Spliceosome

Spliceosome

1. Spliceosome Dr. Emasushan Minj Assistant Professor Department of Botany

2. Spliceosome -mediated RNA splicing mechanism The splicing apparatus for GU-AG introns are the snRNAs called U1, U2, U4, U5 and U6. These are short RNA molecules approx. <250 nucleotides associate with proteins to form small nuclear ribonucleoproteins ( snRNPs ) and attach to the transcript to form a series of complexes . The last one of which, known as the spliceosome . Spliceosome is the structure within which the actual splicing reactions occur.

3. The process of assembly of snRNP and various protein factors occur as follows : The commitment complex (E complex) initiates a splicing activity. This complex comprises U1 which binds to the 5’ splice site by RNA-RNA base-pairing, Branch-Protein Binding Protein (BBP) which binds with U2AF splicing factor, further it binds to the polyprimidine tract and members of SR protein family. The complex E is converted to the A complex when U2 snRNP binds to the branch site. The pre- spliceosome complex (A complex) comprises the commitment complex plus U2-snRNA. At this stage, an association between U1 and U2-snRNP brings the 5’ splice site into close proximity to the branch point.

4. The spliceosome is formed when U4/U6-snRNP and U5-snRNP attach to the pre- spliceosome complex. Following are the formation orders- the B1 complex is formed when U5 and U4/U6- snRNPs binds to the A complex known as spliceosome . It is further converted to the B2 complex after U1-snRNP is released, which leads to the interaction of U6-snRNP with 5’ splice site. This requires hydrolysis of ATP. Dissociation of U4-snRNP leads to the catalytic reaction of U6-snRNP with U2-snRNP and forms catalytic active site (C complex). This brings the 3’ splice site close to the 5’ site and the branch point.

5. All three key positions in the intron are now in proximity and the two transesterifications occur as a linked reaction, possibly catalyzed by U6-snRNP. ATP is required for assembly of the spliceosome , but the transesterification reactions do not require ATP.

6. Spliceosome -mediated RNA splicing mechanism

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