Arrangement of RNA and proteins in the spliceosomal U1 small nuclear ribonucleoprotein particle

In eukaryotic cells, freshly synthesized messenger RNA (pre-mRNA) contains stretches of non-coding RNA that must be excised before the RNA can be tran slated into protein. Their removal is catalysed by the spliceosome, a large complex formed when a number of small nuclear ribonucleoprotein particles (snRNPs) bind sequentially to the pre-mRNA. The first snRNP to bind is call ed U1; other snRNPs (U2, U4/U6 and U5) follow(1). Here we describe the thre e-dimensional structure of human U1 snRNP, determined by single-particle el ectron cryomicroscopy at 10 Angstrom resolution. The reconstruction reveals a doughnut-shaped central element that accommodates the seven Sm proteins common to all snRNPs, supporting a proposed model of circular Sm protein ar rangement(2). By taking earlier biochemical results into account, we were a ble to assign the remaining density of the map to the other known component s of U1 snRNP, deriving a structural model that describes the three-dimensi onal arrangement of proteins and RNA in U1 snRNP.