A 39 kDa fragment of endogenous ASK1 suggests specific cleavage not degradation by the proteasome

Abstract

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.