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 sm all nuclear ribonucleoproteins, required for processes as diverse as pre-mR NA splicing, mRNA degradation and telomere formation. The Lsm proteins from archaea are likely to represent the ancestral Sm/Lsm domain. Here, we pres ent the crystal structure of the Lsm alpha protein from the thermophilic ar chaeon Methanobacterium thermoautrophicum at 2.0 Angstrom resolution. The L sm alpha protein crystallizes as a heptameric ring comprised of seven ident ical subunits interacting via beta -strand pairing and hydrophobic interact ions. The heptamer can be viewed as a propeller-like structure in which eac h blade consists of a seven-stranded antiparallel beta -sheet formed from n eighbouring subunits. There are seven slots on the inner surface of the hep tamer ring, each of which is lined by Asp, Asn and Arg residues that are hi ghly conserved in the Sm/Lsm sequences. These conserved slots are likely to form the RNA-binding site. In archaea, the gene encoding Lsm alpha is loca ted next to the L37e ribosomal protein gene in a putative operon, suggestin g a role for the Lsm alpha complex in ribosome function or biogenesis. (C) 2001 Academic Press.