Nishimoto Madoka, Higashijima Kyoko, Shirouzu Mikako, Grosjean Henri, Bessho Yoshitaka, Yokoyama Shigeyuki
Systems and Structural Biology Center, Yokohama Institute, RIKEN, 1-7-22 Suehiro, Tsurumi, Yokohama 230-0045, Japan.
J Mol Biol. 2008 Nov 21;383(4):871-84. doi: 10.1016/j.jmb.2008.08.068. Epub 2008 Sep 5.
Trm1 catalyzes a two-step reaction, leading to mono- and dimethylation of guanosine at position 26 in most eukaryotic and archaeal tRNAs. We report the crystal structures of Trm1 from Pyrococcus horikoshii liganded with S-adenosyl-l-methionine or S-adenosyl-l-homocysteine. The protein comprises N-terminal and C-terminal domains with class I methyltransferase and novel folds, respectively. The methyl moiety of S-adenosyl-l-methionine points toward the invariant Phe27 and Phe140 within a narrow pocket, where the target G26 might flip in. Mutagenesis of Phe27 or Phe140 to alanine abolished the enzyme activity, indicating their role in methylating G26. Structural analyses revealed that the movements of Phe140 and the loop preceding Phe27 may be involved in dissociation of the monomethylated tRNA*Trm1 complex prior to the second methylation. Moreover, the catalytic residues Asp138, Pro139, and Phe140 are in a different motif from that in DNA 6-methyladenosine methyltransferases, suggesting a different methyl transfer mechanism in the Trm1 family.
Trm1催化两步反应，导致大多数真核生物和古细菌的tRNA中第26位鸟苷的单甲基化和二甲基化。我们报道了来自嗜热栖热菌的Trm1与S-腺苷-L-甲硫氨酸或S-腺苷-L-高半胱氨酸结合的晶体结构。该蛋白质分别由具有I类甲基转移酶和新型折叠结构的N端和C端结构域组成。S-腺苷-L-甲硫氨酸的甲基部分指向一个狭窄口袋内的不变苯丙氨酸27和苯丙氨酸140，目标G26可能会翻转进入该口袋。将苯丙氨酸27或苯丙氨酸140突变为丙氨酸会消除酶活性，表明它们在G26甲基化中的作用。结构分析表明，苯丙氨酸140和苯丙氨酸27之前的环的移动可能参与了第二次甲基化之前单甲基化的tRNA*Trm1复合物的解离。此外，催化残基天冬氨酸138、脯氨酸139和苯丙氨酸140与DNA 6-甲基腺嘌呤甲基转移酶中的基序不同，这表明Trm1家族中存在不同的甲基转移机制。
