The spliceosome is assembled via sequential interactions of pre-mRNA with five small nuclear RNAs and many proteins. Recent determination of cryo-EM structures for several spliceosomal complexes has provided deep insights into interactions between spliceosomal components and structural changes of the spliceosome between steps, but information on how the proteins interact with pre-mRNA to mediate the reaction is scarce. By systematic analysis of proteins interacting with the splice sites (SSs), we have identified many previously unknown interactions of spliceosomal components with the pre-mRNA. Prp8 directly binds over the 5'SS and the branch site (BS) for the first catalytic step, and the 5'SS and 3'SS for the second step. Switching the Prp8 interaction from the BS to the 3'SS requires Slu7, which interacts dynamically with pre-mRNA first, and then interacts stably with the 3'-exon after Prp16-mediated spliceosome remodeling. Our results suggest that Prp8 plays a key role in positioning the 5'SS and 3'SS, facilitated by Slu7 through interactions with Prp8 and substrate RNA to advance exon ligation. We also provide evidence that Prp16 first docks on the intron 3' tail, then translocates in the 3' to 5' direction on remodeling the spliceosome.