  Sm proteins were discovered nearly 20 years ago as a group of small antigenic
proteins ($\approx$ 90-120 residues). Since then, an extensive amount of
biochemical and genetic data have illuminated the crucial roles of these
proteins in forming ribonucleoprotein (RNP) complexes that are used in RNA
processing, e.g., spliceosomal removal of introns from pre-mRNAs. Spliceosomes
are large macromolecular machines that are comparable to ribosomes in size and
complexity, and are composed of uridine-rich small nuclear RNPs (U snRNPs).
Various sets of seven different Sm proteins form the cores of most snRNPs.
Despite their importance, very little is known about the atomic-resolution
structure of snRNPs or their Sm cores. As a first step towards a
high-resolution image of snRNPs and their hierarchic assembly, we have
determined the crystal structures of archaeal homologs of Sm proteins, which we
term Sm-like archaeal proteins (SmAPs).
