Crystal structure of the spliceosomal 15.5kD protein bound to a U4 snRNA fragment.

We have determined the crystal structure of a spliceosomal RNP complex comprising the 15.5kD protein of the human U4/U6.U5 tri-snRNP and the 5' stem-loop of U4 snRNA. The protein interacts almost exclusively with a purine-rich (5+2) internal loop within the 5' stem-loop, giving an unusual RNA fold characterized by two tandem sheared G-A base pairs, a high degree of purine stacking, and the accommodation of a single RNA base, rotated out of the RNA chain, in a pocket of the protein. Apart from yielding the structure of an important entity in the pre-mRNA splicing apparatus, this work also implies a model for the complex of the 15.5kD protein with box C/D snoRNAs. It additionally suggests a general recognition principle in a novel family of RNA binding proteins.