維持基因組的完整性對(duì)細(xì)胞的存活至關(guān)重要,。約翰霍普金斯的研究人員最近發(fā)現(xiàn)了對(duì)細(xì)胞保持染色體完整性至關(guān)重要的一個(gè)蛋白質(zhì)機(jī)器。沒(méi)有這些蛋白質(zhì),，酵母細(xì)胞中就會(huì)發(fā)生染色體和DNA的破壞和損傷——這些破壞在人類(lèi)細(xì)胞中會(huì)導(dǎo)致癌癥的發(fā)生,。研究的結(jié)果公布在7月6日的Current Biology雜志的網(wǎng)絡(luò)版上。
Jef Boeke博士和同事證實(shí),，從酵母細(xì)胞中移除兩種稱(chēng)為sirtuins的蛋白質(zhì)——Hst3p和Hst4p,，能夠?qū)е录?xì)胞對(duì)化學(xué)物質(zhì)和溫度非常敏感并本能地破壞或丟失染色體。在人類(lèi)中,，染色體的這種丟失或缺失能使細(xì)胞的分裂失控,，發(fā)生癌變,。
幾乎每個(gè)人類(lèi)細(xì)胞都含有6英尺長(zhǎng)的壓縮在染色體中的DNA。細(xì)胞分裂時(shí),，所有DNA都必須被精確拷貝并重新恰當(dāng)?shù)赜媒M蛋白進(jìn)行包裝,，以在新的細(xì)胞中形成染色體。
在拷貝過(guò)程中,，新的染色體常常會(huì)自己斷裂并在染色體被裝配完成并分裂成兩個(gè)細(xì)胞前逢合好。所有細(xì)胞都有損傷控制機(jī)制,，能夠感應(yīng)到染色體上的缺口并修復(fù)它們,。
研究人員任務(wù)乙酰化作用以某種放射標(biāo)記新拷貝的DNA,，以使細(xì)胞知道該修復(fù)缺口,。一旦缺口被修補(bǔ)，乙?；鶊F(tuán)就不再需要并在正常細(xì)胞中被移除,。
Sirtuins Hst3p和Hst4p是從組蛋白移除這些特殊的化學(xué)標(biāo)記物（乙酰基）必須的,。乙?；惶砑拥浇M蛋白鏈上的一種叫做lysine-56(賴(lài)氨酸56)的氨基酸上。新的研究證實(shí),，丟失了Hst3p和Hst4p的酵母細(xì)胞中的染色體中,，其lysine-56被高度乙酰化,，幾乎組蛋白中每個(gè)lysine-56（賴(lài)氨酸）都被加上了乙?；?/p>
研究人員首次看到了這種對(duì)組蛋白的巨大影響,。當(dāng)細(xì)胞缺少了這些sirtuins時(shí),，其染色體的組蛋白就會(huì)被乙酰基所飽和,。研究組總結(jié)說(shuō),，細(xì)胞通過(guò)在賴(lài)氨酸56上添加乙酰基來(lái)標(biāo)記出新自造的DNA并警告細(xì)胞該部位可能含有危險(xiǎn)的缺口,。賴(lài)氨酸-56乙?；饔每赡苁羌?xì)胞標(biāo)記受損DNA的一種廣泛的機(jī)制。
As part of a large National Institutes of Health-funded Technology Centers for Networks and Pathways project, Johns Hopkins researchers have discovered protein machinery important for cells to keep chromosomes intact. Without such proteins, their experiments show that yeast cells experience broken chromosomes and DNA damage that in human cells are well known to lead to cancer.
"Maintaining genome integrity is crucial for cell survival," says Jef Boeke, Ph.D., Sc.D., the report's senior author, a professor of molecular biology and genetics and co-director of the High Throughput Biology Center of the Institute for Basic Biomedical Sciences at Hopkins. The report will appear online July 6 in Current Biology.
Boeke and colleagues show that removing from yeast cells two proteins called sirtuins -- Hst3p and Hst4p -- causes cells to become hypersensitive to chemical agents and temperature and to spontaneously break and/or lose chromosomes. In humans, the loss or breakage of chromosomes can cause cells to lose control of when and if they are supposed to divide, becoming cancerous.
Nearly every human cell contains about six feet of DNA packaged into chromosomes. Chromosomes consist of DNA wrapped around a scaffold-like structure made of proteins called histones. Each time a cell divides into two, all of this DNA must be copied exactly and repackaged properly with histones to form chromosomes in the new cell.
During the copying process, new chromosomes often have breaks in them that need to be sealed before the chromosome is considered "finished" and the cell is ready to divide into two. All cells have damage control mechanisms that can sense nicks and breaks in chromosomes -- DNA damage -- and repair them.
"We think acetylation somehow marks the newly copied DNA so the cell knows to repair the breaks," says Boeke. "Once the breaks are repaired, the acetyl groups no longer are needed and are removed in normal cells."
Sirtuins Hst3p and Hst4p are proteins required to remove these specific chemical "decorations" -- called acetyl groups -- from specific sites on histones. The acetyl groups are added to lysine-56, an amino acid in the histone protein chain. Chromosomes in yeast cells missing Hst3p and Hst4p become hyperacetylated on lysine-56 -- it appears that every lysine-56 in every histone has attached an acetyl group.
"This is the first time we've ever seen such a huge effect," says Boeke. "The chromosomes just light up with acetyl groups -- they're just saturated" when cells are missing these sirtuins.
Earlier work showed that yeast cells initially need the lysine-56 decorations to repair breaks or other damage to DNA that occur when the DNA is copied, an essential process that also has the potential to seriously damage DNA. This new work shows that it is even more critical for yeast cells to remove these decorations once repair has been completed. Thus, there is an endless cycle of putting the acetyl groups on whenever there is damage or the danger thereof and taking them off again. Failure to take off the "decorations" leads to loss of entire chromosomes and other problems with the DNA.
Thus, yeast cells need to carefully coordinate acetylation and deacetylation of lysine-56.
The team concludes that by putting an acetyl group on lysine-56, the cell is signaling that its DNA is newly made and as a result possibly contains dangerous breaks. Acetylation on lysine-56 may be a universal mechanism for cells to mark damaged DNA. DNA damage can be caused by exposure to chemical mutagens, chemotherapy or even sunlight.
"There are a million mutagens in our environment," says Boeke. Once cells repair the DNA damage, it is important to shut off repair machinery and return to normal state. The cells require proteins like the sirtuins Hst3p and Hst4p to act as guideposts to help identify dangerous DNA lesions. If the DNA repair machinery does not fix these lesions to maintain chromosome integrity, the cell would lose control of growth or death.
Moving forward, the team hopes to further understand what controls these sirtuins to remove acetyl groups and how hyperacetylation can lead to such dramatic loss of chromosome integrity.
