SUMO-1 represses apoptosis signal-regulating kinase 1 activation through physical interaction and not through covalent modification

This study examined whether small ubiquitin-related modifier-1 (SUMO-1) regulates apoptosis signal-regulating kinase 1 (ASK1). ASK1 interacted with SUMO-1 in vitro as well as in BOSC23 cells. Endogenous ASK1 -SUMO-1 interaction was disrupted following H₂O₂ signal. SUMO-1 overexpression suppressed the self-oligomerization, kinase activity and apoptotic potential of ASK1, whereas SUMO-1 depletion potentiated such activities. SUMO-1(ΔC6), a sumoylation-incompetent mutant lacking carboxy-terminal six amino acids, suppressed ASK1 activation, implying that the suppressive effect of SUMO-1 on ASK1 is independent of sumoylation. ASK1(3M), an ASK1 mutant in which all three lysines in the ψKXE motif were substituted with alanines, still retained the kinase activity and activated the Jun amino-terminal kinase pathway. However, SUMO-1 failed to interact with ASK1(3M) and to suppress ASK1(3M) activation, indicating that the three lysines are important for regulation by SUMO-1. This study shows that SUMO-1 exerts a negative regulatory effect on ASK1 activation through physical interaction and not through covalent modification.