The role of JIP-1 in JNK signalling during stress and apoptosis

Recently, the JIP group of proteins was shown to organise the JNK pathway components into scaffolds. Transient transfection studies have shown that JIP-1 binds to JNK, MKK7, MLK3 and HPK1 and organises the JNK pathway in the form of a scaffold to selectively mediate JNK activation in response to stressful stimuli. An examination of the interaction of endogenous JIP-1 with JNK, MLK3 and HPK1 in vivo demonstrated that JIP-1 interacted with MLK3 and HPK1 in resting N1E-115 cells. In stressed N1E-115 cells, the activation of JNK coincided with the JIP-1-JNK1 interaction while the JIP-1-MLK3 interaction was lost. In the rat brain extracts JIP-1-JNK and MLK3-HPK1 interactions were detected. These data suggest that the JIP-1 scaffold complex is dynamic and the differential interaction of JIP-1 with the JNK pathway kinases may regulate JNK activity in vivo. The exchange of the JNK pathway components with the JIP-1 scaffold during stress and the ability of JIP-1 to oligomerise also suggests a possibility of signal amplification.;The presence of putative caspase-3 and -8 cleavage sites in JIP-1 led us to investigate whether JIP-1 was caspase substrate during apoptosis in vivo. The results showed that JIP-1 was cleaved by caspase-3 in vivo during both receptor- and chemical-induced apoptosis of HeLa cells. An analysis of caspase-3 cleavage-resistant JIP-1b mutants in vitro mapped the caspase-3 cleavage sites as being DLID98/A and DESD405/S.;An examination of JIP-1 cleavage and JNK activity in HeLa cells during apoptosis demonstrated that intact JIP-1 was associated with high levels of JNK activity and the cleavage of JIP-1 was coincident with a decrease in JNK activity. Furthermore, during TRAIL-induced apoptosis, the co-immunoprecipitation of JIP-1 and JJNK1 was coincident with JNK activation, whereas a decrease in JNK activity correlated with a reduced ability of JIP-1 to interact with JNK1. These results suggest that the interaction of JIP-1 with JNK may be required for JNK activation, and that the cleavage of JIP-1 may attenuate JNK signalling, in vivo during apoptosis.;Overall, the results suggests that the interaction of JIP-1 with the JNK pathway components may be necessary for JNK activation in vivo during stress and apoptosis