Open any introductory biology textbook and one of the first things you'll learn is that our DNA spells out the instructions for making 蛋白质, tiny machines that do much of the work in our body's cells. Results from a study published on Jan. 2 in 科学 藐视教科书科学, showing for the first time that the building blocks of a protein, 叫做氨基酸, can be assembled without blueprints – DNA and an intermediate template called messenger RNA (mRNA). A team of researchers has observed a case in which another protein specifies which amino acids are added.
"This surprising discovery reflects how incomplete our understanding of biology is,第一作者彼得·沈说, Ph.D., a postdoctoral fellow in biochemistry at the University of Utah. "Nature is capable of more than we realize."
To put the new finding into perspective, it might help to think of the cell as a well-run factory. Ribosomes are machines on a protein assembly line, linking together amino acids in an order specified by the genetic code. 当事情出错时, 核糖体会停滞, and a quality control crew is summoned to the site. 收拾残局, 核糖体被分解, 蓝图被丢弃, and the partly made protein is recycled.
Yet this study reveals a surprising role for one member of the quality control team, a protein conserved from yeast to man named Rqc2. Before the incomplete protein is recycled, Rqc2 prompts the ribosomes to add just two amino acids (of a total of 20) – alanine and threonine - over and over, 以任何顺序. Think of an auto assembly line that keeps going despite having lost its instructions. It picks up what it can and slaps it on: horn-wheel-wheel-horn-wheel-wheel-wheel-wheel-horn.
"In this case, we have a protein playing a role similar to that filled by mRNA," says 亚当·弗罗斯特.D., Ph.D., assistant professor at University of California, San Francisco (UCSF) and adjunct professor of biochemistry at the University of Utah. He shares senior authorship with Jonathan Weissman, Ph.D., a Howard Hughes Medical Institute investigator and professor of cellular pharmacology and biochemistry at UCSF, 和奥恩·布兰德曼, Ph.D., assistant professor of biochemistry at 斯坦福大学. "I love this story because it blurs the lines of what we thought 蛋白质 could do."
Like a half-made car with extra horns and wheels tacked to one end, a truncated protein with an apparently random sequence of alanines and threonines looks strange, 而且可能无法正常工作. But the nonsensical sequence likely serves specific purposes. The code could signal that the partial protein must be destroyed, or it could be part of a test to see whether the ribosome is working properly. Evidence suggests that either or both of these processes could be faulty in 神经退行性疾病 such as Alzheimer's, Amyotrophic lateral sclerosis (ALS), or Huntington's.
"There are many interesting implications of this work and none of them would have been possible if we didn't follow our curiosity,布兰德曼说。. "The primary driver of discovery has been exploring what you see, and that's what we did. There will never be a substitute for that."
The scientists first considered the unusual phenomenon when they saw evidence of it with their own eyes. They fine-tuned a technique called cryo-electron microscopy to flash freeze, 然后想象一下, the quality control machinery in action. "We caught Rqc2 in the act,弗罗斯特说. “但这个想法太牵强了. 大发娱乐有责任证明这一点."
It took extensive biochemical analysis to validate their hypothesis. New RNA sequencing techniques showed that the Rqc2/ribosome complex had the potential to add amino acids to stalled 蛋白质 because it also bound tRNAs, structures that bring amino acids to the protein assembly line. The specific tRNAs they saw only carry the amino acids alanine and threonine. The clincher came when they determined that the stalled 蛋白质 had extensive chains of alanines and threonines added to them.
"Our job now is to determine when and where this process happens, 当它失败时会发生什么,弗罗斯特说.
沈, 霜, Brandman, and Weissman conducted the work in collaboration with colleagues at the University of Utah (Krishna Parsawar, 詹姆斯·考克斯), University of California at San Francisco (Xueming Li, 程一帆, 马修·拉森), 斯坦福大学（Joseph Park）, and the University of Texas at Austin (Yidan Qin, 艾伦Lambowitz).
The research was supported by grants from the Searle Scholars program, 美国国立卫生研究院, the Howard Hughes Medical Institute, 斯坦福大学, 以及犹他大学.
Rqc2p and 60S ribosomal subunits mediate mRNA-independent elongation of nascent chains. 彼得年代. 沈, Joseph Park, Yidan Qin, Xueming Li, Krishna Parsawar, Matthew H. Larson, James Cox, 程一帆, Alan M. 乔纳森·兰博维茨. 魏斯曼，奥恩·布兰德曼，亚当·弗罗斯特. 科学1月. 2, 2015
被抓在手里
Rqc2 protein adds amino acids to another protein. 这项新发现与教条相反, showing for the first time that the building blocks of a protein, 叫做氨基酸, 能被另一种蛋白质组装吗, 没有基因指令. The Rqc2 protein (yellow) binds tRNAs (dark blue, teal) which add amino acids (bright spot in middle) to a partially made protein (green). The complex binds the ribosome (white).
