Figure 4.

Proposed effects for the action of scaffolds. (A) Schematic for high and low phosphatase activity with and without scaffold. (i) When there is low phosphatase activity and no scaffold present, the kinases can readily activate each other and the signal spreads exponentially along the cascade. (ii) When kinases are bound to scaffold under low phosphatase activity, each kinase is localized so it is only likely to interact with their neighboring substrate kinase and signal amplification is attenuated. (iii) When the level of phosphatase activity is high and there is no scaffold for kinases to complex with, the propogation of the signal is impeded as the kinase will likely encounter a phosphatase and become deactivated before it meets its downstream target. (iv) Under conditions of high phosphatase activity, the scaffold complex facilitates signal amplification due to the enhanced local concentration of kinases. This is because the likelihood for a kinase to successfully interact with its substrate kinase is much higher than for the encounter with a deactivating phosphatase which is not localized to the scaffold. Gray squares represent phosphatases. (B) Trans activation of incomplete scaffold complexes. Kinases bound to different partially occupied scaffolds may activate each other in trans.