PB1 domain-dependent signaling complex is required for extracellular signal-regulated kinase 5 activation.

MEKK2, MEK5, and extracellular signal-regulated kinase 5 (ERK5) are members of a three-kinase cascade for the activation of ERK5. MEK5 is the only MAP2K to express a PB1 domain, and we have shown that it heterodimerizes with the PB1 domain of MEKK2. Here we demonstrate the MEK5 PB1 domain is a scaffold that also binds ERK5, functionally forming a MEKK2-MEK5-ERK5 complex. Reconstitution assays and CFP/YFP imaging (fluorescence resonance energy transfer [FRET]) measuring YFP-MEKK2/CFP-MEK5 and CFP-MEK5/YFP-ERK5 interactions define distinct MEK5 PB1 domain binding sites for MEKK2 and ERK5, with a C-terminal extension of the PB1 domain contributing to ERK5 binding. Stimulus-dependent CFP/YFP FRET in combination with mutational analysis was used to define MEK5 PB1 domain residues critical for the interaction of MEKK2/MEK5 and MEK5/ERK5 required for activation of the ERK5 pathway in living cells. Fusion of the MEK5 PB1 domain to the N terminus of MEK1 confers ERK5 regulation by a MAP2K normally regulating only ERK1/2. The MEK5 PB1 domain confers stringent MAP3K regulation of ERK5 relative to more promiscuous MAP3K control of ERK1/2, JNK, and p38.