Prp24

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Prp24 (Pre-mRNA splicing Protein 24) is a Saccharomyces cerevisiae yeast protein that functions in the formation of base pair interactions between the U6 and U4 snRNPs (small nuclear ribonucleoproteins) to form the U4/U6 di-snRNP in the assembly of the spliceosome. This protein is an octamer containing four RNA recognition motifs (RRMs) that function in the binding of Prp24 to U6 snRNA. These RRMs domains are conserved in structure and sequence in proteins orthologous to Prp24 in Homo sapiens and Schizosaccharomyces pombe, as well as in other proteins containing RRMs.

IntroductionIntroduction

Pre-mRNA SplicingPre-mRNA Splicing

Pre-mRNA splicing is an essential process in eukaryotes that removes non-coding introns from a pre-mRNA transcript and splices coding exons together before the mRNA is exported from the nucleus for translation into a protein. Splicing requires five snRNPs (U1, U2, U4, U5, U6), several other proteins, and the input of energy from ATP. The U1 and U2 snRNPs assemble individually on the pre-mRNA transcript, while U4 and U6 form a U4/U6 di-snRNP before interacting with U5 to form a U4/U6.U5 tri-snRNP that combines with U1 and U2 at the pre-mRNA transcript. U4 and U1 then depart, and after conformational changes and base pair formation with the pre-mRNA the remaining snRNPs form the catalytically active spliceosome. Two transesterification reactions then occur; the first reaction is the nucleophillic attack of the a phosphate group at the end of the 5' exon by the 2' hydroxyl of a specific adenosine at the branch point sequence of the intron. This is then followed by the nucleophillic attack of the phosphorous group linking the 3' exon to the intron by the 3' hydroxyl of the 5' exon. These splicing reactions, in addition to the addition of 7-methylguanosine 5' cap and a 3' polyadenosine tail, results in a mature mRNA transcript that can be exported from the nucleus and translated into protein.

U6 and U4 snRNPsU6 and U4 snRNPs

U6 is considered to be one of the most catalytically important snRNAs in the spliceosome, as it interacts directly with the 5' splice site through base pairing. It is thought to undergoe three conformational changes throughout the entire process of splicing and splicesome assembly; it exists as one conformation as free U6 snRNP, another conformation as part of the U4/U6 di-snRNP and a third conformation associated with U2 and the pre-mRNA. In addition to Prp24, U6 is associated with seven other proteins, Lsm 2-8, which form a ring around the 3' portion of the U6 snRNA (reference Brow 2002).

The U4 snRNA is though to be non-catalytic because it leaves the spliceosome before the transesterification reactions occur. Its function is instead thought to be aiding U6 in maintaining a conformation that will enable it to interact with U2 and the 5' splice site. The U4 snRNP contains the U4 snRNA, a ring complex of the Sm proteins B-G, and the proteins Prp3, Prp4, and Snu13 (reference Brow 2002).

The annealing of the U4 and U6 snRNAs to form the U4/U6 di-snRNP complex, which contains the additional proteins Prp6 and Prp31, is an essential process in the formation of the spliceosome. It enables interaction with the U5 snRNP to form the U4/U6.U5 complex which serves to deliver the U5 and U6 snRNPs to the appropriate sites in the pre-spliceosome complex to form a catalytically active spliceosome upon departure of U1 and U4.

Role of Prp24 in SplicingRole of Prp24 in Splicing

Prp24 is a U6 snNRP protein that functions in the annealing of the U4 and U6 snRNPs during the assembly of the spliceosome. This protein was first identified in a genetic screen as mutated gene that caused an accumulation of pre-mRNA (reference Vijayraghavan et al. 1989). It's first functional role was suggested after several mutant forms of the protein were found to suppress a cold sensitive growth defect caused by mutations in the U4 snRNA (reference Shannon and Guthrie 1991). It is thought that Prp24 helps to stabilize the U6 snRNA and hold it in a conformation that promotes base pairing interactions with the U4 snRNA to form the stem I and stem II structures of the U4/U6 di-snRNP. Prp24 departs from the U4/U6 complex before the formation of the U4/U6.U5 tri-snRNP, but it has been suggested that Prp24 may also play a role in the dissociation of U4 from U6 during the base pairing of U6 with U2 and the 5' splice site. This additional role for Prp24, however, has not been experimentally supported.

StructureStructure

spinqualitylabelsall models X-ray crystallographic structure of the first three RRMs of Prp24 Show:Asymmetric Unit Biological Assembly Drag the structure with the mouse to rotate   Export Animated Image

The key structural elements of Prp24 are the conserved RNA recognition motifs (RRMs). These motifs are found in many proteins with RNA binding properties and are contain conserved RNP elements that are recognizable from their primary sequence. For several years, Prp24 was thought to contain three RRMs, termed RRM 1, RRM 2, and RRM 3 (green link)(reference Shannon and Guthrie 1991). However, analysis of homologs of Prp24 from several different species allowed the identification of a fourth RRM in Prp24 of S. cerevisiae, albeit one that was much less highly conserved and not easily recognizable by its RNP-consensus domain (reference Rader and Guthrie 2002).

RNA Recognition MotifsRNA Recognition Motifs

Prp24 contains four RRMs, RRM 1, RRM 2, RRM 3, and RRM 4. These motifs have a canonical structure of a platform of four &beta strands with two &alpha***** helices on one side of the &beta sheet plane (green link).