Prp8: Difference between revisions

Pre-mRNA splicing is carried out by one of the most complex pieces of cellular machinery known to date; the Spliceosome. It is a large, catalytic protein-RNA complex responsible for removing the intronic sequences of pre-mRNA and “splicing” together the exonic sequences to form mature mRNA. The spliceosome is made up of approximately 145 distinct spliceosomal proteins, and five small nuclear RNAs (snRNAs) <ref name='Comprehensive proteomic analysis of the human spliceosome'>DOI:10.1038/nature01031</ref>. The five snRNAs have been subsequently named U1, U2, U4, U5, and U6. The spliceosome carries out a two-step transesterification reaction through a series of chemical steps in order to remove the intronic sequences from the pre-mRNA, which is facilitated by the movements, rearrangements, and dynamic exchanges of the snRNAs and splicing associated proteins <ref name='Messenger RNA Splicing in Yeast: Clues to Why the Spliceosome is a Ribonucleoprotein'>PMID:1853200</ref>.  

First, the U1 snRNP recognizes and binds to the 5’ splice site of the pre-mRNA and the U2 snRNP recognizes and binds to the branch site of the intronic sequence, which contains a conserved adenine nucleotide, which is collectively known as complex A <ref name='Biochemical Mechanisms of Constitutive and Regulated PRE-mRNA Splicing'>DOI:10.1146/annurev.cb.07.110191.003015</ref>. Subsequently, the U4/U6 U5 tri-snRNP moves in and associates with the intronic sequence (now known as complex B)<ref name='Biochemical Mechanisms of Constitutive and Regulated PRE-mRNA Splicing'/>. This causes the U4 and U1 snRNPs to be displaced, forming the catalytically active spliceosome (complex B*) and the remaining U2, U5, and U6 snRNP facilitate the two transesterification reactions, dissociate from the now mature mRNA, and are recycled for subsequent use in another splicing reaction <ref name='Biochemical Mechanisms of Constitutive and Regulated PRE-mRNA Splicing'/>.

As stated above, the chemical mechanism pre-mRNA splicing follows is a two-step transesterification reaction. With the release of U1 and U4, following the subsequent rearrangement of the remaining snRNPs, the 2’ hydroxyl of the conserved adenine nucleotide carries out a nucleophilic attack on the 5’ splice site, which cuts the backbone of the mRNA, freeing the first exon <ref name='Messenger RNA Splicing in Yeast: Clues to Why the Spliceosome is a Ribonucleoprotein'>PMID:1853200</ref>. Now the 5’ end of the intron is covalently bonded to the adenine nucleotide, forming a lariat structure <ref name='Messenger RNA Splicing in Yeast: Clues to Why the Spliceosome is a Ribonucleoprotein'>PMID:1853200</ref>. Finally, the 3’ OH of the first exon nucleophilically attacks the 5’ end of the second exon, displacing the intron and splicing the exons together <ref name='Messenger RNA Splicing in Yeast: Clues to Why the Spliceosome is a Ribonucleoprotein'>PMID:1853200</ref>. This is followed by the subsequent release of the lariat structure and the remaining spliceosome dissociates <ref name='Messenger RNA Splicing in Yeast: Clues to Why the Spliceosome is a Ribonucleoprotein'>PMID:1853200</ref>.

=Structure of Prp8=