Figure 1.

Critical events in early spliceosome assembly steps. U1 snRNP interacts with the functional 5' splice site (5'ss), SF1 recognizes branchpoint sequence (BPS), and U2 snRNP auxiliary factors bind the polypyrimidine tract (Py-tract) between the BPS and AG dinucleotide. After U2 snRNP binding at the 3' splice site (3'ss), SF1 and U2AF heterodimer detach from the spliceosome followed by the joining of the U4/U6.U5 tri-snRNPs in an ATP-dependent manner to convert pre-spliceosome to mature spliceosome.