(C) Anti-HA immunoreactive bands corresponding to the precursor and mature APOPT1 species are detected in fibroblasts exposed to H2O2. Note that the upper band, corresponding to the precursor APOPT1-HA species, is present in the sample collected 8 hr after the exposure to H2O2, whereas only the mature species is detected in samples collected at 24 and 48 hr, suggesting that over time the precursor APOPT1-HA species has been translocated across the inner mitochondrial membrane and quantitatively processed into the mature species by cleavage of a 39 amino acid N-terminal MTS.