MEK Partner 1 (MP1) was initially identified as a scaffold protein that tethers MEK1 and ERK1 to facilitate their activation. MP1 and other MAPK scaffolds promote phosphorylation efficiency and specificity, localize their bound pathway components to particular subcellular sites, and serve as nodes of signal integration for regulation of MAPK pathways by other signaling events. With the assistance of adaptor protein p14, the bound MP1 complex localizes to late endosomes and is thought to regulate endosomal traffic and cellular proliferation during tissue homeostasis.