Mitogen-activated protein (MAP) kinases play central roles in transmitting extracellular and intracellular information in a wide variety of situations in eukaryotic cells. Their activities are perturbed in a large number of diseases, and their activating kinases are currently therapeutic targets in cancer. MAPKs are highly conserved among all eukaryotes. MAPKs were first cloned from the yeastSaccharomyces cerevisiae. Yeast has five MAPKs and one MAPK-like kinase. The mating MAPK Fus3 is the best characterized yeast MAPK. Members of all subfamilies of human MAPKs can functionally substituteS. cerevisiaeMAPKs, providing systems to use genetic approaches to study the functions of either yeast or human MAPKs and to identify functionally relevant amino acid residues that enhance or reduce the effects of therapeutically relevant inhibitors and regulatory proteins. Here, we describe an assay to measure Fus3 activity in immune complexes prepared fromS. cerevisiaeextracts. The assay conditions are applicable to other MAPKs, as well.
