A common structural mechanism for RNA recognition by the SF3B complex in mRNA splicing and export

The SF3B complex plays a critical role in branch point adenosine recognition during pre-mRNA splicing. Its largest subunit SF3B1 is frequently mutated in cancers, leading to aberrant alternative splicing. Besides its function in pre-mRNA splicing, the SF3B complex also binds mature or intronless mRNAs to facilitate their nuclear export. Notably, the RNA motifs recognized by the SF3B complex exhibit no apparent sequence similarities, raising the question of how the SF3B complex recognizes diverse mRNA sequences for various cellular activities. Here we report the cryo-EM structures of the human SF3B complex associated with either intronless histone mRNAs or intron-U2 snRNA. These structures unveil that both mRNA molecules adopt a similar conformation featuring a bulged adenosine and bind the SF3B complex in a remarkably resembling manner, suggesting that SF3B recognizes the specific shape rather than the sequence of its RNA targets. Further cryo-EM and molecular dynamics analyses of the hotspot-mutant SF3B complexes bound to intron-U2 snRNA demonstrate that the SF3B1K700E and SF3B1R625H mutations similarly repel the attachment of the intronic polypyrimidine tract around the mutation sites, leading to reduced RNA-binding affinity. Altogether, our study provides structural insights into the RNA-recognition mechanism of the SF3B complex and suggests that the cancer-associated SF3B1 mutations could potentially affect multiple cellular processes including mRNA splicing and export, which advances our understanding of the pathogenic mechanisms of the SF3B1 mutations.