Eukaryotic cells use the extracellular signal regulated kinase (ERK) cascade to 
connect cell-surface receptors to intracellular targets. Although various 
signals are routed through the ERK pathway, cells respond accordingly to a given 
stimulus. To regulate proper signal transduction, scaffolds and adaptors are 
employed to organize specific signaling units. The scaffold protein MP1 (MEK1 
partner) assembles a scaffold complex in the ERK cascade. We show that p14 
functions as an adaptor protein, which is required and sufficient to localize 
MP1 to endosomes. Reduction of MP1 or p14 protein levels by siRNAi results in 
defective signal transduction. Therefore, our results suggest that the endosomal 
localization of the p14/MP1-MAPK scaffold complex is crucial for signal 
transduction.