The protease activity of mutant NE is not required to activate UPR-induced apoptosis. (A) The indicated NE cDNA was transiently transfected into RBL cells, and 24 hours later cell extracts were prepared from bulk cultures. NE protease activity was quantified using a chromogenic, NE-sensitive substrate. Shown is NE protease activity after correcting for the number of GFP⁺ cells analyzed. (B,C) Cultured human granulocytic precursors were transiently transfected with the indicated NE cDNA. (B) BiP mRNA expression relative to rRNA was determined on sorted GFP⁺ cells 12 hours after transfection. (C) The percentage of GFP⁺ 7AAD⁻ cells that were annexin V⁺ at 24 hours after transfection is shown. Data represent the mean (± SD) of 4-5 independent experiments. *P < .05 compared with WT-transfected cells. NS indicates nonsignificant.