ERK Signals: Scaffolding Scaffolds?

Abstract

ERK1/2 MAP Kinases become activated in response to multiple intra- and extra-cellular stimuli through a signaling module composed of sequential tiers of cytoplasmic kinases. Scaffold proteins regulate ERK signals by connecting the different components of the module into a multi-enzymatic complex by which signal amplitude and duration are fine-tuned, and also provide signal fidelity by isolating this complex from external interferences. In addition, scaffold proteins play a central role as spatial regulators of ERKs signals. In this respect, depending on the subcellular localization from which the activating signals emanate, defined scaffolds specify which substrates are amenable to be phosphorylated. Recent evidence has unveiled direct interactions among different scaffold protein species. These scaffold-scaffold macro-complexes could constitute an additional level of regulation for ERK signals and may serve as nodes for the integration of incoming signals and the subsequent diversification of the outgoing signals with respect to substrate engagement.

Keywords: ERK; MAP kinase; protein-protein interactions; scaffold protein; signaling pathways.

Figure 1

Allosteric regulation of KSR2. A regulatory RAF interacts with KSR in cis to induce a conformational switch on MEK to expose its activation loop, subject to phosphorylation by RAF in trans. In the KSR2–MEK1 hetero-tetramer (left), the inaccessible activation segment of MEK1 is released through the interaction of KSR2 with RAF, induced by a conformational change, allowing a “catalytic” RAF to phosphorylate MEK (right).

Figure 2

Scaffolding promotes signal amplification in the presence of phosphatases. High phosphatase activity in the absence (A) or presence (B) of scaffold proteins. When there are no scaffolds, the signal will be strongly down-regulated by phosphatases. Scaffold proteins enhance the local concentration of kinases and shields them from dephosphorylation, facilitating signaling.

Figure 3

Scaffold proteins as spatial regulators of ERK signaling. In response to stimulation, phosphorylated ERK monomers detach from MEK and may follow three destinies: (1) translocate as monomers to the nucleus; (2) dimerize freely in the cytoplasm, and (3) specific scaffolds act as dimerization platforms in a sublocalization–specific fashion, where ERK dimers are assembled and the new complexes can interact with different cytoplasmic pools of substrates.

Figure 4

Hypothetical model showing how alterations on scaffolds levels can impact on the functions of other scaffolds. (A) Overexpression of the red scaffold attenuates signals from itself and from the blue scaffold, that competes for the same pools of kinases. (B) Depletion of the red scaffold attenuates its own signals but promotes signaling by the blue scaffold as a consequence of the increment on available kinases that increase the number of complete blue scaffold complexes.

Figure 5

Scaffold-Scaffold interactions as nodes for signal integration. (A) Working independently, defined scaffold proteins respond to specific stimuli and convey signals to a limited number of ERK substrates. (B) Scaffold complexes composed of two (or more) scaffold proteins, where trans-phosphorilation among the different kinase tiers would be feasible, would facilitate signal integration, serving as nodes for various incoming signals and for the diversification of outgoing signals with respect to the number of substrates.

Figure 6

Scaffold-Scaffold interactions may compensate deficiencies in kinases, facilitating signaling. Provided that trans-phosphorylation was possible, complexes formed by two (or more) partially occupied scaffolds would be able to complement each other's kinase deficiencies, so incomplete scaffold complexes, apparently impaired for supporting efficient signaling, would be capable of improving the flux of signals.