Djordjevic S, Stock A M
Howard Hughes Medical, Institute Center for Advanced Biotechnology and Medicine, Department of Biochemistry, University of Medicine and Dentistry of New Jersey, Piscataway, New Jersey, 08854, USA.
Structure. 1997 Apr 15;5(4):545-58. doi: 10.1016/s0969-2126(97)00210-4.
Flagellated bacteria swim towards favorable chemicals and away from deleterious ones. The sensing of chemoeffector gradients involves chemotaxis receptors, transmembrane proteins that detect stimuli through their periplasmic domains and transduce signals via their cytoplasmic domains to the downstream signaling components. Signaling outputs from chemotaxis receptors are influenced both by the binding of the chemoeffector ligand to the periplasmic domain and by methylation of specific glutamate residues on the cytoplasmic domain of the receptor. Methylation is catalyzed by CheR, an S-adenosylmethionine-dependent methyltransferase. CheR forms a tight complex with the receptor by binding a region of the receptors that is distinct from the methylation site. CheR belongs to a broad class of enzymes involved in the methylation of a variety of substrates. Until now, no structure from the class of protein methyltransferases has been characterized.
The structure of the Salmonella typhimurium chemotaxis receptor methyltransferase CheR bound to S-adenosylhomocysteine, a product and inhibitor of the methylation reaction, has been determined at 2.0 A resolution. The structure reveals CheR to be a two-domain protein, with a smaller N-terminal helical domain linked through a single polypeptide connection to a larger C-terminal alpha/beta domain. The C-terminal domain has the characteristics of a nucleotide-binding fold, with an insertion of a small antiparallel beta sheet subdomain. The S-adenosylhomocysteine-binding site is formed mainly by the large domain, with contributions from residues within the N-terminal domain and the linker region.
The CheR structure shares some structural similarities with small molecule DNA and RNA methyltransferases, despite a lack of sequence similarity among them. In particular, there is significant structural preservation of the S-adenosylmethionine-binding clefts; the specific length and conformation of a loop in the alpha/beta domain seems to be required for S-adenosylmethionine binding within these enzymes. Unique structural features of CheR, such as the beta subdomain, are probably necessary for CheR's specific interaction with its substrates, the bacterial chemotaxis receptors.
鞭毛细菌会朝着有利的化学物质游动，远离有害的化学物质。化学效应物梯度的感知涉及趋化性受体，这些跨膜蛋白通过其周质结构域检测刺激，并通过其细胞质结构域将信号传递给下游信号组件。趋化性受体的信号输出既受化学效应物配体与周质结构域的结合影响，也受受体细胞质结构域上特定谷氨酸残基甲基化的影响。甲基化由CheR催化，CheR是一种依赖S-腺苷甲硫氨酸的甲基转移酶。CheR通过结合受体上与甲基化位点不同的区域与受体形成紧密复合物。CheR属于参与多种底物甲基化的一类广泛的酶。到目前为止，尚未对这类蛋白质甲基转移酶的结构进行表征。
已确定与甲基化反应的产物及抑制剂S-腺苷同型半胱氨酸结合的鼠伤寒沙门氏菌趋化性受体甲基转移酶CheR的结构，分辨率为2.0埃。该结构显示CheR是一种双结构域蛋白，较小的N端螺旋结构域通过单个多肽连接与较大的C端α/β结构域相连。C端结构域具有核苷酸结合折叠的特征，插入了一个小的反平行β折叠亚结构域。S-腺苷同型半胱氨酸结合位点主要由大结构域形成，N端结构域和连接区的残基也有贡献。
尽管CheR结构与小分子DNA和RNA甲基转移酶之间缺乏序列相似性，但它们在结构上有一些相似之处。特别是，S-腺苷甲硫氨酸结合裂隙有显著的结构保留；α/β结构域中一个环的特定长度和构象似乎是这些酶中S-腺苷甲硫氨酸结合所必需的。CheR的独特结构特征，如β亚结构域，可能是CheR与其底物细菌趋化性受体特异性相互作用所必需的。
