Abstract
Emaravirus is a recently described genus of negative-strand RNA plant viruses. Emaravirus P4 protein localizes to plasmodesmata, suggesting that it could be a viral movement protein (MP). In the current study, we showed that the P4 protein of raspberry leaf blotch emaravirus (RLBV) rescued the cell-to-cell movement of a defective potato virus X (PVX) that had a deletion mutation in the triple gene block 1 movement-associated protein. This demonstrated that RLBV P4 is a functional MP. Sequence analyses revealed that P4 is a distant member of the 30K superfamily of MPs. All MPs of this family contain two highly conserved regions predicted to form β-strands, namely β1 and β2. We explored by alanine mutagenesis the role of two residues of P4 (Ile106 and Asp127) located in each of these strands. We also made the equivalent substitutions in the 29K MP of tobacco rattle virus, another member of the 30K superfamily. All substitutions abolished the ability to complement PVX movement, except for the I106A substitution in the β1 region of P4. This region has been shown to mediate membrane association of 30K MPs; our results show that it is possible to make non-conservative substitutions of a well-conserved aliphatic residue within β1 without preventing the membrane association or movement function of P4.
Highlights
The genus Emaravirus is a very recently established grouping of negative-strand RNA plant viruses for which European mountain ash ringspot-associated virus (EMARAV) is the type species (Mielke & Muehlbach, 2007)
We showed that the raspberry leaf blotch virus (RLBV) 42 kDa P4 protein fused to GFP or monomeric red fluorescent protein localized to the plasma membrane and colocalized with TMV 30K movement protein (MP) to PD in the cell wall (McGavin et al, 2012)
These results showed that, as expected, the tobacco rattle virus (TRV) 29K protein could complement the movement deficiency of the potato virus X (PVX)-GFP-DMP TGB1 mutant
Summary
The genus Emaravirus is a very recently established grouping of negative-strand RNA plant viruses for which European mountain ash ringspot-associated virus (EMARAV) is the type species (Mielke & Muehlbach, 2007). Other accepted species in the genus are Fig mosaic virus and Rose rosette virus, and putative members are maize red stripe virus (MRSV; referred to as High Plains virus), pigeon pea sterility mosaic virus (PPSMV), raspberry leaf blotch virus (RLBV) and redbud yellow ringspot virus (RYRSV). Some of these viruses have been shown to have a roughly spherical morphology of 80– 100 nm diameter with a surrounding envelope contained within vesicles thought to be derived from host membranes (Ebrahim-Nesbat & Izadpanah, 1992). The emaravirus RNA segments each encode a single putative protein, three of which are predicted to function as an RNA-dependent RNA polymerase (encoded by RNA1), a glycoprotein precursor protein (RNA2) and a nucleocapsid protein (RNA3)
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