MAPKSP1 is a scaffold protein which operates in the extracellular signal-regulated kinase (ERK) cascade. MAPKSP1 protein is restricted to late endosomes by the mitogen-activated protein-binding protein-interacting protein, and binds specifically to MAP kinase kinase MAP2K1/MEK1, MAP kinase MAPK3/ERK1, and MAP kinase MAPK1/ERK2.