Molecular Mimicry in Translation

蛋白质合成的终止发生在核糖体上，作为对“解码”位点（A位点）中的终止密码子而不是有义密码子的响应。多肽释放因子（RFs）在这一过程中起着至关重要的作用。尽管终止过程和RF活性在20世纪60年代后期在体外被发现，但许多机制和装置仍然不清楚。Caskey、Capecchi、Nirenberg及其同事的先驱工作揭示了体外大肠杆菌的密码子特异性RFs，RF 1（用于UAG/UAA）和RF 2（用于UGA/UAA）的参与。然而，直到最近，一直缺乏证据RF和终止密码子之间的直接联系。在研究过程中，我们发现了一个相当于tRNA反密码子的RFs肽决定簇，通过RF 1和RF 2之间的保守结构域的交换，发现了一个可以转换识别特异性的结构域。系统的遗传和生化分析表明，三肽 ...更多信息 RF 1的Pro-Ala-Thr和RF 2的Ser-Pro-Phe决定RF的特异性。第一个和第三个氨基酸分别独立地区分第二个和第三个嘌呤碱基，因此被称为三肽“反密码子”。令人惊讶的是，从发现遗传密码到破译其64个密码子的基本机制需要40年的时间，在新生多肽释放后，由核糖体、脱酰化的tRNA、RF和mRNA组成的终止后复合物需要解离以进行下一轮蛋白质合成。核糖体再循环因子（RRF），与延伸因子EF-G一致，是终止后复合物解体所必需的。RRF的晶体结构最近已经被包括我的小组在内的三个小组解决了。这三种分子由两个结构域（结构域1和结构域2）组成，通过两个环（铰链）桥接，并且<Phe>除了与氨基酸结合的3'端之外几乎完全与tRNA结合。已经提出RRF是一个近乎完美的tRNA模拟物，可以解释终止后核糖体复合物的机械解体。然而，RRF在结构上不同于tRNA，因为RRF的铰链形成柔性的“鹅颈”肘，而tRNA手肘是刚性的，并且RRF的这种柔性对其功能至关重要。我们假设，自然界可能没有创造出这样一种类似于tRNA的蛋白质来简单地取代tRNA，除非蛋白质需要追求一些tRNA无法实现的功能。少

The termination of protein synthesis takes place on the ribosomes as a response to a stop, rather than a sense, codon in the 'decoding' site (A site). Polypeptide release factors (RFs) play an essential role in this process. Although the termination process and the RF activity were discovered in vitro in the late 1960's, much of the mechanism and the apparatus have remained obscure. The pioneer works by Caskey, Capecchi, Nirenberg and colleagues have uncovered the involvement of codon-specific RFs, RF1 (for UAG/UAA) and RF2 (for UGA/UAA), of Escherichia coli in vitro. However, evidence has been lacking for direct contact between RFs and stop codons until recently. During the course of this study, we have discovered a peptide determinant in RFs equivalent to the anticodon of tRNA.Swaps of each of the conserved domains between RF1 and RF2 led to the identification of a domain that could switch recognition specificity. Systematic genetic and biochemical analyses showed that the tripeptide … More s Pro-Ala-Thr in RF1 and Ser-Pro-Phe in RF2 determine RF specificity. The first and third amino acids independently discriminate the second and third purine bases, respectively, hence referred to as a tripeptide 'anticodon'. It is surprising that it took 4 decades since the discovery of the genetic code to figure out the basic mechanisms behind the deciphering of its 64 codons.After release of nascent polypeptides, the posttermination complex composed of the ribosome, deacylated tRNA, RF and mRNA needs to be dissociated for the next round of protein synthesis. Ribosome recycling factor (RRF), in concert with elongation factor EF-G, is required for disassembly of the posttermination complex. The crystal structure of RRF has recently been solved by three groups including my group. These three molecules are composed of two domains, domain 1 and domain 2, bridged by two loops (a hinge), and superimpose almost perfectly with tRNA^<Phe> except for the amino acid-binding 3' end. It has been proposed that RRF is a near perfect tRNA mimic to explain the mechanistic disassembly of the posttermination ribosomal complex. RRF, however, is architecturally different from tRNA in that the hinge of RRF forms a flexible 'gooseneck' elbow, while the elbow of tRNA is rigid, and that this flexibility of RRF is vital for its function. We assume that nature may not have created such protein of a tRNA mimic to simply substitute for tRNA unless protein is required to pursue some function(s) that tRNA cannot do. Less