ATF3 facilitates expression of GADD34 in response to ER stress. ATF3 was transiently expressed in PEK −/− MEF cells, and PEK +/+ and transfected (tsf) cells were subjected to the presence (+) or absence (−) of ER stress for the indicated number of hours. Vec indicates that the parent expression plasmid alone was introduced into the PEK −/− MEF cells. Whole-cell lysates were prepared from these transfected cells, and phosphorylated eIF2α (eIF2α-P), total eIF2α, and GADD34 levels were measured by immunoblot analysis. Tg, thapsigargin.