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)的终止密码子而不是正义密码子的反应。多肽释放因子在这一过程中起着至关重要的作用。虽然在20世纪60年代末，S在体外就发现了终止过程和RF活性，但其机制和装置仍不清楚。Caskey、Capecchi、Nirenberg及其同事的开创性工作在体外发现了大肠杆菌密码子特异的RF1(针对UAG/UAA)和RF2(针对UGA/UAA)的参与。然而，直到最近，还缺乏RFS和STOP密码子之间直接接触的证据。在本研究过程中，我们发现RFS中有一个相当于tRNA反密码子的肽决定簇，RF1和RF2之间的每个保守结构域的交换导致了一个可以切换识别特异性的结构域的确定。系统的遗传和生化分析表明，三肽…Rf1中较多的S Pro-Ala-Thr和Rf2中的SerPro-Phe决定了RF的特异性。第一和第三个氨基酸分别独立地区分第二和第三个嘌呤碱基，因此被称为三肽‘反密码子’。令人惊讶的是，自发现遗传密码以来，人们花了40年的时间才弄清楚其密码子破译的基本机制。在新生多肽释放后，由核糖体、脱酰tRNA、RF和mRNA4组成的终止后复合体需要解离，才能进行下一轮蛋白质合成。核糖体循环因子(RRF)与延伸因子EF-G共同作用，是分解终止后复合体所必需的。最近，包括我的小组在内的三个小组已经解决了RRF的晶体结构问题。这三个分子由两个结构域组成，结构域1和结构域2，由两个环(一个铰链)连接，除了与氨基酸结合的3‘端外，几乎与tRNA^&lt；Phe&gt；有人提出，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体的机械分解。然而，RRF在结构上与tRNA的不同之处在于，RRF的铰链形成了一个灵活的“鹅颈”肘部，而tRNA的肘部是刚性的，RRF的这种灵活性对其功能至关重要。我们假设，大自然可能没有创造出这种tRNA模拟的蛋白质来简单地替代tRNA，除非蛋白质需要追求tRNA无法完成的某些功能(S)。较少

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