New Cdc2 Tyr 4 phosphorylation by dsRNA‐activated protein kinase triggers Cdc2 polyubiquitination and G2 arrest under genotoxic stresses

Cell division cycle 2 (Cdc2) protein is an essential subunit of M‐phase kinase (MPK), which has a key role in G2/M transition. Even though the control of MPK activity has been well established with regard to the phosphorylation of Cdc2 at Thr 14 and/or Tyr 15 and Thr 161, little is known about the proteolytic control of Cdc2. In this study, we observed that Cdc2 was downregulated under genotoxic stresses and that double‐stranded RNA‐activated protein kinase (PKR) was involved in the process. The PKR‐mediated Tyr4 phosphorylation triggered Cdc2 ubiquitination. Phospho‐mimic mutations at the Tyr 4 residue (Y4D or Y4E) caused significant ubiquitination of Cdc2 even in the absence of PKR. Our findings demonstrate that (i) PKR, Ser/Thr kinase, phosphorylates its new substrate Cdc2 at the Tyr 4 residue, (ii) PKR‐mediated Tyr 4‐phosphorylation facilitates Cdc2 ubiquitination and proteosomal degradation, (iii) unphosphorylated Tyr 4 prevents Cdc2 ubiquitination, and (iv) downstream from p53, PKR has a crucial role in G2 arrest and triggers Cdc2 downregulation under genotoxic conditions.