Which component is responsible for pushing the A residue outward in splicing?

The component that plays a crucial role in pushing the A residue outward during splicing is referred to as the A complex. This complex is a part of the spliceosome machinery, which is responsible for the removal of introns from pre-mRNA.

The A complex is formed after the binding of the U2 small nuclear ribonucleoprotein (snRNP) to the branch point site in the pre-mRNA, which contains the adenosine (A) residue. This interaction induces a conformational change that positions the A residue correctly for the catalysis of the first transesterification reaction in the splicing process. In this step, the 2' hydroxyl group of the A residue attacks the phosphate bond at the 5' splice site, leading to the cleavage of the pre-mRNA and the formation of a lariat structure.

In summary, the A complex is critical for initiating the splicing process by facilitating the correct positioning of the A residue, ultimately allowing for effective catalysis in the formation of mature mRNA.