A common core RNP structure shared between the small nucleoar box C/D RNPsand the spliceosomal U4 snRNP

The box C/D snoRNAs function in directing 2'-O-methylation and/or as chaper ones in the processing of ribosomal RNA. We show here that Snu13p (15.5kD i n human), a component of the U4/U6.U5 tri-snRNP, is also associated with th e box C/D snoRNAs. Indeed, genetic depletion of Snu13p in yeast leads to a major defect in RNA metabolism. The box C/D motif can be folded into a stem -internal loop-stem structure, almost identical to the 15.5kD binding site in the U4 snRNA. Consistent with this, the box C/D motif binds Snu13p/15.5k D in vitro. The similarities in structure and function observed between the U4 snRNP (chaperone for U6) and the box C/D snoRNPs raises the interesting possibility that these particles may have evolved from a common ancestral RNP.