Exploring the functional diversification of the C4 proteins encoded by geminiviruses

双生病毒编码的C4蛋白是双生病毒家族中致病性决定因子，也是双生病毒家族中分化最大的蛋白。我们最近描述了来自番茄黄曲叶病毒（TYLCV）的C4定位于两个不同的亚细胞区室，即质膜（PM）/胞间连丝（PD）和叶绿体。PM定位的C4可以通过靶向PD处的两种植物受体样激酶来抑制RNA干扰的细胞间移动;叶绿体定位的C4干扰下游防御激活，抑制病原体感知时水杨酸（SA）生物合成的激活。因此，来自TYLCV的C4在抑制植物防御中起双重作用，说明病毒蛋白的亚细胞区室化如何成为多功能性的基础。尽管C4在迄今为止分析的所有物种中都是必不可少的，但该病毒家族中大多数C4蛋白的性质和作用仍有待确定。在双生病毒蛋白中，C4似乎是例外的：当所有其他蛋白都受到负选择或纯化选择时，C4似乎处于正选择之下;双生病毒基因组中的其他位置同源物显示出共同的或大部分重叠的亚细胞定位，但来自不同双生病毒的C4出现在不同的细胞区室中;并且迄今为止还没有描述C4蛋白的共同的一般功能。综上所述，这些观察结果使我们假设C4正在探索其潜在的功能景观，获得新的亚细胞靶向和相互作用，并在不同的病毒物种中产生不同的特性和功能。进一步支持这一观点的是，发现在给定的双生病毒物种中C4编码序列的替换足以产生毒力的定量增加、获得对卫星分子的独立性或抗性的破坏。在这个建议中，我们打算探索不同的双生病毒编码的C4蛋白的多样性，在其亚细胞定位，毒力功能，相互作用，宿主靶点，并在植物中的表达引起的发育改变，并深入了解其在感染过程中的作用的分子机制。我们预计，相关的主机过程/途径/蛋白质可能会出现收敛目标不同的C4蛋白通过独立进化的策略。

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.