The c-Jun NH2-terminal kinase (JNK) belongs to a subgroup of the mitogen-activated protein kinase (MAPK) superfamily and is activated in response to a variety of stresses and cytokines in mammalian cells. To gain insight into the JNK cascade, we have undertaken a genetic analysis of the JNK signaling pathway in C. elegans. The genes for the C. elegans homolog of JNK,
jnk-1 (formerly named
sak-1), and its direct activator,
jkk-1 (formerly named
sek-2), were isolated based on their abilities to function in the yeast Hog1 MAP kinase pathway. JKK-1 is a novel member of the MAP kinase kinase (MAPKK) superfamily. Both
jnk-1 and
jkk-1 are expressed in most neurons. To understand the role of JKK-1, we isolated a
jkk-1 deletion allele that lacks most of the kinase domains. If JKK-1 functions as MAPKK of JNK-1 in C. elegans, the latter should not be activated in
jkk-1 mutant animals. To test this possibility, we examined the kinase activity of JNK-1 immunoprecipitated from wild-type and mutant animals. After immunoprecipitating JNK-1 from extracts of wild-type adults with an anti-JNK-1 antibody, we performed in vitro kinase assays using c-Jun as a substrate. JNK-1 prepared from wild-type animals phosphorylated c-Jun. However, JNK-1 immunoprecipitated from
jkk-1 mutant adults exhibited little kinase activity. These results indicate that JNK-1 activity is positively regulated by JKK-1, supporting the idea that they constitute part of a functional JNK cascade.
jkk-1 mutants were viable, but exhibited defects in body movement. Wild-type C. elegans moves by propagating waves of alternating dorsal and ventral flexions along its body length, which produces regular sinusoidal tracks on a bacterial lawn. In contrast, the mutant animals left tracks with increased amplitude relative to those left by the wild type. Paths also meandered more, seldom running in a straight trajectory for a long distance, and resulting in a much shorter migration distance during a given period of time. Other behaviors, including pharyngeal pumping, egg laying, and defecation, were normal in
jkk-1 mutants. Thus, JKK-1 is specifically involved in the modulation of coordinated locomotion. To determine whether the locomotion defects are due to abnormal development or abnormal cell function, we generated a plasmid which places the
jkk-1 gene under the control of heat-shock promoter and integrated it to the
jkk-1 strain. These transgenic mutants were defective in movement without heat treatment. When heat-treated as adults, the movement defects were rescued. These results suggest that the movement defects are not due to a developmental abnormality, but rather to a defect in neuronal cell function. Thus, the JNK pathway modulates coordinated movement in C. elegans as a result of its role in neuronal function.