The crystal structure of a heptameric archaeal Sm protein: Implications for the eukaryotic snRNP core

Sm proteins form the core of small nuclear ribonucleoprotein particles (snR NPs), making them key components of several mRNA-processing assemblies, inc luding the spliceosome. We report the 1.75-Angstrom crystal structure of Sm AP, an Sm-like archaeal protein that forms a heptameric ring perforated by a cationic pore. In addition to providing direct evidence for such an assem bly in eukaryotic snRNPs, this structure (i) shows that SmAP homodimers are structurally similar to human Sm heterodimers, (ii) supports a gene duplic ation model of Sm protein evolution, and (iii) offers a model of SmAP bound to single-stranded RNA (ssRNA) that explains Sm binding-site specificity. The pronounced electrostatic asymmetry of the SmAP surface imparts directio nality to putative SmAP-RNA interactions.