Three-dimensional Structure Determined for a Subunit of Human tRNA Splicing Endonuclease (Sen15) Reveals a Novel Dimeric Fold

Splicing of eukaryal intron-containing tRNAs requires the action of the heterotetrameric splicing endonuclease, which is composed of two catalytic subunits, Sen34 and Sen2, and two structural subunits, Sen15 and Sen54. Here we report the solution structure of the human tRNA splicing endonuclease subunit HsSen15. To facilitate the structure determination, we removed the disordered 35 N-terminal and 14 C-terminal residues of the full-length protein to produce HsSen15(36–157). The structure of HsSen15(36–157), the first for a subunit of a eukaryal splicing endonuclease, revealed that the protein possesses a novel homodimeric fold. Each monomer consists of three α-helices and a mixed antiparallel/parallel β-sheet, arranged in a topology similar to that of the C-terminal domain of Methanocaldococcus jannaschii endonuclease. The dimeric interface is dominated by a β-barrel structure, formed by face-to-face packing of two, three-stranded β-sheets. Each of the β-sheets results from reciprocal parallel pairing of one β-strand from one subunit with two other β-strands from the symmetric subunit. The structural model provides insights into the functional assembly of the human tRNA splicing endonuclease.