Host Factors in the Infection Cycle of Bamboo mosaic virus

Abstract

To complete the infection cycle efficiently, the virus must hijack the host systems in order to benefit for all the steps and has to face all the defense mechanisms from the host. This review involves a discussion of how these positive and negative factors regulate the viral RNA accumulation identified for the Bamboo mosaic virus (BaMV), a single-stranded RNA virus. The genome of BaMV is approximately 6.4 kb in length, encoding five functional polypeptides. To reveal the host factors involved in the infection cycle of BaMV, a few different approaches were taken to screen the candidates. One of the approaches is isolating the viral replicase-associated proteins by co-immunoprecipitation with the transiently expressed tagged viral replicase in plants. Another approach is using the cDNA-amplified fragment length polymorphism technique to screen the differentially expressed genes derived from N. benthamiana plants after infection. The candidates are examined by knocking down the expression in plants using the Tobacco rattle virus-based virus-induced gene silencing technique following BaMV inoculation. The positive or negative regulators could be described as reducing or enhancing the accumulation of BaMV in plants when the expression levels of these proteins are knocked down. The possible roles of these host factors acting on the accumulation of BaMV will be discussed.

Keywords: Bamboo mosaic virus; defense proteins; host factors; virus movement; virus replication.

FIGURE 1

A schematic representation of the model for BaMV RNA-replicase-host factors interaction in the replication steps. (1) Once BaMV RNA entering the cell, the viral replicase is translated using the host translation system. (2) The 3′ untranslated region of BaMV genomic RNA shown as (+)vRNA is interacted with several host factors which regulate BaMV replication positively including chloroplast phosphoglycerate kinase (chlPGK), heat shock protein 90 (Hsp90), thioredoxin transferase GSTU4, and negatively indicated with red arrows including elongation factor 1a (eEF1a) and glyceraldehyde 3-phosphate dehydrogenase (GAPDH). Another host factor a putative methyltransferase (PMTS1) is also shown as a negative regulator for BaMV replication through the interaction with viral replicase. (3) The (+)vRNA is transported to the chloroplasts through the interaction with chlPGK for replication. (4) Some of host factors required for BaMV replication can be transported into chloroplasts by the endomembrane trafficking system using one of the Rabs, NbRabG3f. (5) The minus-strand RNA shown as (-)vRNA is synthesized inside the chloroplasts. (6) The plus-strand (+)vRNA is then synthesized following with (-)vRNA synthesis.

FIGURE 2

A schematic view of the hypothetical model for BaMV movement. The intracellular trafficking of BaMV movement complex is proposed to form the vesicle and trafficking through the cytoskeleton toward the plasmodesmata (PD). The steps of the BaMV movement are illustrated as (1) the movement proteins of BaMV, TGBp2 and TGBp3, are synthesized on the endoplasmic reticulum (ER) and transported via vesicles possibly regulated by one of the Rab-GTPases; (2) the newly synthesized viral RNA is assembled with the capsid protein (CP), movement protein TGBp1, and the viral replicase to form a competent viral replication complex (VRC); (3) the VRC and possibly some other host factors are recruited to TGBp2/TGBp3-containing vesicle to form a movement complex and trafficking toward the PD; (4) the host factor STKL localized on the plasma membrane might control the gate of the PD; (5) the host factor CK2α targeting the CP of VRC and release the vRNA from the VRC to the neighbor cell for further translation or replication; (6) TGBp2 and TGBp3 are released from the movement complex after disassembly on the PD and might shuttle back from the plasma membrane to late endosome/multivesicle bodies/prevacuolar compartments (LE/MVB/PVC) with the help of RabGAP1 to activate the Rab (unidentified yet); (7) RabG3f is possibly involved in shuttling the viral movement proteins back to Golgi and ER. One of the host factors, TRXh2, is shown to play a negative role indicated as (-) in hindering the movement through the interaction with viral movement protein TGBp2.