| Protein phosphorylation—a reversible covalent process—has evolved in eukaryotes as one of the major posttranslational modifications that targets numerous transcription factors. Apoptosis (programmed cell death) is an irreversible intrinsic mechanism essential for removing unwanted cells, and maintaining tissue homeostasis in multicellular organisms. Activation of caspases is a major event during programmed cell death by which more than 280 proteins are cleaved. YY1 (Yin-Yang 1) is a multifunctional zinc finger transcription factor that has been implicated in different cellular processes such as proliferation, embryogenesis, differentiation, tumorigenesis, and apoptosis. YY1 has been shown to be a phosphoprotein and several studies have reported that phosphorylation regulates its activities. Also, YY1 was found to be one of only a few transcription regulators that is a target for cleavage by caspases in vitro and in vivo, although very little is known about the mechanisms that regulate its cleavage during cell death. Here, we identify serine 118 in the N-terminal domain of YY1, as the site of CK2α phosphorylation, proximal to a caspase cleavage site. CK2 inhibitors, as well as knockdown of CK2α by siRNA, reduce S118 phosphorylation in vivo and enhance YY1 cleavage under apoptotic conditions, whereas increasing CK2α activity by overexpression in vivo elevates S118 phosphorylation. A serine to alanine substitution at serine 118 also increases the cleavage of YY1 during apoptosis when compared to wild-type YY1. Taken together, we have discovered a regulatory link between YY1 phosphorylation at serine 118 and regulation of its cleavage during programmed cell death.;In response to genotoxic stress, eukaryotic cells activate a set of DNA-damage kinase cascades that are initiated by sensor proteins such as ATM (ataxia-telangiectasia mutated), ATR (ataxia-telangiectasia mutated and Rad3-related kinases), or DNA-PK (DNA-dependent protein kinase). These sensor proteins phosphorylate various targets including Chk1, Chk2, and p53. To study the signaling pathway(s) regulating the cleavage of YY1 under apoptotic conditions, we have used ATR-deficient fibroblasts, as well as knockout cell lines where Chk2, DNA-PKcs, p53 and ATM are ablated. Cisplatin, a DNA damaging agent, was added to these cells to induce cell death. Cisplatin-induced YY1 cleavage and apoptosis are found to be independent of DNA-PK and Chk2, delayed in ATM and p53 knockout cell lines, and suppressed in ATR-deficient fibroblasts and DLD-Chk1 heterozygous cells. Consistently, inhibition of ATR with a selective ATR inhibitor prevents cisplatin-induced YY1 cleavage. Together, these results suggest a critical role of ATR-Chk1 in regulating YY1 cleavage by caspases during apoptosis. |
