在ISC 1：酵母遗传学
When future generations tell the story of how medical scientists found a cure for cancer or other diseases resulting from DNA damage, the first chapters may center on an unassuming and unlikely character—yeast.
“We work with the same stuff you use to bake bread,” Oliver Kerscher says. “It’s cheap and it gives you good results. I can take you in my lab and teach you everything you need to know to work with it in three weeks.”
Kerscher和他的学生研究染色体周期中基因的相互作用，这是一个复杂而优雅的过程，细胞通过这个过程复制自己。 Errors in the process can lead to DNA damage, resulting in health problems that range from cancer to Down’s syndrome to spontaneous abortions. 通过研究酵母细胞复制周期中的缺陷，他们希望更好地了解人类DNA损伤的根源。 Kerscher说，酵母丰富且易于培养，对基础科学研究还有另一个更重要的品质。
“Yeast can tell us a lot about this process, because about 60 percent of the genes that exist in yeast cells are homologous to genes in humans—meaning they actually look like the genes that play roles in similar processes in humans,” he explained. “So if I can understand the role of a certain gene involved in the chromosome segregation in yeast, there is a good chance I can tell you the function of that gene in humans.”
想象一个生物施乐
Kerscher, an assistant professor of biology at the College of William and Mary, points out that a properly functioning chromosome cycle is important, as the process is no less than the means by which the body’s cells replenish themselves. 想象一下一个生物的施乐，理想情况下，副本应该与原件无法区分。 The content that’s being replicated from what biologists call the mother cell to the daughter cell is DNA.
Kerscher描述了一个多步骤的过程，通过这个过程，母细胞复制它的染色体，将两组染色体对齐，然后分裂出子细胞。 这个过程的特点是细胞在检查点检查自己。
“It stops the cell cycle and says, OK, let’s see if we have everybody. 我们的DNA是健康的？ 没有DNA损伤吗？,” he explained. “Only when the cell has checked that there is no DNA damage, then it will go on with the 下一个 step.”
如果细胞检测到有什么问题，它会派出一个酶修复小组来修复损伤。 Kerscher said that the checkpoints and repair systems make for very robust quality control—but nothing is perfect. 即使每十万次细胞分裂中只有一次发生错误，一个含有大约50万亿个细胞的身体也是脆弱的。
癌症是如何开始的
“Trouble can arise if you make one serious mistake on the wrong gene. You can imagine that on a sunny day, you go out and accumulate a lot of UV damage in your skin’s DNA. 如果你的DNA修复蛋白缺失了，你就无法修复DNA。 未修复的DNA会导致染色体周期出现问题，即下一个分裂周期。 The damage can start to accumulate,” Kerscher said. “Cancer is actually a multi-step process.”
染色体循环的步骤是由一组分子、蛋白质、酶和基因调控的，它们在这个优雅的过程中都有各自的功能。 Kerscher’s lab focuses on the role of individual genes in the chromosome cycle in yeast.
“We have at least one or two cases in the lab of genes that we first have identified in yeast and that now appear to play a very important role in humans,” he said. “The role of these genes was not identified because we studied humans or human cell life—but the role of the genes was identified because we understood the function in yeast first.”
For instance, his lab has been focusing on the function of a yeast gene known as HEX3, usually found bound to its “buddy protein” SLX8. Kerscher解释说，HEX3/SLX8复合物可能作为修复受损DNA的酶的生化终止物。
“You not only need to make sure that DNA damage-repair comes on, but you also have to make sure that it gets turned off again,” he said, “because these DNA repair enzymes meddle with DNA. You don’t want them unsupervised and hanging out with DNA, because they can take DNA and recombine it, meaning they can re-shuffle the genetic information. So you need to keep them in a locked box and you let them out only if you really, really need them.”
HEX3/SLX8 identifies the repair enzymes after their work is done and, as Kerscher suggests, may send them to the “molecular wastebasket”—the proteasome—by adding a “death tag” of ubiquitin to the molecule. 在整个染色体周期的背景下，泛素扮演着交通警察的角色。 Kerscher解释说，这个循环中的许多步骤都被一个分子停止标志所停止。 “Ubiquitin,” he explained, “says ‘get rid of that stop sign.’”
The lab has discovered that another protein known as SUMO serves as a signal to attract HEX3/SLX8 to its target proteins in a manner that Kerscher says might be “almost like a search and destroy mission” within the cell.
“The search comes in because HEX3/SLX8 looks for SUMOylated target proteins that meddle with DNA damage,” he said. “The proeins already have a little flag on them, a SUMO flag” which may mark them for destruction via HEX3/SLX8 mediated ubiquitination.
搜索和重组
Kerscher explained that the HEX3/SLX8 complex’s search-and-destroy mission may actually be a search-and-reorganize mission, in which the DNA target proteins are biochemically reorganized into another part of the cell by the ubiquitin, rather than being sent to the molecular wastebasket of the proteasome.
“I think that’s our claim to fame. We were the first guys to show that SUMOlation leads to ubiquitination,” he said. “It doesn’t sound like it affects your life every day—but let me tell you, if you don’t have ubiquitin in your cell, you end up with a whole bunch of trouble, because a lot of proteins that are not supposed to be in your cell start to accumulate. You can have neurodegenerative diseases, as one example, but you can also skew cells within the cell cycle if you don’t have proper ubiquitination, because the synchronization of the cell cycle is actually mediated by ubiquitin.”
The study of yeast is valuable because the workings of the yeast chromosome cycle are quite similar to the same cycle in mammals—including humans. They’re similar, but not identical, because some of the functions performed in our never-ending cell cycle are performed by different molecules than those in yeast. 例如，酵母中HEX3/SLX8复合体的功能是由一个称为RNF4的无伴基因处理的。
我们自己的RNF4和酵母中HEX3/SLX8之间的关系惊人地密切。 Laura Boutwell ’09, an undergraduate researcher in Kerscher’s lab, was able to knock out the HEX3/SLX8 from some yeast samples and replace it with the human RNF4 gene. Kerscher said the cell cycle continued just as expected, only with the human gene in the driver’s seat.
“So we can take a human gene, put it into the yeast cell and it shows a function. Now if you have a better way to show the function of the human gene, I don’t know what it could be,” Kerscher said. “So we can do genetics, we can do cell biology, we can do biochemistry and we can also do molecular biology with yeast. It’s extremely powerful and you don’t have to do research on humans and we don’t have to kill any animals…and it’s easy to learn.”