A pathway linking translation stress to checkpoint kinase 2 signaling in Neurospora crassa

Checkpoint kinase 2 (CHK-2) is a key component of the DNA damage response (DDR). CHK-2 is activated by the PIP3-kinase-like kinases (PI3KKs) ataxia telangiectasia mutated (ATM) and ataxia telangiectasia and Rad3-related protein (ATR), and in metazoan also by DNA-dependent protein kinase catalytic subunit (DNA-PKcs). These DNA damage-dependent activation pathways are conserved and additional activation pathways of CHK-2 are not known. Here we show that PERIOD-4 (PRD-4), the CHK-2 ortholog of Neurospora crassa, is part of a signaling pathway that is activated when protein translation is compromised. Translation stress induces phosphorylation of PRD-4 by a PI3KK distinct from ATM and ATR. Our data indicate that the activating PI3KK is mechanistic target of rapamycin (mTOR). We provide evidence that translation stress is sensed by unbalancing the expression levels of an unstable protein phosphatase that antagonizes phosphorylation of PRD-4 by mTOR complex 1 (TORC1). Hence, Neurospora mTOR and PRD-4 appear to coordinate metabolic state and cell cycle progression.