein is incorporated into the endoplasmic reticulum, where chaperones (BiP and calnexin) (Hammond et al. 1994) facilitate its proper folding and assembly into trimers before being transported and fused into the Golgi complex. As it moves through the cell, it undergoes further posttranslational modifications including glycosylation (Schmidt et al. 1979) before being transported to the cholesterol- and sphingolipid-rich lipid rafts in the basolateral plasma membrane. The M protein is synthesized mainly as a soluble protein in the cytoplasm and is also membrane bound, although in smaller amounts. However, both forms of M protein are recruited to the assembly of nucleocapsid/M complexes at the host plasma membrane, from where virions will bud. This budding process is facilitated by the interaction of M with host-encoded proteins responsible for the formation of multivesicular bodies (MVB) and their release from the plasma membrane.

Clinically significant is the human pathogenic rabies virus, causative agent of the almost invariably fatal rabies.

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