| Abstract
| - As part of a high-throughput, structural proteomic project we have used NMR spectroscopyto determine the solution structure and ascertain the function of a previously unknown, conserved protein(MtH895) from the thermophilic archeon Methanobacterium thermoautotrophicum. Our findings indicatethat MtH895 contains a central four-stranded β-sheet core surrounded by two helices on one side and athird on the other. It has an overall fold superficially similar to that of a glutaredoxin. However, detailedanalysis of its three-dimensional structure along with molecular docking simulations of its interactionwith T7 DNA polymerase (a thioredoxin-specific substrate) and comparisons with other known membersof the thioredoxin/glutaredoxin family of proteins strongly suggest that MtH895 is more akin to athioredoxin. Furthermore, measurement of the pKa values of its active site thiols along with directmeasurements of the thioredoxin/glutaredoxin activity has confirmed that MtH895 is, indeed, a thioredoxinand exhibits no glutaredoxin activity. We have also identified a group of previously unknown proteinsfrom several other archaebacteria that have significant (34−44%) sequence identity with MtH895. Theseproteins have unusual active site -CXXC- motifs not found in any known thioredoxin or glutaredoxin. Onthe basis of the results presented here, we predict that these small proteins are all members of a new classof truncated thioredoxins.
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