Understanding the plant ESCRT machinery and its role in tombusvirus-induced mitochondrial multivesicular body biogenesis
Carnation Italian ringspot virus (CIRV) is a positive-strand RNA virus that assembles its membrane-bound replication complexes at mitochondria in plant cells. This process is accompanied by extensive inward invagination of the mitochondrial outer membrane, leading to the formation of cytosol-filled spherules, wherein viral RNA synthesis occurs. The mechanism by which CIRV is able to induce spherule formation is unknown, however growing evidence suggests that the host-cell ESCRT (Endosomal Sorting Complex Required for Transport) machinery – a multi-protein complex normally involved in late endosome maturation – may be involved. ESCRT consists of ~30 soluble proteins that form sub-complexes assembled at the late endosomal surface, and function in multivesicular body (MVB) biogenesis. While ESCRT is relatively well characterized in yeasts and mammals, comparably little is known about ESCRT in plants. Hence, as an initial step towards understanding the potential role of ESCRT in CIRV replication, we examined the protein-protein interaction network, subcellular localization, and gene expression profiles of the Arabidopsis thaliana ESCRT components. Overall, the results from these studies suggest that ESCRT organization and function is relatively well conserved in plants compared to other eukaryotes. We also observed that ESCRT is important for CIRV replication, as expression of dominant-negative versions of several key ESCRT components reduced CIRV replication efficiency in plant cells. Moreover, the Arabidopsis ESCRT-I component, Vps23A is recruited from late endosomes to mitochondria in plant cells expressing the CIRV replicase protein, p36, and recruitment of Vps23A was shown to be mediated by sequences located at the N terminus of p36. It was also shown that recruitment of Vp23A to mitochondria by p36 does not require the Ubiquitin E2 Variant domain of Vps23A, which is in contrast to recruitment of ESCRT by retroviruses during viral budding in mammalian cells. Taken together, these results support the hypothesis that CIRV recruits ESCRT by a novel mechanism in order to carry out its replication, a finding that may lend important insight to aspects of normal ESCRT function in plants.