Abstract
Cucurbit aphid-borne yellows virus (CABYV) is a polerovirus (Luteoviridae family) with a capsid composed of the major coat protein and a minor component referred to as the readthrough protein (RT). Two forms of the RT were reported: a full-length protein of 74 kDa detected in infected plants and a truncated form of 55 kDa (RT*) incorporated into virions. Both forms were detected in CABYV-infected plants. To clarify the specific roles of each protein in the viral cycle, we generated by deletion a polerovirus mutant able to synthesize only the RT* which is incorporated into the particle. This mutant was unable to move systemically from inoculated leaves inferring that the C-terminal half of the RT is required for efficient long-distance transport of CABYV. Among a collection of CABYV mutants bearing point mutations in the central domain of the RT, we obtained a mutant impaired in the correct processing of the RT which does not produce the RT*. This mutant accumulated very poorly in upper non-inoculated leaves, suggesting that the RT* has a functional role in long-distance movement of CABYV. Taken together, these results infer that both RT proteins are required for an efficient CABYV movement.
Highlights
Cucurbit aphid-borne yellows virus (CABYV) is a member of the Polerovirus genus in the Luteoviridae family [1]
Absence of detection of the RT* protein in polerovirus-infected plants may result from a lack of sensitivity of the antibodies used for its detection, or from a rapid degradation of the protein in its unassembled form
Contrary to the complete RT that was and reproducibly found in phloem exudate sampled from infected plants, the 55 kDa product was not always detected, neither at the same RT*/coat protein (CP) ratio in phloem exudate collected from infected C. sativus
Summary
Cucurbit aphid-borne yellows virus (CABYV) is a member of the Polerovirus genus in the Luteoviridae family [1]. Poleroviruses encode a movement protein (P4) that, in spite of having cellular and biochemical characteristics of viral movement proteins [12,13,14,15,16], cannot support virus movement outside phloem cells This protein was shown to be host-dependent, suggesting the existence of a P4-independent transport of poleroviruses in some plants [17,18]. Plant infection with RTengineered mutants of TuYV and Potato leafroll virus (PLRV, Polerovirus genus) unable to encapsidate the RT* suggested that the complete RT protein cannot achieve in trans the transport function carried by the incorporated RT* [21,22]. In addition to its complex role in virus movement and phloem limitation, the RT protein is a key factor in aphid transmission since it intervenes in virus transport across the aphid gut cells, specifies intestinal tropism and interacts with aphid endosymbionts [7,11,20,21,22,23,24,25,26]
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