p58IPK is an inhibitor of the eIF2α kinase GCN2 and its localization and expression underpin protein synthesis and ER processing capacity

One of the key cellular responses to stress is the attenuation of mRNA translation and protein synthesis via the phosphorylation of eIF2 alpha eukaryotic translation initiation factor 2 alpha). This is mediated by four eIF2 alpha kinases and it has been suggested that each kinase is specific to the cellular stress imposed. In the present study, we show that both PERK PKR-like endoplasmic reticulum kinase/eIF2 alpha kinase 3) and GCN2 general control non-derepressible 2/eIF2 alpha kinase 4) are required for the stress responses associated with conditions encountered by cells overexpressing secreted recombinant protein. Importantly, whereas GCN2 is the kinase that is activated following coldshock/hypothermic culturing of mammalian cells, PERK and GCN2 have overlapping functions since knockdown of one of these at the mRNA level is compensated for by the cell by up-regulating levels of the other. The protein p58(IPK) {also known as DnaJ3C [DnaJ heat-shock protein hsp) 40 homologue, subfamily C, member 3]} is known to inhibit the eIF2 alpha kinases PKR dsRNA-dependent protein kinase/eIF2 alpha kinase 2) and PERK and hence prevent or delay eIF2 alpha phosphorylation and consequent inhibition of translation. However, we showthat p58(IPK) is a general inhibitor of the eIF2 alpha kinases in that it also interacts with GCN2. Thus forced overexpression of cytoplasmic p58 delays eIF2 alpha phosphorylation, suppressesGCN2 phosphorylation and prolongs protein synthesis under endoplasmic reticulum (ER), hypothermic and prolonged culture stress conditions. Taken together, our data suggest that there is considerable cross talk between the eIF2a kinases to ensure that protein synthesis is tightly regulated. Their activation is controlled by p58 and the expression levels and localization of this protein are crucial in the capacity the cells to respond to cellular stress via control of protein synthesis rates and subsequent folding in the ER.