Abstract: Modified MEK1 peptides and modified MEK2 peptides, polynucleotides encoding those peptides, and methods for purifying the peptides and crystallizing them as peptide: cofactor: ligand complexes have been discovered. The three-dimensional structures of MEK1 peptide and MEK2 peptide, including the cofactor- and ligand-binding pockets, and uses of this information, for example, in molecular replacement and the modification, design and screening of compounds that may associate with MEK1, MEK2, or peptides structurally related thereto, have also been discovered.