FC0109
MAPK kinase MKK4  -  p38α

Biological function
Mitogen-activated protein kinase (MAPK) signaling transmits extracellular proliferative signals into the nucleus. In the kinase cascade, MAPK kinase kinase phosphorylates MAPK kinase, which in turn phosphorylates MAPK. MKK4 can specifically activate both p38 or JNK to initiate cell proliferation, differentiation, migration or cell death. The p38 complex is stimulated by proinflammatory cytokines.

Domain organization/sequence features
Regulatory domain (1-86) recognizes the cognate MAPK, followed by the catalytic domain. The docking motif (40-48 AA) consists of three hydrophobic residues: K/R₂₋₃ −X₁₋₆ −Φ_L− X₁₋₃−Φ_A−X−Φ_B , where corresponds to a hydrophobic residue. Kinase-specificity sequence (KIS, 49-62 AA) flanks the docking motif encodes selectivity for different signaling partners.

Structural evidence
Relaxation dispersion (RD) and chemical exchange saturation transfer (CEST) indicates a two-site exchange model via the canonical motif (40-48) as well as the KIS domain, with nearly identical bound populations of the two regions. Based on R2 data, the KIS domain undergoes nanosecond motions within the complex.

Biochemical evidence
Although the docking site motif is essential for binding of p38α, the KIS domain encodes specificity toward different phosphatases. The KIS domain was shown to enhance affinity and specificity towards hematopoietic tyrosine phosphatase.

Structure/Mechanism
NMR relaxation experiments indicate a dynamic interaction profile for the MKK4:p38α signaling complex. The docking site motif of MKK4 binds at the docking groove of p38α, while the KIS domain makes contacts with a region located at the bottom of the C-lobe of p38α, which is largely composed of surface- exposed hydrophobic residues such as V273, I275, G276, A277, P279, and L280. While the docking site motif adopts a specific, rigid conformation within the complex and anchors the regulatory domain to p38α, the KIS domain experiences fast dynamics in the complex controlled by fluctuating hydrophobic contacts.

Mechanism category
tethering

Significance
Fuzziness enables a bivalent binding mode, and where dynamic interactions regulate signaling specificity.

Submitted by
Malene Ringkjobing Jensen   malene.ringkjobing-jensen@ibs.fr