Crystal structure of a heptameric Sm-like protein complex from archaea: implications for the structure and evolution of snRNPs

The Sm/Lsm proteins associate with small nuclear RNA to form the core of small nuclear ribonucleoproteins, required for processes as diverse as pre-mRNA splicing, mRNA degradation and telomere formation. The Lsm proteins from archaea are likely to represent the ancestral Sm/Lsm domain. Here, we present the crystal structure of the Lsmα protein from the thermophilic archaeon Methanobacterium thermoautotrophicum at 2.0 Å resolution. The Lsmα protein crystallizes as a heptameric ring comprised of seven identical subunits interacting via β-strand pairing and hydrophobic interactions. The heptamer can be viewed as a propeller-like structure in which each blade consists of a seven-stranded antiparallel β-sheet formed from neighbouring subunits. There are seven slots on the inner surface of the heptamer ring, each of which is lined by Asp, Asn and Arg residues that are highly conserved in the Sm/Lsm sequences. These conserved slots are likely to form the RNA-binding site. In archaea, the gene encoding Lsmα is located next to the L37e ribosomal protein gene in a putative operon, suggesting a role for the Lsmα complex in ribosome function or biogenesis.