Exploring the functional diversification of the C4 proteins encoded by geminiviruses

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The C4 protein encoded by geminiviruses is an essential pathogenicity determinant and the most divergent protein in this virus family. We have recently described that C4 from Tomato yellow leaf curl virus (TYLCV) localizes in two distinct subcellular compartments, namely plasma membrane (PM)/plasmodesmata (PD) and chloroplasts. PM-localized C4 can suppress the cell-to-cell movement of RNA interference by targeting two plant receptor-like kinases at PD; chloroplast-localized C4 interferes with the downstream activation of defence, suppressing activation of salicylic acid (SA) biosynthesis upon pathogen perception. Therefore, C4 from TYLCV plays a dual role in the suppression of plant defences, illustrating how subcellular compartmentalization of viral proteins can underlie multifunctionality. Although C4 is essential in all species analyzed to date, the properties and roles of most C4 proteins in this virus family remain to be determined. Among geminiviral proteins, C4 seems to be exceptional: while all other proteins are subjected to negative or purifying selection, C4 seems to be under positive selection; the other positional homologues in the geminiviral genome display common or mostly overlapping subcellular localizations, but C4 from different geminiviruses appears in distinct cellular compartments; and no common, general function has been described for C4 proteins so far. Taken together, these observations lead us to hypothesize that C4 is exploring its potential functional landscape, acquiring novel subcellular targeting and interactors and giving rise to divergent properties and functions in different viral species. Further supporting this idea is the finding that replacement of the C4-coding sequence in a given geminivirus species is sufficient to produce a quantitative increase in virulence, acquisition of independence from a satellite molecule, or breakdown of resistance. In this proposal, we intend to explore the diversity in the C4 proteins encoded by different geminiviruses, in terms of their subcellular localization, virulence functions, interactome, host targets, and developmental alterations caused by their expression in planta, and to gain insight into the molecular mechanisms underlying their role during the infection. We anticipate that relevant host processes/pathways/proteins might emerge as convergently targeted by different C4 proteins through independently evolved strategies.