Martin Georges, Doublié Sylvie, Keller Walter
Department of Cell Biology, Biozentrum, University of Basel, Klingelbergstrasse 70, Basel, Switzerland.
Biochim Biophys Acta. 2008 Apr;1779(4):206-16. doi: 10.1016/j.bbagrm.2007.12.003. Epub 2007 Dec 14.
Poly(A) polymerases were identified almost 50 years ago as enzymes that add multiple AMP residues to the 3' ends of primer RNAs without use of a template from ATP as cosubstrate and with release of pyrophosphate. Based on sequence homology of a signature motif in the catalytic domain, poly(A) polymerases were later found to belong to a superfamily of nucleotidyl transferases acting on a very diverse array of substrates. Enzymes belonging to the superfamily can add from single nucleotides of AMP, CMP or UMP to RNA, antibiotics and proteins but also homopolymers of many hundred residues to the 3' ends of RNA molecules. The recently reported structures of several nucleotidyl transferases facilitate the study of the catalytic mechanisms of these very diverse enzymes. Numerous structures of CCA-adding enzymes have now revealed all steps in the formation of a CCA tail at the 3' end of tRNAs. In addition, structures of poly(A) polymerases and uridylyl transferases are now available as binary and ternary complexes with incoming nucleotide and RNA primer. Some of these proteins undergo significant conformational changes after substrate binding. This is proposed to be an indication for an induced fit mechanism that drives substrate selection and leads to catalysis. Insights from recent structures of ternary complexes indicate an important role for the primer molecule in selecting the incoming nucleotide.
多聚腺苷酸聚合酶在近50年前就被鉴定为这样一类酶，它们在不使用模板的情况下，以ATP作为共底物，将多个AMP残基添加到引物RNA的3'末端，并释放出焦磷酸。基于催化结构域中一个特征基序的序列同源性，多聚腺苷酸聚合酶后来被发现属于一个核苷酸转移酶超家族，该超家族作用于非常多样的底物。属于该超家族的酶可以将单个的AMP、CMP或UMP核苷酸添加到RNA、抗生素和蛋白质上，也可以将数百个残基的同聚物添加到RNA分子的3'末端。最近报道的几种核苷酸转移酶的结构有助于研究这些非常多样的酶的催化机制。现在，多种添加CCA的酶的结构已经揭示了tRNA 3'末端CCA尾巴形成的所有步骤。此外，多聚腺苷酸聚合酶和尿苷酰转移酶的结构现在也有了与进入的核苷酸和RNA引物形成的二元和三元复合物的形式。其中一些蛋白质在底物结合后会发生显著的构象变化。这被认为是诱导契合机制的一个迹象，该机制驱动底物选择并导致催化作用。来自三元复合物最新结构的见解表明，引物分子在选择进入的核苷酸方面起着重要作用。
