Effect of mutation and phosphorylation of type I keratins on their caspase-mediated degradation

Type I keratins K18 and K19 undergo caspase-mediated degradation during apoptosis. Two known K18 caspase cleavage sites are aspartates in the consensus sequences VEVDA and DALDS, located within the rod domain and tail domain, respectively. Several K14 (another type I keratin) mutations within the caspase cleavage motif have been described in patients with epidermolysis bullosa simplex. Here we use extensive mutational analysis to show that K19 and K14 are caspase substrates and that the ability to undergo caspase-mediated digestion of K18, K19, or K14 is highly dependent on the location and nature of the mutation within the caspase cleavage motif. Caspase cleavage of K14 occurs at the aspartate of VEMDA, a consensus sequence found in type I keratins K12-17 with similar but not identical sequences in K18 and K19. For K18, apoptosis-induced cleavage occurs sequentially, first at (393)DALD and then at (234)VEVD. Hyperphosphorylation of K18 protects from caspase-3 in vitro digestion at (234)VEVD but not at (393)DALD. Hence, keratins K12-17 are likely caspase substrates during apoptosis. Keratin hyperphosphorylation, which occurs early in apoptosis, protects from caspase-mediated K18 digestion in a cleavage site-specific manner. Mutations in epidermolysis bullosa simplex patients could interfere with K14 degradation during apoptosis, depending on their location.