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We have carried out a systematic analysis of the proteins that interact with specific intron and exon sequences during each stage of mammalian spliceosome assembly. This was achieved by site-specifically labeling individual nucleotides within the 5' and 3' splice sites, the branchpoint sequence (BPS), or the exons with 32P and identifying UV-cross-linked proteins in the E, A, B, or C spliceosomal complex. Significantly, two members of the SR family of splicing factors, which are known to promote E-complex assembly, cross-link within exon sequences to a region approximately 25 nucleotides upstream from the 5' splice site. At the 5' splice site, cross-linking of the U5 small nuclear ribonucleoprotein particle protein, U5(200), was detected in both the B and C complexes. As observed in yeast cells, U5(200), also cross-links to intron/exon sequences at the 3' splice site in the C complex and may play a role in aligning the 5' and 3' exons for ligation. With label at the branch site, we detected three distinct proteins, designated BPS72,BpS70, and BPS56, which replace one another in the E, A, and C complexes. Another dynamic exchange was detected with pre-mRNA labeled at the AG dinucleotide of the 3' splice site. In this case, a protein, AG100,cross-links in the A complex and is replaced by another protein, AG75, in the C complex. The observation that these proteins are specifically associated with critical pre-mRNA sequence elements in functional complexes at different stages of spliceosome assembly implicates roles for these factors in key recognition events during the splicing pathway.

Identification of Proteins That Interact with Exon Sequences, Splice Sites, and the Branchpoint Sequence during Each Stage of Spliceosome Assembly