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Human spliceosomal U1 small nuclear ribonucleoprotein particles (snRNPs), which consist of U1 small nuclear RNA and ten proteins, recognize the 5' splice site within precursor messenger RNAs and initiate the assembly of the spliceosome for intron excision. An electron density map of the functional core of U1 snRNP at 5.5 Å resolution has enabled us to build the RNA and, in conjunction with site-specific labelling of individual proteins, to place the seven Sm proteins, U1-C and U1-70K into the map. Here we present the detailed structure of a spliceosomal snRNP, revealing a hierarchical network of intricate interactions between subunits. A striking feature is the amino (N)-terminal polypeptide of U1-70K, which extends over a distance of 180 Å from its RNA binding domain, wraps around the core domain consisting of the seven Sm proteins and finally contacts U1-C, which is crucial for 5'-splice-site recognition. The structure of U1 snRNP provides insights into U1 snRNP assembly and suggests a possible mechanism of 5'-splice-site recognition.
In eukaryotes, the majority of protein-coding genes are interrupted by non-coding sequences known as introns. The entire length of a gene, including its introns, is transcribed into precursor mRNAs (pre-mRNAs). The introns are excised and the exons spliced together to form mRNA with continuous protein-coding sequences by a large RNA-protein assembly called the spliceosome1-3. The major components of the spliceosome are five types of snRNP (U1, U2, U4, U5 and U6), each containing one of five spliceosomal U-type small nuclear RNAs (snRNAs), seven Sm or Lsm proteins and particle-specific proteins1-6. Assembly of the spliceosome is initiated by the binding of U1 snRNP to the 59 splice site in pre-mRNA, followed by an ATPdependent binding of U2 snRNP to the branch-point sequence within the intron. Upon further binding of the tri-snRNP, which contains U4,U6andU5snRNPs, the spliceosomeundergoes extensive rearrangements to become catalytically active1-3,5,6.
Mammalian U1 snRNP consists of U1 snRNA, seven Sm proteins (Sm-B/Sm-B9, Sm-D1, Sm-D2, Sm-D3, Sm-E, Sm-F and Sm-G) and three U1-specific proteins (U1-70K, U1-A and U1-C). The base-pairing between the 59 end of U1 snRNA7,8 and the 59 splice site in pre-mRNA has a crucial role in 59-splice-site recognition9-11. In higher eukaryotes, splicing factors, such as SR proteins, bound to splice enhancer sequences directU1snRNPthrough protein-protein interactions12,13 to the vicinity of 59...