Transfected human ASK1 produces a 150kDa protein. However, we have detected
endogenous ASK1 predominantly as 39kDa and 50kDa C-terminal and 75kDa and
110kDa N-terminal fragments in a panel of non-transfected cancer cell lines and HUVEC
endothelial cells. This suggests that in non-apoptotic cells, endogenous ASK1 protein is
normally cleaved at a number of specific sites, some of which are in the kinase domain.
Transfected ASK1 protein is known to be degraded by the proteasome. In contrast, the
cleavage of endogenous ASK1 is independent of the proteasome as treatment with the
proteasome inhibitor, lactacystin did not inhibit cleavage. Cisplatin treatment decreased
the amount of 39kDa C-terminal ASK1 fragment in mutant p53 cell lines suggesting a
decrease in cleavage associated with apoptosis. Transfected ASK1 may therefore not
accurately reflect the role of endogenous ASK1.