Structure and assembly of the spliceosomal small nuclear ribonucleoproteinparticles

The spliceosome is a macromolecular assembly that carries out the excision of introns from nuclear pre-mRNAs. It consists of four large RNA-protein co mplexes, called the U1, U2, U4/U6 and U5 small nuclear ribonucleoproteins ( snRNPs), and many protein factors. Crystal structures of seven protein comp onents and fragments of the U1 and U2 small nuclear RNAs have been determin ed in the form of RNA-protein and protein-protein complexes. Together with electron microscopy studies of the snRNPs, these structures have begun to p rovide important insights into the architecture of the snRNPs and the mecha nisms of RNA-protein and protein-protein recognition.