A HEAT-repeat superhelix within SF3b155 plays a key role in branch site recognition.

  • Major finding: A HEAT-repeat superhelix within SF3b155 plays a key role in branch site recognition.

  • Concept: Cancer-related mutations in SF3b155 primarily occurred in HD residues important for tertiary structure.

  • Impact: The SF3b crystal structure provides mechanistic insight into the role of SF3b155 mutations in cancer.

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SF3b is a multiprotein complex component of the spliceosome that is essential for branch site recognition and selection during splicing. Aberrant branch site recognition can result in accumulation of aberrant transcripts and promote transformation. Mutations in the HEAT-repeat domain (HD) of the largest SF3b subunit, SF3b155 (also known as SF3B1), promote alternative branch site selection and use of cryptic 3′ splice sites and are common in myelodysplastic syndrome, chronic lymphocytic leukemia, and some solid tumors. However, only low-resolution structural data is available for the human SF3b complex, and the mechanism by which alternative branch site selection occurs is not clear. Cretu and colleagues determined the crystal structure of the human core SF3b complex (SF3b155–SF3b130–SF3b14b–SF3b10) to a 3.1 Å resolution. SF3b130 contained three β-propeller domains (BPA, BPB, and BPC) that acted as a protein scaffold, and SF3b10 bound primarily to BPA. The HD of SF3b155 was found to be comprised of 20 tandem repeats that form a right-handed superhelix induced by interactions with SF3b130, SF3b10, and SF3b14b, which form a multipartite scaffold that induces and maintains the SF3b155 structural conformation. SF3b14b interacted with two highly conserved residues at the N- and C-terminus of the SF3b155 superhelix, maintaining the superhelical conformation. Further, the HD superhelix of SF3b155 formed an RNA-binding platform in complex with SF3b14b and the branch-site binding protein p14 for branch site recognition and interacted with the essential splicing factor U2AF65. The majority of cancer-associated mutations in SF3b155 occur in the HEAT repeats that are involved in forming its tertiary structure, suggesting that these mutations might lead to aberrant splicing by disrupting interactions between the SF3b155 HD superhelix and pre-mRNA or spliceosomal proteins. Altogether, the determination of the SF3b complex structure provides insight into the potential underlying mechanism of cancer-associated alternative branch site selection.

Cretu C, Schmitzová J, Ponce-Salvatierra A, Dybkov O, De Laurentiis EI, Sharma K, et al. Molecular architecture of SF3b and structural consequences of its cancer-related mutations. J Mol Cell 2016;64:307–19.

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