PCTA: A new player in TGF-β signaling

Transforming growth factor β (TGF-β) regulates a wide variety of biological activities by binding to cell surface serine/threonine kinase receptors. Canonical TGF-β signaling is mediated by Smad proteins, which transduce the TGF-β signal from the cell surface into the nucleus to regulate transcription. Upon TGF-β binding and receptor activation, the TGF-β receptor phosphorylates Smad2 and Smad3. SARA (Smad anchor for receptor activation) and cPML (cytoplasmic promyelocytic leukemia protein) recruit Smad2 and Smad3 for phosphorylation by the TGF-β receptor. cPML is sequestered in the nucleus by the homeodomain protein TGIF (TG-interacting factor), a negative regulator of TGF-β signaling. Recently, PCTA (PML competitor for TGIF association) has been shown to compete with cPML for binding to TGIF, resulting in the accumulation of cPML in the cytoplasm, where it mediates the interaction between Smad2/3 and SARA and coordinates the phosphorylation of Smad2 and Smad3 by the TGF-β receptor. Accordingly, PCTA promotes TGF-β.mediated transcriptional regulation and growth inhibition. Thus, PCTA defines a new regulator in TGF-β signaling.