Phosphorylation of splicing factor SF1 on Ser20 by cGMP-dependent protein kinase regulates spliceosome assembly

Splicing factor 1 (SF1) functions at early stages of pre-mRNA splicing and contributes to splice site recognition by interacting with the essential sp licing factor U2AF65 and binding to the intron branch site. We have identif ied an 80 kDa substrate of cGMP-dependent protein kinase-I (PKG-I) isolated from rat brain, which is identical to SF1. PKG phosphorylates SF1 at Ser20 , which inhibits the SF1-U2AF65 interaction leading to a block of pre-splic eosome assembly. Mutation of Ser20 to Ala or Thr also inhibits the interact ion with U2AF65, indicating that Ser20 is essential for binding. SF1 is pho sphorylated in vitro by PKG, but not by cAMP-dependent protein kinase A (PK A), Phosphorylation of SF1 also occurs ill cultured neuronal cells and is i ncreased on Ser20 in response to a cGMP analogue. These results suggest a n ew role for PKG in mammalian pre-mRNA splicing by regulating in a phosphory lation-dependent manner the association of SF1 with U2AF65 and spliceosome assembly.