National Laboratory of Biomacromolecules, Institute of Biophysics, Chinese Academy of Sciences, Beijing, China.
PLoS One. 2011 Mar 2;6(3):e17346. doi: 10.1371/journal.pone.0017346.
Type I DNA methyltransferases contain one specificity subunit (HsdS) and two modification subunits (HsdM). The electron microscopy model of M.EcoKI-M₂S₁ methyltransferase shows a reasonable closed state of this clamp-like enzyme, but the structure of the open state is still unclear. The 1.95 Å crystal structure of the specificity subunit from Thermoanaerobacter tengcongensis (TTE-HsdS) shows an unreported open form inter-domain orientation of this subunit. Based on the crystal structure of TTE-HsdS and the closed state model of M.EcoKI-M₂S₁, we constructed a potential open state model of type I methyltransferase. Mutational studies indicated that two α-helices (aa30-59 and aa466-495) of the TTE-HsdM subunit are important inter-subunit interaction sites in the TTE-M₂S₁ complex. DNA binding assays also highlighted the importance of the C-terminal region of TTE-HsdM for DNA binding by the TTE-M₂S₁ complex. On the basis of structural analysis, biochemical experiments and previous studies, we propose a dynamic opening and closing mechanism for type I methyltransferase.
I 型 DNA 甲基转移酶包含一个特异性亚基（HsdS）和两个修饰亚基（HsdM）。M.EcoKI-M₂S₁ 甲基转移酶的电子显微镜模型显示了这种类似夹钳的酶的合理关闭状态，但开放状态的结构仍不清楚。来自热厌氧菌（TTE）的特异性亚基（TTE-HsdS）的 1.95 Å 晶体结构显示了该亚基未报告的开放形式的结构域间取向。基于 TTE-HsdS 的晶体结构和 M.EcoKI-M₂S₁ 的封闭状态模型，我们构建了 I 型甲基转移酶的潜在开放状态模型。突变研究表明，TTE-HsdM 亚基的两个α-螺旋（aa30-59 和 aa466-495）是 TTE-M₂S₁ 复合物中重要的亚基间相互作用位点。DNA 结合实验还强调了 TTE-HsdM 的 C 末端区域对于 TTE-M₂S₁ 复合物结合 DNA 的重要性。基于结构分析、生化实验和先前的研究，我们提出了 I 型甲基转移酶的动态开启和关闭机制。
