The pre-RNA in eukaryotes is subjected to excision of the introns and ligation of the exons, resulting in formation of a continuous reading frame from encoding from the N- terminal to C-terminal end of the polypeptide. This process involves the transient formation of an organelle called the spliceosome The classical spliceosome contains five small nuclear Ribonucleoproteins, snRNPs (U1,2,4,5,6), that form a the spliceosome. The identification of excision points and ligation points along the RNA is provided for by specific RNA sequences in the mRNA code. There are three such sites, that denote an intron; the 5" splice site, 3" splice site and the branch point. Each snRNP contains a snRNA (small nuclear RNA) and at least 7 protein subunits.
What is the function of the snRNAs in the snRNP?
What are the roles the proteins play in the biological activity of the spliceosome?
Small nuclear RNA (snRNA) is one of many small RNA species confined to the nucleus; several of the snRNAs are involved in splicing or other RNA processing reactions.
ReplyDeleteThe snRNPs involved in splicing are U1, U2, U5, U4, and U6. They are named according to the snRNAs that are present. Each snRNP contains a single snRNA and several (<20) proteins. The U4 and U6 snRNPs are usually found as a single (U4/U6) particle. A common structural core for each snRNP consists of a group of 8 proteins, all of which are recognized by an autoimmune antiserum called anti-Sm; conserved sequences in the proteins form the target for the antibodies. The other proteins in each snRNP are unique to it. The Sm proteins bind to the conserved sequence PuAU36Gpu, which is present in all snRNAs except U6. The U6 snRNP contains instead a set of Sm-like (Lsm) proteins. The Sm proteins must be involved in the autoimmune reaction, although their relationship to the phenotype of the autoimmune disease is not clear.
Some of the proteins in the snRNPs may be involved directly in splicing; others may be required in structural roles or just for assembly or interactions between the snRNP particles. About one third of the proteins involved in splicing are components of the snRNPs. Increasing evidence for a direct role of RNA in the splicing reaction suggests that relatively few of the splicing factors play a direct role in catalysis; most are involved in structural or assembly roles.
http://bioscience.jbpub.com/cells/MBIO5245.aspx
DeleteWhen researching this topic, I came across this figure:
ReplyDeletehttp://www.mhhe.com/biosci/genbio/enger/student/olc/art_quizzes/genbiomedia/0266.jpg
The image does not show the protein subunit, but I believe it does provide a good visualization of how the spliceosome works to remove introns.
ReplyDelete"Small nuclear ribonucleic acid (snRNA), also commonly referred to as U-RNA, is a class of small RNA molecules that are found within... the cell nucleus in eukaryotic cells. The length of an average snRNA is approximately 150 nucleotides. They are transcribed by either RNA polymerase II or RNA polymerase III, and studies have shown that their primary function is in the processing of pre-messenger RNA (hnRNA) in the nucleus. They have also been shown to aid in the regulation of transcription factors (7SK RNA) or RNA polymerase II (B2 RNA), and maintaining the telomeres."
http://en.m.wikipedia.org/wiki/Small_nuclear_RNA
snRNAs are able to recognize certain sequences in the pre-mRNA which signal a splice site (i believe it's through complementarity). After recognition, the introns can be excised by either proteins associated with the snRNA (as part of the spliceosome) or since many RNAs have been shown to have catalytic properties, they may induce the excision themselves.
ReplyDeleteUsing the U1 snRNP as an example, the U1 spliceosomal RNA acts as the U1’s snRNA. This component is required for base pairing with the 5’ splice site of introns during RNA splicing.
ReplyDeleteIt has been determined that the U1 snRNA associates specifically with the general transcription initiation factor TFIIH which is a core RNA splicing component. This process is most likely a regulator of transcription.
ReplyDeleteSnRNA is a class of small RNA that are found within theSnRNA is a class of small RNA that are found within the nucleus cell nucleus of the eukaryotic cells.They are transcribed by either RNA polymerase II or RNA polymerase III.snRNA are associated wSnRNA is a class of small RNA that are found within theSnRNA is a class of small RNA that are found within the nucleus cell nucleus of the eukaryotic cells.They are transcribed by either RNA polymerase II or RNA polymerase III.snRNA are associated with with a set of specific proteins called snRNP.
ReplyDeleteThe spicesome is a large,protein-RNA complex that consist of five small RNAs(U1,U2,U4,U5, and U6) and over 150 proteins.The snRNPs,along with their associated proteins,form ribonucleoprotein complexes (snRNPs),which bind to specific sequences on the pre-mRNA subtrate.This intricate process results in two sequential transesterification reactions.These reactions produce a free lariat intron and ligate two exons to form a mature mRNA.
snRNAs are also involved are also involved in rRNAs processing
snRNPs may be involved directly with splicing;others may be required in structural roles or just assembly or interactions between the snRNP particles.
snRNAs are a class of eukaryotic small RNA molecules found in the nucleus, usually as ribonucleoproteins, and apparently involved in processing heterogeneous nuclear RNA.They are also involved in the maturation of primary transcripts of messenger RNAs (mRNAs). They are transcribed by either RNA polymerase II or RNA polymerase III. The spliceosome is a protein-RNA complex that consists of five small nuclear RNAs (U1, U2, U4, U5, and U6) and also has over 150 proteins.
ReplyDeleteThe proteins have the potential to impact numerous aspects of the modified RNA, including structure, thermal stability and biochemical interactions.
Since everyone has done a great job at answer the question posted, I thought I would just post some interesting videos about spliceosome.
ReplyDeletehttps://www.youtube.com/watch?v=wmdPymy6MeY
https://www.youtube.com/watch?v=FVuAwBGw_pQ
https://www.youtube.com/watch?v=aVgwr0QpYNE
https://www.youtube.com/watch?v=U_5yJYRvh8A
Here's an interesting questions- what happened to U3? From what I remember, the subunits were named in order of being discovered and U3 was obviously found 3rd. The U3 was found to be a non-coding sequence to direct traffic during pre-rRNA processing.
ReplyDelete