Structure and Activity of a Novel Archaeal β-CASP Protein with N-Terminal KH Domains

MTH1203, a β-CASP metallo-β-lactamase family nuclease from the archaeon Methanothermobacter thermautotrophicus, was identified as a putative nuclease that might contribute to RNA processing. The crystal structure of MTH1203 reveals that, in addition to the metallo-β-lactamase nuclease and the β-CASP domains, it contains two contiguous KH domains that are unique to MTH1203 and its orthologs. RNA-binding experiments indicate that MTH1203 preferentially binds U-rich sequences with a dissociation constant in the micromolar range. In vitro nuclease activity assays demonstrated that MTH1203 is a zinc-dependent nuclease. MTH1203 is also shown to be a dimer and, significantly, this dimerization enhances the nuclease activity. Transcription termination in archaea produces mRNA transcripts with U-rich 3' ends that could be degraded by MTH1203 considering its RNA-binding specificity. We hypothesize that this nuclease degrades mRNAs of proteins targeted for degradation and so regulates archaeal RNA turnover, possibly in concert with the exosome.