What is the function of the B complex in splicing?

The B complex in splicing plays a critical role in the assembly of the splicing machinery at the pre-mRNA. This complex is formed during the splicing process after the initial recognition of the splice sites by the U1 small nuclear ribonucleoprotein (snRNP) and U2 snRNP.

One of the main functions of the B complex is to facilitate the interaction between U1 and U2 snRNPs, which is essential for the formation of the spliceosome. By bringing these components together, the B complex ensures that the spliceosome is correctly assembled, which is necessary for subsequent steps of splicing, including the recognition of the branch point and the active site formation.

In terms of context, while the active site for splicing and the recruitment of additional splicing factors are also functions that occur during the splicing process, the primary role of the B complex is specifically focused on the initial assembly and interaction of U1 and U2. The removal of introns is a product of the splicing process, rather than a specific function of the B complex itself, as this is accomplished during later stages of spliceosome activity.