The JNK MAP kinase (MAPK) pathway plays a pivotal role in the various stress responses of evolutionarily diverse species. In C. elegans, a JNK-like MAPK pathway composed of MLK-1 MAPKKK, MEK-1 MAPKK, and KGB-1 MAPK is known to act in a heavy metal stress response. However, the upstream or downstream components of this pathway remain unknown. Here, to identify novel components of the KGB-1 pathway, we have conducted genetic and biochemical experiments as follow. It has been shown that the KGB-1 pathway is negatively regulated by VHP-1, a dual-specificity MAPK phosphatase, and that mutations defective in the KGB-1 pathway suppress the growth arrest caused by a loss-of-function mutation in the
vhp-1 gene. Therefore, we performed
vhp-1 suppressor screen to isolate genes functioning in the KGB-1 pathway. As a result, we isolated 73 lines of
vhp-1 suppressors. Expression of Constitutive Active form of MEK-1 (MEK-1-CA) causes lethal phenotype by hyperactivation of KGB-1 and this lethality is suppressed by the
kgb-1 mutation but not by the
mlk-1 mutation. Out of 73
vhp-1 suppressors, only one line suppressed the lethal phenotype caused by MEK-1-CA. This mutation was found to occur in the
kgb-1 gene. Further biochemical analysis also revealed that 18 lines out of the other 72
vhp-1 suppressors decreased the activated KGB-1 levels, suggesting that these 18 mutations are defective in components upstream of KGB-1. We are currently characterizing these suppressor mutations.