Exon-independent recruitment of SRSF1 is mediated by U1 snRNP stem-loop 3

EMBO J. 2022 Jan 4;41(1):e107640. doi: 10.15252/embj.2021107640. Epub 2021 Nov 15.


SRSF1 protein and U1 snRNPs are closely connected splicing factors. They both stimulate exon inclusion, SRSF1 by binding to exonic splicing enhancer sequences (ESEs) and U1 snRNPs by binding to the downstream 5' splice site (SS), and both factors affect 5' SS selection. The binding of U1 snRNPs initiates spliceosome assembly, but SR proteins such as SRSF1 can in some cases substitute for it. The mechanistic basis of this relationship is poorly understood. We show here by single-molecule methods that a single molecule of SRSF1 can be recruited by a U1 snRNP. This reaction is independent of exon sequences and separate from the U1-independent process of binding to an ESE. Structural analysis and cross-linking data show that SRSF1 contacts U1 snRNA stem-loop 3, which is required for splicing. We suggest that the recruitment of SRSF1 to a U1 snRNP at a 5'SS is the basis for exon definition by U1 snRNP and might be one of the principal functions of U1 snRNPs in the core reactions of splicing in mammals.

Keywords: RNA splicing; RNA-protein interaction; SRSF1; U1 snRNP; exon definition.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Exons / genetics*
  • HeLa Cells
  • Humans
  • Models, Biological
  • Nucleic Acid Conformation*
  • Protein Binding
  • RNA Precursors / metabolism
  • RNA Splice Sites / genetics
  • RNA, Small Nuclear / chemistry
  • RNA, Small Nuclear / metabolism
  • Ribonucleoprotein, U1 Small Nuclear / metabolism*
  • Serine-Arginine Splicing Factors / metabolism*


  • RNA Precursors
  • RNA Splice Sites
  • RNA, Small Nuclear
  • Ribonucleoprotein, U1 Small Nuclear
  • SRSF1 protein, human
  • U1 small nuclear RNA
  • Serine-Arginine Splicing Factors