THE INTRACISTERNAL A-PARTICLE PROXIMAL ENHANCER-BINDING PROTEIN ACTIVATES TRANSCRIPTION AND IS IDENTICAL TO THE RNA-BINDING AND DNA-BINDINGPROTEIN P54(NRB) NONO/

The long terminal repeats of murine intracisternal A particles (IAPs) contain an IAP proximal enhancer (IPE) element that is inactive in mur ine F9 embryonal carcinoma cells and active in the parietal endoderm c ell line PYS-2. The element binds efficiently to a 60-kDa IPE-binding protein (IPEB) present in PYS-2 cells but poorly to F9 proteins, sugge sting a role for IPEB in regulating IAP expression, We have purified c alf thymus IPEB, which binds to the IPE and transactivates a reporter gene in HeLa cell extracts, Based on the peptide sequence of the purif ied calf IPEB, we have cloned a 420-bp cDNA and showed that the encode d protein is the homolog of human p54(nrb) and mouse NonO, which are c haracterized by the presence of two RNA recognition motifs. We Show th at p54(nrb) is an IPE-binding transcription activator with its DNA-bin ding and activation domains in the N- and C-terminal halves, respectiv ely, The activation domain of p54(nrb) is active in HeLa, PYS-2, and F 9 cells, whereas p54(nrb) as a whole molecule is active in HeLa and PY S-2 cells but not in F9 cells. Thus, the lack of activity of p54(nrb) in F9 cells is due to an ineffective DNA-binding domain, We demonstrat e that p54(nrb) also binds to a pre-mRNA. Based on the close sequence relatedness of this protein to PSF, which is required for pre-mRNA spl icing in vitro, we discuss the possibility that p54(nrb) has dual role s in transcription and splicing.