What’s the spliceosome assembly pathway?

The commitment complex progresses to pre-spliceosome (the A complex) in the presence of ATP. Recruitment of U5 and U4/U6 snRNPs converts the A complex to the mature spliceosome (the B1 complex). The B1 complex is next converted to the B2 complex in which U1 snRNP is released to allow U6 snRNA to interact with the 5′ splice site. When U4 dissociates from U6 snRNP, U6 snRNA can pair with U2 snRNA to form the catalytic active site. Both transesterification reactions take place in the activated spliceosome (the C complex). The splicing reaction is reversible at all steps.