The first signaling scaffold protein discovered was the Ste5 protein from the yeastSaccharomyces cerevisiae. Three distinct domains of Ste5 were shown to associate with the protein kinasesSte11, Ste7, and Fus3 to form a multikinase complex.
Function
Scaffold proteins act in at least four ways: tethering signaling components, localizing these components to specific areas of the cell, regulating signal transduction by coordinating positive and negative feedback signals, and insulating correct signaling proteins from competing proteins.
Tethering signaling components
This particular function is considered a scaffold's most basic function. Scaffolds assemble signaling components of a cascade into complexes. This assembly may be able to enhance signaling specificity by preventing unnecessary interactions between signaling proteins, and enhance signaling efficiency by increasing the proximity and effective concentration of components in the scaffold complex. A common example of how scaffolds enhance specificity is a scaffold that binds a protein kinase and its substrate, thereby ensuring specific kinase phosphorylation. Additionally, some signaling proteins require multiple interactions for activation and scaffold tethering may be able to convert these interactions into one interaction that results in multiple modifications. Scaffolds may also be catalytic as interaction with signaling proteins may result in allosteric changes of these signaling components. Such changes may be able to enhance or inhibit the activation of these signaling proteins. An example is the Ste5 scaffold in the mitogen-activated protein kinase pathway. Ste5 has been proposed to direct mating signaling through the Fus3 MAPK by catalytically unlocking this particular kinase for activation by its MAPKK Ste7.
Localization of signaling components in the cell
Scaffolds localize the signaling reaction to a specific area in the cell, a process that could be important for the local production of signaling intermediates. A particular example of this process is the scaffold, A-kinase anchor proteins, which target cyclic AMP-dependent protein kinase to various sites in the cell. This localization is able to locally regulate PKA and results in the local phosphorylation by PKA of its substrates.
Many hypotheses about how scaffolds coordinate positive and negative feedback come from engineered scaffolds and mathematical modeling. In three-kinase signaling cascades, scaffolds bind all three kinases, enhancing kinase specificity and restricting signal amplification by limiting kinase phosphorylation to only one downstream target. These abilities may be related to stability of the interaction between the scaffold and the kinases, the basal phosphataseactivity in the cell, scaffold location, and expression levels of the signaling components.
Insulating correct signaling proteins from inactivation
Signaling pathways are often inactivated by enzymes that reverse the activation state and/or induce the degradation of signaling components. Scaffolds have been proposed to protect activated signaling molecules from inactivation and/or degradation. Mathematical modeling has shown that kinases in a cascade without scaffolds have a higher probability of being dephosphorylated by phosphatases before they are even able to phosphorylate downstream targets. Furthermore, scaffolds have been shown to insulate kinases from substrate- and ATP-competitive inhibitors.
Scaffold protein summary
Huntingtin protein
protein co-localizes with ATM repair protein at sites of DNA damage. Huntingtin is a scaffolding protein in the ATM oxidative DNA damage response complex. Huntington’s disease patients with aberrant huntingtin protein are deficient in repair of oxidative DNA damage. Oxidative DNA damage appears to underlie Huntington’s disease pathogenesis. Huntington’s disease is likely caused by the dysfunction of mutant huntingtin scaffold protein in DNA repair leading to increased oxidative DNA damage in metabolically active cells.
