Abstract
BackgroundtRNA m1A58 methyltransferases (TrmI) catalyze the transfer of a methyl group from S-adenosyl-L-methionine to nitrogen 1 of adenine 58 in the T-loop of tRNAs from all three domains of life. The m1A58 modification has been shown to be essential for cell growth in yeast and for adaptation to high temperatures in thermophilic organisms. These enzymes were shown to be active as tetramers. The crystal structures of five TrmIs from hyperthermophilic archaea and thermophilic or mesophilic bacteria have previously been determined, the optimal growth temperature of these organisms ranging from 37°C to 100°C. All TrmIs are assembled as tetramers formed by dimers of tightly assembled dimers.ResultsIn this study, we present a comparative structural analysis of these TrmIs, which highlights factors that allow them to function over a large range of temperature. The monomers of the five enzymes are structurally highly similar, but the inter-monomer contacts differ strongly. Our analysis shows that bacterial enzymes from thermophilic organisms display additional intermolecular ionic interactions across the dimer interfaces, whereas hyperthermophilic enzymes present additional hydrophobic contacts. Moreover, as an alternative to two bidentate ionic interactions that stabilize the tetrameric interface in all other TrmI proteins, the tetramer of the archaeal P. abyssi enzyme is strengthened by four intersubunit disulfide bridges.ConclusionsThe availability of crystal structures of TrmIs from mesophilic, thermophilic or hyperthermophilic organisms allows a detailed analysis of the architecture of this protein family. Our structural comparisons provide insight into the different molecular strategies used to achieve the tetrameric organization in order to maintain the enzyme activity under extreme conditions.
Highlights
TRNA m1A58 methyltransferases (TrmI) catalyze the transfer of a methyl group from S-adenosyl-Lmethionine to nitrogen 1 of adenine 58 in the T-loop of tRNAs from all three domains of life
Structural comparison of TrmI proteins The archaeal and bacterial m1A58 MTases have similar size and architecture The crystal structures of three bacterial and one archaeal TrmIs have previously been reported (Table 1): from Mycobacterium tuberculosis, a mesophilic bacterium (MtTrmI, initially called Rv2118c) [22,25], Thermus thermophilus, a thermophilic bacterium (TtTrmI)[26], Aquifex aeolicus (PDB code 2YVL, AaTrmI), a hyperthermophilic bacterium and Pyroccocus abyssi, a hyperthermophilic archaeon (PaTrmI) that lives in an environment of extreme pressure [27]
In the present study, we aimed at performing a detailed structural analysis to investigate the structural mechanisms underlying stability in TrmIs from organisms spanning a large variety of optimal growth conditions
Summary
TRNA m1A58 methyltransferases (TrmI) catalyze the transfer of a methyl group from S-adenosyl-Lmethionine to nitrogen 1 of adenine 58 in the T-loop of tRNAs from all three domains of life. The m1A58 modification has been shown to be essential for cell growth in yeast and for adaptation to high temperatures in thermophilic organisms These enzymes were shown to be active as tetramers. Different structural features have been shown to contribute to protein thermostability, such as an increased number of hydrogen bonds, more ionic interactions, greater hydrophobic interactions, a more compact and rigid packing, and the presence of disulfide bridges [11-14]. These studies revealed that there is no single universal mechanism that promotes stability, and the molecular mechanisms behind thermostability can vary from one protein to the other [1,11,12]
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