Tpa1 (for termination and polyadenylation) from Saccharomyces cerevisiae is a component of a messenger ribonucleoprotein (mRNP) complex at the 3′ untranslated region of mRNAs. It comprises an N-terminal Fe(II)- and 2-oxoglutarate (2OG) dependent dioxygenase domain and a C-terminal domain. The N-terminal dioxygenase domain of a homologous Ofd1 protein from Schizosaccharomyces pombe was proposed to serve as an oxygen sensor that regulates the activity of the C-terminal degradation domain. Members of the Tpa1 family are also present in higher eukaryotes including humans. Here we report the crystal structure of S. cerevisiae Tpa1 as a representative member of the Tpa1 family. Structures have been determined as a binary complex with Fe(III) and as a ternary complex with Fe(III) and 2OG. The structures reveal that both domains of Tpa1 have the double-stranded β-helix fold and are similar to prolyl 4-hydroxylases. However, the binding of Fe(III) and 2OG is observed in the N-terminal domain only. We also show that Tpa1 binds to poly(rA), suggesting its direct interaction with mRNA in the mRNP complex. The structural and functional data reported in this study support a role of the Tpa1 family as a hydroxylase in the mRNP complex and as an oxygen sensor.

We performed extensive density functional calculations on various metallofullerene complexes and their polyanions to gain insight into novel η1 and η2[6:5] metal (M)–C60 bonding modes. For LnMC60 (L = ligand), the η1 mode is calculated to be the most stable, followed by η2[6:5] and η2[6:6] for –3 anions, in contrast to η2[6:6] >> η2[6:5] ≈ η1 for neutral cases. This observation is responsible for the transformation from η2[6:6] to η1 for LnM3C60, such as [Os3(CO)9C60], upon successive electron reductions. Our energy partitioning analysis (EPA) indicates that the π‐type character of η2[6:6] is much larger than that of η2[6:5]. An electron addition decreases the π‐type interaction of both the η2[6:6] and η2[6:5] modes by about 35 %, whereas it has little effect on σ‐type interactions. Because of the large proportion of π‐character in η2[6:6] coordination, the stability of η2[6:6] coordination decreases steeply as electron reductions continue. On the basis of the EPA results, we could explain why the reaction of [Os3(CO)8(CNR)(μ3‐η2[6:6],η2[6:6],η2[6:6]‐C60)] (R = CH2Ph) with CNR (4e donor) produces [Os3(CO)8(CNR)(μ3‐CNR)(μ3‐η1,η2[6:5],η1‐C60)]. The η1 and η2[6:5] bonding modes of M–C60 are crucial to fully understand the bonding nature of M–C60 bonds in exohedral metallofullerene complexes.

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