Taylor I A, Davis K G, Watts D, Kneale G G
Biophysics Laboratories, University of Portsmouth, Portsmouth.
EMBO J. 1994 Dec 1;13(23):5772-8. doi: 10.1002/j.1460-2075.1994.tb06915.x.
The type IC DNA methyltransferase M.EcoR124I is a complex multisubunit enzyme that recognizes the non-palindromic DNA sequence GAAN6RTCG. Small angle X-ray scattering has been used to investigate the solution structure of the methyltransferase and of complexes of the enzyme with unmethylated and hemimethylated 30 bp DNA duplexes containing the specific recognition sequence. A major change in the quaternary structure of the enzyme is observed following DNA binding, based on a decrease in the radius of gyration from 56 to 40 A and a reduction in the maximum dimension of the enzyme from 180 to 112 A. The structural transition observed is independent of the methylation state of the DNA. CD shows that there is no change in the secondary structure of the protein subunits when DNA is bound. In contrast, there is a large increase in the CD signal arising from the DNA, suggesting considerable structural distortion which may allow access to the bases targeted for methylation. We propose that DNA binding induces a large rotation of the two HsdM subunits towards the DNA, mediated by hinge bending domains in the specificity subunit HsdS.
I型DNA甲基转移酶M.EcoR124I是一种复杂的多亚基酶，可识别非回文DNA序列GAAN6RTCG。小角X射线散射已用于研究甲基转移酶以及该酶与含有特定识别序列的未甲基化和半甲基化30 bp DNA双链体复合物的溶液结构。基于回转半径从56 Å减小到40 Å以及酶的最大尺寸从180 Å减小到112 Å，观察到DNA结合后酶的四级结构发生了重大变化。观察到的结构转变与DNA的甲基化状态无关。圆二色性表明，结合DNA时蛋白质亚基的二级结构没有变化。相反，DNA产生的圆二色性信号大幅增加，表明存在相当大的结构畸变，这可能允许接近靶向甲基化的碱基。我们提出，DNA结合会诱导两个HsdM亚基通过特异性亚基HsdS中的铰链弯曲结构域向DNA发生大的旋转。
