Integrating signals from RTKs to ERK/MAPK.

Signals received at the cell surface must be properly transmitted to critical targets within the cell to achieve the appropriate biological response. This process of signal transduction is often initiated by receptor tyrosine kinases (RTKs), which function as entry points for many extracellular cues and play a critical role in recruiting the intracellular signaling cascades that orchestrate a particular response. Essential for most RTK-mediated signaling is the engagement and activation of the mitogen-activated protein kinase (MAPK) cascade comprised of the Raf, MEK and extracellular signal-regulated kinase (ERK) kinases. For many years, it was thought that signaling from RTKs to ERK occurred only at the plasma membrane and was mediated by a simple, linear Ras-dependent pathway. However, the limitation of this model became apparent with the discovery that Ras and ERK can be activated at various intracellular compartments, and that RTKs can modulate Ras/ERK signaling from these sites. Moreover, ERK scaffolding proteins and signaling modulators have been identified that play critical roles in determining the strength, duration and location of RTK-mediated ERK signaling. Together, these factors contribute to the diversity of biological responses generated by RTK signaling.