Functional Genomics of Insect Baculoviruses

昆虫杆状病毒侵染了农业和森林的许多害虫，以及蚊子和苍蝇等滋扰害虫，使它们成为生物防治的主要媒介。杆状病毒也被用作有效的表达载体(如Cervarx疫苗)。其庞大的dsDNA基因组编码了大约150个在病毒复制周期中重要的基因。其中一些杆状病毒基因的功能是已知的，其中一些与初始感染、病毒粒子蛋白的产生、细胞外排出、细胞和宿主调节以及病毒传播有关。 对其他基因的了解要少得多。其中包括me53基因，这是一种编码53 kDa蛋白质的主要早期基因。Me53在感染后不久和以后被转录。我们证明了它是一种重要的病毒蛋白，因为如果没有它，病毒的复制就会受到严重影响。由于它具有与转录因子一致的蛋白质结构域，我们认为它必须在细胞核内发挥作用，以调节转录。我们发现ME53不仅位于细胞核(正如转录因子所预期的那样)，而且令人惊讶的是，它还位于细胞外围的病灶中，在那里它与病毒包膜蛋白GP64共存。由此推测，ME53可能帮助病毒粒子穿透这些ME53/GP64的细胞膜。但我们仍然被这个问题困住了，我53的角色(S)是什么？ 我们提出了两个假设，一个是关于它在转录调控中的作用，另一个是关于病毒出口。我们的目标之一是跟踪ME53缺失病毒(DeltaME53)与野生型病毒相比在病毒复制周期方面的差异。我们将开始用电子显微镜进行超微结构比较(例如，观察病毒源基质以及病毒粒子在细胞核和细胞膜中的组装/运输)。我们还将比较病毒(和细胞)的转录图谱。为了区分ME53的核特异性功能和细胞膜功能，第二个目标是定位ME53的核转位(NTD)和膜转位(MTD)所需的结构域。通过突变这些易位结构域，我们希望评估ME53在不同细胞内位置上的作用。例如，突变的NTD将防止核转位，所观察到的任何差异(例如，NTD突变体的某些病毒甚至宿主基因缺乏调控)肯定是由于其核功能，反之亦然。虽然ME53与GP64和衣壳蛋白VP39相关，但我们怀疑它也与其他病毒(或宿主)蛋白相互作用，例如帮助ME53转运到细胞核或质膜。为此，我们将使用遗传、酵母双杂交筛选和生化“下拉”实验寻找ME53蛋白质伙伴(ME53互动体)，以从细胞制剂中移除ME53，但相关蛋白质仍附着。来自这些“下拉”的结合蛋白将通过非常精确的光谱光度测量来识别。这之后将进行进一步的突变，试图确定ME53中对与不同伙伴结合至关重要的结构域(并使伙伴本身发生突变)。无法与某些伙伴结合的ME53突变将被评估它们对病毒复制的影响。在结合结构域突变体ME53和正常ME53之间注意到的任何差异都必须与该特定蛋白质的重要性有关。 通过这项工作，我们可以更好地了解ME53在杆状病毒生命周期中的功能。虽然这些发现推动了基础科学的发展，但我们无法预测这种知识是否会有任何实用或商业价值。由于这些病毒被用作生物防治剂，或许这一知识可能被用来提高它们的效力，更好地抵御加拿大的害虫，或者改善疫苗的生产。

Insect baculoviruses infect many insect pests of agriculture and forests and nuisance pests like mosquitoes and blackflies giving them a major role as biological control agents. Baculoviruses are also used as efficient expression vectors (e.g. Cervarix vaccine). Its large dsDNA genome encodes some 150 genes important in the virus replication cycle. The functions of some of these baculovirus genes are known, some for initial infection, production of virion proteins, egress from the cell, cell and host regulation and virus transmission. Much less is known about other genes. Among these is the me53 gene, a major early gene encoding a 53 kDa protein. Me53 is transcribed shortly after infection and later. We showed it was an important viral protein because in its absence virus replication is severely impacted. Since it has protein domains consistent with transcription factors, we felt it must work in the nucleus to regulate transcription. We found that not only is ME53 located in the nucleus (as expected of a transcription factor) but surprisingly also in foci at the cell periphery where it colocalizes with a viral envelope protein GP64. From this we surmised that ME53 might help virions bud through the membrane at these ME53/GP64 foci. But we are still stuck with the question, what is the role(s) of ME53? We posited two hypotheses, one on its role in transcriptional regulation and one on virus egress. One of our objectives is to follow differences in the virus replication cycle of ME53 deleted viruses (deltaME53) compared to wild type virus. We will start our search by electron microscopy for an ultrastructural comparison (e.g. looking at virogenic stroma and assembly/transit of virions in the nucleus and at the cell membrane). We will also compare virus (and cellular) transcriptional profiles. To differentiate the nuclear-specific function from the cell membrane one of ME53, a second objective is to map domains of ME53 needed for nuclear translocation (NTD) and those for membrane translocation (MTD). By mutating those translocation domains we hope to assess the roles of ME53 at different intracellular sites. For example mutating NTD will prevent nuclear translocation and any differences observed (e.g. lack of regulation of certain viral, or even host, genes for the NTD mutant), must be due to its nuclear function, and vice versa for the MTD mutants. While ME53 associates with GP64 and the capsid protein VP39, we suspect it also interacts with other viral (or host) proteins, for example to help ME53 translocate to the nucleus or plasma membrane. For this we will be looking for ME53 protein partners (the ME53 interactome) using a genetic, yeast two hybrid screen, and biochemical “pull down” experiments to remove ME53 from a cell preparation but with associated proteins still attached. Binding proteins from those “pull downs” will be identified by very precise spectroscophotometric measures. This would be followed by further mutagenesis to try to identify domains in ME53 important for binding to the different partners (and mutate the partners themselves). ME53 mutations unable to bind to certain partners will then be assessed for their affect on virus replication. Any differences noticed between the binding domain mutant ME53 and normal ME53 must relate to the importance of that particular protein. From this work we should better understand the function of ME53 in the baculovirus life cycle. While such discoveries advance fundamental science we cannot predict if there will be any practical or commercial value to this knowledge. As these viruses are used as biological control agents, perhaps this knowledge might be used to improve their efficacy and better protect against Canadian pest insects, or improve vaccine production.