Question

What are the two components of spliceosomes?

Answer 1

• The commitment complex (E complex):- initiates a splicing activity. This complex comprises U1-snRNP, which binds to the 5' splice site by RNA-RNA base pairing , Branch Point Binding Protein (PBP) which binds branch point, the splicing factor U2AF, which binds with the polypyrimidine tract and members of SR protein family. The name SR protein is given due to the presence of an Arg-Ser-rich region. SR proteins interact with one another via their Arg-Ser-rich regions and also bind to RNA. Binding of U1 snRNP to the 5'splice site is the first step in splicing. The E complex 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-snRNP, the latter attached to the branch site. At this stage, an association between U1-snRNP and the U2-snRNP brings the 5' splice site into close proximity to the branch point.

• The spliceosome is formed when U4/U6-snRNP ( a single snRNP containing two snRNAs) and U5-snRNP attach to the pre-spliceosome complex. Following formation of the A complex, the other snRNPs involved in splicing associate with the complex in a definite order. The B1 complex is formed when U5-snRNPs binds to the A complex. This complex is regarded as the spliceosome, since it contains the components needed for the splicing reaction. It is converted to the B2 complex after U1 snRNP is released. The release of U1 snRNP allows U6 snRNP to interact with the 5'splice site. The catalytic reaction is triggered by the release of U4. This requires hydrolysis of ATP. When U4 dissociates from U6 snRNP, U6 snRNP can pair with U2 snRNA to form the catalytic active site (C complex). this resultss in additional interactions that bring the 3' splice site to close to the 5' site and the branch point. All three positions in the intron are now in proximity and the two transesterifications occur as a linked reaction, possibly catalyzed by U6-snRNP, completing the splicing process. During splicing, ATP is required for assembly of the spliceosome, but the transesterification reactions do not require ATP.