Updated publication reference for PubMed record(s): 29547722. CDKs and MAPKs phosphorylate similar sites yet generally have distinct functions/substrates. We report here an unanticipated system of cooperative regulation by CDK and MAPK, involving collaborative multi-site phosphorylation of a single substrate. The budding yeast protein Ste5 is a signaling scaffold for the pheromone-activated G1 arrest pathway. Upon cell cycle entry, CDK activity inhibits Ste5 via phosphorylation at numerous sites flanking its membrane-binding domain. Ste5 is also regulated by negative feedback from the pathway MAPK, though the mechanism was unknown. Using quantitative time-lapse microscopy, we examined Ste5 membrane recruitment dynamics at different cell cycle stages. Surprisingly, at stages where we expected Ste5 recruitment would be blocked, we observed transient recruitment followed by a steep decline, depending on both CDK and MAPK activities. The collective results of mutagenesis, mass spectrometry, and electrophoretic analyses suggest that the CDK and MAPK target shared sites, and that the substrate is most extensively phosphorylated when both kinases are active and able to bind their respective docking sites on Ste5. This collaborative phosphorylation can diversify regulatory options, ranging from mild tuning to strong blocking, and can yield distinct patterns of regulatory dynamics at different cell cycle stages.