The invention provides isolated p49/STRAP protein, and isolated nucleic acids encoding a p49/STRAP protein. The inventors have discovered a new protein, named p49/STRAP that is expressed in cardiac tissue and other tissues in mammals. The p49/STRAP protein binds to serum response factor (SRF) and regulates transcription of SRF-responsive genes in the heart. p49/STRAP is also discovered to inhibit tumor cell proliferation, and thus the invention provides a method of inhibiting cancer cell proliferation by contacting the cells with p49/STRAP.