近日，美国沙克生物研究所（Salk Institute for Biological Studies）的研究人员发现感冒病毒（cold virus）产生的一种小分子蛋白E4-ORF3可破坏与生长、复制和癌症有关的细胞器。这种蛋白可在细胞核中形成三维网状结构，捕获目标细胞器。这一发现为癌症的治疗开辟了一条新的道路，相关论文发表在10月11日的《Cell》杂志上。

　　E4-ORF3是腺病毒编码的一种致癌蛋白，它可组阻止肿瘤抑制蛋白p53与其靶基因的结合。P53被认为是基因组卫士（guardian of the genome），在正常情况下可通过使DNA发生损伤的细胞自我毁灭而抑制肿瘤的发生。在几乎所有类型的人类癌症中，p53的肿瘤抑制作用都丧失，使得癌细胞逃脱正常的生长控制。E4-ORF3可抑制p53，使得腺病毒可以在感染的人类细胞中进行复制。

　　研究的领导者Clodagh O'Shea说，癌症是一个黑匣子，弄清楚小DNA肿瘤病毒（DNA tumor virus）蛋白和细胞的肿瘤抑制复合体（tumor suppressor complexes）之间的相互作用才能找到黑匣子的钥匙。但若不清楚这种蛋白质的结构，我们便无法理解为何它可胜过肿瘤抑制基因。

　　两年前，O'Shea发现E4-ORF3可为腺病毒的增殖铺平道路，这一作用通过抑制帮助细胞抵御这一病毒的基因实现。E4-ORF3可在细胞中自组装称无序、网状结构，继而捕获并灭活肿瘤抑制蛋白。

　　此外，研究者解析了E4-ORF3在细胞核中形成的聚合物的超微结构。这项研究成果可帮助科学家开发新的小分子药物，这种药物通过结合并扰乱帮助癌细胞生长和扩散的细胞元件发挥作用。

　　为避免杀死健康细胞需设计"肿瘤爆炸病毒"（tumor-busting virus），这种改造的病毒仅可破坏癌细胞，因其仅能在p53受抑制的癌细胞中复制。当一个癌细胞被杀死，它可释放出已复制的病毒，然后寻找并杀死扩散至全身的癌细胞。

　　设计这种病毒需要使得E4-ORF3失去灭活正常细胞p53的作用，否则E4-ORF3在杀死癌细胞的同时也杀死了正常细胞。

　　Cold viruses point the way to new cancer therapies

　　Horng D. Ou1, Witek Kwiatkowski2, Thomas J. Deerinck5, Andrew Noske5, Katie Y. Blain2, Hannah S. Land3, Conrado Soria1, Colin J. Powers1, Andrew P. May10, Xiaokun Shu7, 8, 11, Roger Y. Tsien7, 8, 9, James A.J. Fitzpatrick4, Jeff A. Long3, Mark H. Ellisman5, 6, Senyon Choe2, Clodagh C. O'Shea

　　Evolution of minimal DNA tumor virus' genomes has selected for small viral oncoproteins that hijack critical cellular protein interaction networks. The structural basis for the multiple and dominant functions of adenovirus oncoproteins has remained elusive. E4-ORF3 forms a nuclear polymer and simultaneously inactivates p53, PML, TRIM24, and MRE11/RAD50/NBS1 (MRN) tumor suppressors. We identify oligomerization mutants and solve the crystal structure of E4-ORF3. E4-ORF3 forms a dimer with a central β core, and its structure is unrelated to known polymers or oncogenes. E4-ORF3 dimer units coassemble through reciprocal and nonreciprocal exchanges of their C-terminal tails. This results in linear and branched oligomer chains that further assemble in variable arrangements to form a polymer network that partitions the nuclear volume. E4-ORF3 assembly creates avidity-driven interactions with PML and an emergent MRN binding interface. This reveals an elegant structural solution whereby a small protein forms a multivalent matrix that traps disparate tumor suppressors.

　　Salk researchers discovered that a small protein produced by cold viruses disables large cellular machines involved in growth, replication and cancer. These proteins accomplish this by forming a three-dimensional web inside a cell's nucleus (yellow) that traps these components. The findings point the way to new ways to target and destroy tumors. Credit: Salk Institute for Biological Studies

　　Cold viruses generally get a bad rap—which they've certainly earned—but new findings by a team of scientists at the Salk Institute for Biological Studies suggest that these viruses might also be a valuable ally in the fight against cancer.

