Figure 2. Spliceosome assembly and the involvement of adRP associated splicing factors

During the early step of spliceosome assembly, the U1snRNP and the U2snRNP interact with the conserved sequences at the 5' splice site (GU) and the branch point adenosine (A) of the 3' splice site, respectively, to form the prespliceosome. Next, the U4/U6 snRNP associates with the U5snRNP to form the U4/U6.U5 tri-snRNP, which is incorporated into the spliceosome. The formation of the U4/U6.U5 tri-snRNP complex requires PRPF8, PRPF3 and PRPF31, among other splicing factors. U1 is displaced from the 5' splice site, and U4 dissociates from U4/U6 snRNP, allowing U6 to interact with the 5' splice site and with the U2snRNP. In the mature spliceosome, splice sites are aligned in the catalytic center, and the two sequential trans-esterification reactions occur (Figure 1), resulting in the removal of the intron [95]. (Adapted by permission from Macmillan Publishers Ltd: Patel AA, Steitz JA. Splicing double: insights from the second spliceosome. Nature Reviews Molecular Cell Biology 4: 960-970, 2003)