Smu1 and RED are required for activation of spliceosomal B complexes assembled on short introns.

Human pre-catalytic spliceosomescontain several proteins that associate transiently just prior to spliceosomeactivation and are absent in yeast, suggesting that this critical step is more complex in higher eukaryotes. We demonstrate via RNAi coupled with RNA-Seqthat two of these human-specific proteins, Smu1 and RED, function both as alternative splicingregulators and as general splicing factorsand are required predominantly for efficient splicingof short introns. In vitro splicingassays reveal that Smu1 and RED promote spliceosomeactivation, and are essential for this step when the distance between the pre-mRNA’s 5′ splicesite (SS) and branch site (BS) is sufficiently short. This Smu1-RED requirement can be bypassed when the 5′ and 3′ regions of short intronsare physically separated. Our observations suggest that Smu1 and RED relieve physical constraints arising from a short 5′SS-BS distance, thereby enabling spliceosomesto overcome structural challenges associated with the splicingof short introns. Human spliceosomecomponents Smu1 and RED regulate alternative splicing. Here the authors show that Smu1 and RED are also required for constitutive splicingof short introns.