　　Adenovirus, a type of cold virus, has developed molecular tools—proteins—that allow it to hijack a cell's molecular machinery, including large cellular machines involved in growth, replication and cancer suppression. The Salk scientists identified the construction of these molecular weapons and found that they bind together into long chains (polymers) to form a three-dimensional web inside cells that traps and overpowers cellular sentries involved in growth and cancer suppression. The findings, published October 11 in Cell, suggest a new avenue for developing cancer therapies by mimicking the strategies employed by the viruses.

　　"Cancer was once a black box," says Clodagh O'Shea, an assistant professor in Salk's Molecular and Cell Biology Laboratory, who led the study. "The key that opened that box was revealing the interactions between small DNA tumor virus proteins and cellular tumor suppressor complexes. But without knowing the structure of the proteins they use to attack cells, we were at a loss for how these tiny weapons win out over much larger tumor suppressors."

　　O'Shea's team studied E4-ORF3, a cancer-causing protein encoded by adenovirus, which prevents the p53 tumor suppressor protein from binding to its target genes. Known as the "guardian of the genome," p53 normally suppresses tumors by causing cells with DNA damage—a hallmark of cancer—to self-destruct. The p53 tumor suppressor pathway is inactivated in almost every human cancer, allowing cancer cells to escape normal growth controls. Similarly, by inactivating p53, the E4-ORF3 protein enables adenovirus replication in infected human cells to go unchecked.

　　E4-ORF3 self-assembles inside cells into a disordered, web-like structure that captures and inactivates different tumor suppressor protein complexes. Horng Ou, a postdoctoral researcher in O'Shea's laboratory, says E4-ORF3 is unusual. "It doesn't resemble any known proteins that assemble polymers or that function in cellular tumor suppressor pathways," he says. "Most cellular polymers and filaments form uniform, rigid chains. But E4-ORF3 is the virus's Swiss army knife—it assembles into something that is highly versatile. It has the ability to build itself into all sorts of different shapes and sizes that can capture and deactivate the many defenses of a host cell."

　　In collaboration with scientists from the National Center for Microscopy and Imaging Research at University of California, San Diego, led by Mark Ellisman, the center's director, O'Shea's team used new techniques to reveal the ultrastructure of the remarkable polymer that E4-ORF3 assembles in the nucleus—something that previously had proven difficult since the polymer is effectively invisible using conventional electron microscopy. "What you see is the E4-ORF3 polymer bending and weaving and twisting its way through the nucleus," she says. "It does appear to have a single repeating pattern and creates a matrix that captures several different tumor suppressors and silences p53 target genes."

　　Initially, E4-ORF3 forms a dimer, made up of only two subunits. In this form, E4-ORF3 largely ignores its cellular targets. The researchers theorized that when E4-ORF3 assembles into a polymer, however, it binds to tumor suppressor targets far more aggressively. To test this theory, they genetically fused E4-ORF3 polymer mutants to lamin, a cellular protein that assembles intermediate filaments that provide stability and strength to cells. They showed that the lamin-E4-ORF3 fusion protein assembled into cylinder-like superstructures in the nucleus that bind and disrupt PML, a protein complex that suppresses tumors.

　　The Salk findings may help scientists develop small molecules—the basis for the vast majority of current drugs—capable of destroying tumors by binding and disrupting large and complex cellular components that allow cancer cells to grow and spread. Understanding how viruses overcome healthy cells may also help scientists engineer tumor-busting viruses, which offer a new and potentially self-perpetuating cancer therapy. Such modified viruses would destroy only cancer cells, because they could only replicate in cells in which the p53 tumor suppressor has been deactivated. When a cancer cell is destroyed it would release additional copies of the engineered viruses, which would seek out and kill remaining cancer cells that have spread throughout the body.

　　Engineering these viruses requires disabling the ability of the E4-ORF3 protein to inactivate p53 in healthy cells—otherwise, the virus could destroy healthy cells as well as cancer cells. At the same time, E4-ORF3 has certain important functions in allowing the virus to replicate in the first place, so it can't be completely removed from the virus's arsenal. Thus, the Salk researcher's work on understanding the protein's precise structure, functions and interactions is crucial to engineering viruses in which E4-ORF3's abilities have been precisely modified.