Deep trefoil knot implicated in RNA binding found in an archaebacterial protein

The M. thermoautotrophicum MT1 gene is conserved in archaea, it lies in a ribosomal protein operon, and it codes for 268 amino acid protein of unknown function. We report here the structure of MT1 that is novel from several standpoints: (i) the structure contains a novel topological unit -- a deep C-terminal trefoil knot first observed in a TIM barrel-like fold, archaebacterial proteins and rarely observed in other proteins; (ii) structurally, it contains only five ({beta}{alpha}) units, and the arrangements of its hydrophobic and hydrophilic surfaces are opposite to that found in classical TIM barrel proteins; (iii) functionally, although it lacks typical features found in enzymes of the barrel family, it has strongly conserved residues clustered on the surface that form a potential catalytic-site; (iv), the structure provides a first example of barrel-like fold linked to an RNA-binding domain, suggesting an extension of TIM barrel functionality to nucleic acid binding and/or catalysis.