Spatiotemporal regulation of MKK4 dictates switch-like JNK activation and binary cell-fate decisions.

The JNK pathway converts graded (analogue) stress stimuli into a switch-like (digital) response, thereby dictating binary, all-or-none cell-fate decisions such as survival or death. However, the underlying mechanism remains unclear. Here, we report that the stress-induced modulation of MKK4 spatiotemporal dynamics serves as an analogue-to-digital converter for JNK signalling. Under steady-state conditions, MKK4 shuttles slowly between the nucleus and cytoplasm, but its shuttling rate increases markedly under stress via JNK-mediated feedback regulation. Experimental and mathematical analyses reveal that the increased shuttling rate, coupled with the predominant nuclear localisation of MKK4, cooperatively generates a switch-like JNK activation in response to graded stress stimuli. Disruption of this mechanism provokes graded JNK activity proportional to stress intensity, thereby aberrantly triggering apoptosis, pro-inflammatory cytokine production, and developmental abnormalities, even under mild stress in human cells and zebrafish embryos. Our findings reveal the fundamental molecular mechanism that maintains biological homeostasis under fluctuating environmental conditions.