Dominant negative murine serum response factor: Alternative splicing within the activation domain inhibits transactivation of serum response factor binding targets

Primary transcripts encoding the MADS box superfamily of proteins, such as MEF2 in animals and ZEMa in plants, are alternatively spliced, producing se veral isoformic species. We show here that murine serum response factor (SR F) primary RNA transcripts are alternatively spliced at the fifth exon, del eting approximately one-third of the C-terminal activation domain. Among th e different muscle types examined, visceral smooth muscles have a very low ratio of SRF Delta 5 to SRF. Increased levels of SRF Delta 5 correlates wel l with reduced smooth muscle contractile gene activity within the elastic a ortic arch, suggesting important biological roles for differential expressi on of SRF Delta 5 variant relative to wild-type SRF. SRF Delta 5 forms DNA binding-competent homodimers and heterodimers. SRF Delta 5 acts as a natura lly occurring dominant negative regulatory mutant that blocks SRF-dependent skeletal alpha-actin, cardiac alpha-actin, smooth alpha-actin, SM22 alpha, and SRF promoter-luciferase reporter activities. Expression of SRF Delta 5 interferes with differentiation of myogenic C2C12 cells and the appearance of skeletal alpha-actin and myogenin mRNAs. SRF Delta 5 repressed the seru m-induced activity of the c-fos serum response element. SRF Delta 5 fused t o the yeast Gal4 DNA binding domain displayed low transcriptional activity, which was complemented by overexpression of the coactivator ATF6. These re sults indicate that the absence of exon 5 might be bypassed through recruit ment of transcription factors that interact with extra-exon 5 regions in th e transcriptional activating domain. The novel alternatively spliced isofor m of SRF, SRF Delta 5, may play an important regulatory role in modulating SRF-dependent gene expression.