Activation of caspase-like activity and poly (ADP-ribose) polymerase degradation during sporulation in Aspergillus nidulans

Mycelium vacuolization, protein degradation, and as the final stage autolysis, often accompanies developmental changes in fungi and similarities between autolysis and apoptosis have previously been suggested. Caspases are the key executors of apoptosis and in this study caspase-like activities were detected in protein extracts from Aspergillus nidulans during sporulation. This was shown by hydrolysis of the fluorescent DEVD- and IETD-AFC peptide substrates specific for caspase 3- and 8-like activities, respectively. These activities were repressed by the caspase 3 and 8 specific irreversible peptide inhibitors DEVD-fmk and IETD-fmk, but were not affected by the unspecific inhibitor E-64. Isoelectric focusing of protein extracts followed by activity staining revealed the presence of two bands with caspase-like activity. One of the proteins degraded both caspase 3 and caspase 8 specific substrates whereas the other only degraded the caspase 8 substrate. Searches in an A. nidulans genome database revealed two genes encoding metacaspase proteins with predicted sizes of 45 kDa that could be responsible for the measured caspase-like activities. The searches also found a single gene encoding a poly (ADP-ribose) polymerase (PARP) protein with a predicted size of 81 kDa. PARP is one of the known target proteins inactivated by caspase degradation in animal cells. Western blotting of fungal extracts using a bovine PARP antibody confirmed the presence of a fungal PARP-like protein of about 81 kDa. By Western blotting it was shown that this PARP-like protein band was present only at early time points until the start of conidia formation and the accompanying increase in caspase-like activity. Thereafter, a degradation product of about 60 kDa appeared indicating that the degradation of the fungal PARP-like protein was specific. The PARP antibody also recognized an 85 kDa protein band that was not degraded, and which conceivably represents a modified form of the 81 kDa PARP. Fungal extracts high in caspase-like activity could degrade both the fungal 81 kDa PARP and bovine PARP. In the presence of the caspase 3 inhibitor DEVD-fmk this degradation was delayed. Thus, as in animal apoptotic cells, caspase activities are involved in fungal mycelium self-activated proteolysis.