The study investigates the role of mitogen-activated protein (MAP) kinase (ERK1 and ERK2) in regulating cell motility. It demonstrates that MAP kinase activation is essential for cell migration on extracellular matrix proteins, such as collagen, by phosphorylating myosin light chain kinase (MLCK), which in turn enhances the phosphorylation of myosin light chains (MLC). Inhibition of MAP kinase activity reduces MLCK function and MLC phosphorylation, leading to decreased cell migration. Conversely, expression of mutationally active MAP kinase increases MLCK and MLC phosphorylation, enhancing cell migration. In vitro experiments show that ERK1 and ERK2 can directly phosphorylate MLCK, increasing its activity and sensitivity to calmodulin. This study defines a signaling pathway initiated by integrin or growth factor receptor ligation, leading to increased myosin phosphorylation and cell migration. The findings suggest that MAP kinase activation can directly impact the cell's motility machinery, independent of gene transcriptional events.The study investigates the role of mitogen-activated protein (MAP) kinase (ERK1 and ERK2) in regulating cell motility. It demonstrates that MAP kinase activation is essential for cell migration on extracellular matrix proteins, such as collagen, by phosphorylating myosin light chain kinase (MLCK), which in turn enhances the phosphorylation of myosin light chains (MLC). Inhibition of MAP kinase activity reduces MLCK function and MLC phosphorylation, leading to decreased cell migration. Conversely, expression of mutationally active MAP kinase increases MLCK and MLC phosphorylation, enhancing cell migration. In vitro experiments show that ERK1 and ERK2 can directly phosphorylate MLCK, increasing its activity and sensitivity to calmodulin. This study defines a signaling pathway initiated by integrin or growth factor receptor ligation, leading to increased myosin phosphorylation and cell migration. The findings suggest that MAP kinase activation can directly impact the cell's motility machinery, independent of gene transcriptional events.