Splicing Modulators Act at the Branch Point Adenosine Binding Pocket Defined by the PHF5A-SF3b Complex

Developing small-molecule splicing modulators represents a promising therapeutic approach for various diseases. Natural products such as pladienolide, herboxidiene, and spliceostatin have been identified as potent splicing modulators that target SF3B1 in the SF3b subcomplex. Using integrated chemogenomic, structural and biochemical approaches, we show that PHF5A, another core component of the SF3b complex, is also targeted by these compounds. Mutations in PHF5A-Y36, SF3B1-K1071, SF3B1-R1074, and SF3B1-V1078 confer resistance to these modulators, suggesting a common site of interaction. Whole-transcriptome RNA-seq analysis reveals that PHF5A-Y36C has minimal effect on basal splicing but alters the action of splicing modulators from inducing intron-retention to exon-skipping. Relative intron strength to splicing inhibition correlates with the differential in GC content between adjacent introns and exons, leading to this differential global splicing pattern. We determine the crystal structure of human PHF5A and find that Y36 is located on the surface in a region of high sequence conservation. Structural analysis of the cryo-EM spliceosome Bact complex shows that these mutations cluster in a well-defined pocket surrounding the branch point adenosine suggesting a possible competitive mode of action for these splicing modulators, which interact with the aromatic side-chain of PHF5A-Y36. Collectively, we propose that PHF5A-SF3B1 forms a central node for binding to these small-molecule splicing modulators, offering insights to modulate splicing. Overall design: 24 samples, including HCT-116 cells +/- treatment with E7107 with and without PHF5A Y36C mutation. Each condition has 6 replicates.