HeLa cells were cultured in MEM media supplemented with 10% fetal calf serum, 1% penicillin-streptomycin, 1% glutamine and 1% non-essential amino acids, in a humidified incubator with 5% CO 2 at 37uC. Aerolysin was purified as described [41] . Cells were continuously treated with 2 ng/mL (0.02 nM) of proaerolysin. At different time points, cells were washed with PBS and lysed at 4uC in 0.25 M sucrose supplemented with proteases inhibitor (Roche, Germany) using a needle. The whole cell extracts were subjected to SDS-PAGE and Western blotting. XBP1 (R-14) antibody was from Santa Cruz Inc. Band intensities were quantified, after background removal, using ImageJ software (NIH). The loading in each lane was normalized relative to the intensity of a nonspecific antibody-reacting band on the blot.