SR proteins play critical roles in the major pre-mRNA
splicing pathway. A second pathway processes U12-dependent
AT-AC introns. We demonstrate, by biochemical complementation,
the requirement for SR proteins in splicing of AT-AC introns.
Whereas SR proteins were sufficient to activate splicing
of a P120 AT-AC intron, splicing of a sodium channel AT-AC
intron required an additional nuclear fraction. Individual
recombinant SR proteins promoted splicing of both substrates,
but displayed marked preferences. SR proteins supported
basal AT-AC splicing, and also splicing stimulation via
a downstream enhancer or conventional 5′ splice site.
Analysis of chimeric transcripts revealed that information
dispersed throughout exons and introns dictates SR protein
specificity and the requirement for the additional nuclear
fraction. Thus, SR proteins function in both major and
minor splicing pathways, and in coordinating the activities
of both spliceosomes via exon definition. These results
suggest that despite the substantial differences in intron
consensus sequences and in four of the five snRNPs in each
spliceosome, at least some of the interactions involving
SR proteins are conserved between the two pathways.