The structural characteristics of EMS1 (human cortactin) suggest that it may link signaling events to reorganization of the actin cytoskeleton. Interestingly, the EMS1 gene is commonly amplified and overexpressed in several human cancers, which may alter their invasive or metastatic properties. An 80 to 85-kDa mobility shift of EMS1 correlates with an alteration in subcellular distribution and is likely to represent an important regulatory event. In HEK 293 cells, epidermal growth factor treatment or cell detachment induced this shift, and this was blocked by the mitogen-activated protein/extracellular signal-regulated kinase kinase (MEK) inhibitor PD98059. Furthermore, expression of a constitutively active form of MEK induced the shift, indicating that MEK activation was both sufficient and necessary for this modification. The epidermal growth factor-induced shift correlated with increased phosphorylation on serine and threonine residues of the same tryptic phosphopeptides detected under basal conditions. Deletion of the helical-proline-rich region of the protein blocked the mobility shift and EMS1 phosphorylation. In vitro kinase assays demonstrated that the extracellular signal-regulated kinases represent candidate kinases for this region, although other MEK-regulated enzymes must also participate. These data identify MEK as an important intermediate involved in EMS1 phosphorylation and highlight the helical-proline-rich region as a key regulatory domain.