Jc. Deak et al., FAS-INDUCED PROTEOLYTIC ACTIVATION AND INTRACELLULAR REDISTRIBUTION OF THE STRESS-SIGNALING KINASE MEKK1, Proceedings of the National Academy of Sciences of the United Statesof America, 95(10), 1998, pp. 5595-5600
The stress-activated protein kinase (SAPK, alternatively JNK) is activ
ated rapidly by cell stress stimuli such as inflammatory cytokines and
oxidative stress, and more slowly by the initiation of the apoptotic
cell death response by events such as ligation of the Fas protein. Mit
ogen-activated protein kinase/Erk kinase kinase-l (MEKK1) is an activa
tor of SAPK, serving as a SAPK-kinase-kinase through intermediate phos
phorylation of the SAPK kinase SEK1, By sequencing proteolytic cleavag
e products of MEKK1, we found that the proapoptotic protease caspase 3
(CPP32) cleaves MEKK1 after residue D68 both in vivo and in vitro. Cl
eavage of MEKK1 after D68 is blocked by viral and chemical protease in
hibitors, Cleavage of MEKK1 at D68 changes the intracellular distribut
ion of the protein from a Triton insoluble compartment to a Triton-sol
uble compartment, reflected in a redistribution from a particulate to
a diffuse cytoplasmic staining seen by immunofluorescence. Activation
of both SAPK and MEKK1 after Fas ligation is prevented by both viral a
nd chemical caspase 3 inhibitors, which in contrast fail to block acti
vation of SAPK by rapidly acting cell stresses, Stress factor-induced
SAPK signaling is not dependent on caspase 3 function. We propose that
two mechanisms of stress signaling through MEKK1 exist. One is rapid,
independent of proteases, and occurs in the particulate Triton-insolu
ble compartment. The other is more slowly activated and involves liber
ation of particulate MEKK1 by proteolytic cleavage and activation by c
aspase 3.