Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, CA 94720, USA.
Mol Cell. 2011 Mar 4;41(5):600-8. doi: 10.1016/j.molcel.2011.02.004.
Messenger RNA decay plays a central role in the regulation and surveillance of eukaryotic gene expression. The conserved multidomain exoribonuclease Xrn1 targets cytoplasmic RNA substrates marked by a 5' monophosphate for processive 5'-to-3' degradation by an unknown mechanism. Here, we report the crystal structure of an Xrn1-substrate complex. The single-stranded substrate is held in place by stacking of the 5'-terminal trinucleotide between aromatic side chains while a highly basic pocket specifically recognizes the 5' phosphate. Mutations of residues involved in binding the 5'-terminal nucleotide impair Xrn1 processivity. The substrate recognition mechanism allows Xrn1 to couple processive hydrolysis to duplex melting in RNA substrates with sufficiently long single-stranded 5' overhangs. The Xrn1-substrate complex structure thus rationalizes the exclusive specificity of Xrn1 for 5'-monophosphorylated substrates, ensuring fidelity of mRNA turnover, and posits a model for translocation-coupled unwinding of structured RNA substrates.
信使 RNA 衰变在真核基因表达的调控和监测中起着核心作用。保守的多结构域外切核糖核酸酶 Xrn1 以未知的机制靶向被 5'单磷酸标记的细胞质 RNA 底物，进行连续的 5'-3'降解。在这里，我们报告了一个 Xrn1-底物复合物的晶体结构。单链底物通过 5'-末端三核苷酸之间的堆积固定在位，而一个高度碱性的口袋特异性识别 5'磷酸。参与结合 5'-末端核苷酸的残基的突变会损害 Xrn1 的连续性。底物识别机制允许 Xrn1 将连续水解与 RNA 底物中足够长的单链 5'突出端的双链解链偶联。因此，Xrn1-底物复合物结构合理化了 Xrn1 对 5'-单磷酸化底物的特异性，确保了 mRNA 周转的保真度，并提出了一个用于结构 RNA 底物的易位偶联解旋的模型。
