The Sm domain is an ancient RNA-binding motif with oligo(U) specificity

Sm and Sm-like proteins are members of a family of small proteins that is w idespread throughout eukaryotic kingdoms. These proteins form heteromers wi th one another and bind, as heteromeric complexes, to various RNAs, recogni zing primarily short U-rich stretches. Interestingly, completion of several genome projects revealed that archaea also contain genes that may encode S m-like proteins. Herein, we studied the properties of one Sm-like protein d erived from the archaebacterium Archaeoglobus fulgidus and overexpressed in Escherichia coli. This single small protein closely reflects the propertie s of an Sm or Sm-like protein heteromer. It binds to RNA with a high specif icity for oligo(U), and assembles onto the RNA to form a complex that exhib its, as judged by electron microscopy, a ring-like structure similar to the ones observed with the Sm core ribonucleoprotein and the like Sm (LSm) pro tein heteromer. Importantly, multivariate statistical analysis of negative- stain electron-microscopic images revealed a sevenfold symmetry far the obs erved ring structure, indicating that the proteins form a homoheptamer. The se results support the structural model of the Sm proteins derived from cry stallographic studies on Sm heterodimers and demonstrate that the Sm protei n family evolved from a single ancestor that was present before the eukaryo tic and archaeal kingdoms separated.