The Interferon-inducible P202a Protein Modulates NF-kappaB Activity by Inhibiting the Binding to DNA of P50/p65 Heterodimers and P65 Homodimers While Enhancing the Binding of P50 Homodimers

p202a is a member of the interferon-inducible murine p200 family of proteins. These proteins share 1 or 2 partially conserved 200 amino acid segments of the a or the b type. The known biological activities of p202a include among others the regulation of muscle differentiation, cell proliferation, and apoptosis. These biological activities of p202a can be correlated with the inhibition of the activity of several transcription factors. Thus, the binding of p202a results in the inhibition of the sequence-specific binding to DNA of the c-Fos, c-Jun, E2F1, E2F4, MyoD, myogenin, and c-Myc transcription factors. This study concerns the mechanisms by which p202a inhibits the activity of NF-kappaB, a transcription factor involved among others in host defense, inflammation, immunity, and the apoptotic response. NF-kappaB consists of p50 and p65 subunits. We demonstrate that p202a can inhibit in vitro and in vivo the binding to DNA of p65 homodimers and p50/65 heterodimers, whereas it increases the binding of p50 homodimers. Thus p202a can impair NF-kappaB activity both by inhibiting the binding to DNA of the transcriptionally active p65 homodimers and p50/p65 heterodimers and by boosting the binding of the repressive p50 homodimers. p202a can bind p50 and p65 in vitro and in vivo, and p202a can be part of the p50 homodimer complex bound to DNA. p50 binds in p202a to the a type segment, whereas p65 binds to the b type segment. Transfected ectopic p202a increases the apoptotic effect of tumor necrosis factor (at least in part) by inhibiting NF-kappaB and its antiapoptotic activity.