Fig. 1

Schematic of the process of pre-mRNA splicing and major spliceosome assembly. Initial assembly into Complex E involves binding of the U1 snRNP (U1) to the 5’splice site (ss), while non-snRNP splicing factor 1 (SF1) and U2AF bind to the branchpoint sequence and polypyrimidine tract, respectively [16]. Subsequently, U2 snRNP is recruited by SF1 and U2AF, replaces SF1 to bind to the branchpoint, and initiates the formation of Complex A. The recruitment of U2 then facilitates enlistment of the U4/U6-U5 tri-snRNP that is pre-assembled from the U4/U6 and U5 snRNPs, thus forming the pre-catalytic Complex B. Next, destabilisation of U4 and U1 leads to the dissociation of U4, while U6 replaces U1 at the 5’ss and gives rise to the activated spliceosome. This catalytically activated Complex B initiates the first step in splicing, giving rise to Complex C that then cleaves the 5’ss, releasing the first exon to fold and the 5’ss can now join to the branchpoint, forming a lariat within the intron. Following the lariat formation is the second step in splicing; cleavage of the intron at the 3’ss, release of the lariat and the ligation of the two bordering exons. Upon completion of the final step, the spliceosome dissociates so that the snRNPs may be recycled and splicing of a subsequent intron occurs. This is repeated until all the introns from the mRNA are removed, thus giving rise to the formation of the mature mRNA transcript [17, 18]. Following intron excision and ligation of the exons, the U snRNPs are recycled. 5’ss, 3’ss, bp and polypyrimidine tracts are shown in the line representing the intron. Exons are shown as magenta boxes. Adapted from Pitout (2019) [19].