Mitogen-Activated Protein Kinase Scaffold Protein 1 (MAPKSP1) was identified as an interacting protein that belongs to the LAMTOR3 family. MAPKSP1 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. MAPKSP1 interacts with MAP2K1/MEK1 and MAPK2 and enhances the activation of MAPK2, and thus is thought to function as an adaptor to enhance the efficiency of the MAP kinase cascade.