Nonvisual Arrestins Function as Simple Scaffolds Assembling the MKK4-JNK3α2 Signaling Complex

Arrestins make up a small family of proteins with four mammalian members that play key roles in the regulation of multiple G protein-coupled receptor-dependent and -independent signaling pathways. Although arrestins were reported to serve as scaffolds for MAP kinase cascades, promoting the activation of JNK3, ERK1/2, and p38, the molecular mechanisms involved were not elucidated, and even the direct binding of arrestins with MAP kinases was never demonstrated. Here, using purified proteins, we show that both nonvisual arrestins directly bind JNK3 alpha 2 and its upstream activator MKK4, and that the affinity of arrestin-3 for these kinases is higher than that of arrestin-2. Reconstitution of the MKK4-JNK3 alpha 2 signaling module from pure proteins in the presence of different arrestin-3 concentrations showed that arrestin-3 acts as a "true" scaffold, facilitating JNK3 alpha 2 phosphorylation by bringing the two kinases together. Both the level of JNK3 alpha 2 phosphorylation by MKK4 and JNK3 alpha 2 activity toward its substrate ATF2 increase at low and then decrease at high arrestin-3 levels, yielding a bell-shaped concentration dependence expected with true scaffolds that do not activate the upstream kinase or its substrate. Thus, direct binding of both kinases and true scaffolding is the molecular mechanism of action of arrestin-3 on the MKK4-JNK3 alpha 2 signaling module.