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Cell cycle progression from G2 phase into mitosis is regulated by a complex network of mechanisms, all of which finally control the timing of Cyclin B/CDK1 activation. PLK1 regulates a network of events that contribute to regulating G2/M phase progression. Here we have used a proteomics approach to identify proteins that specifically bind to the Polobox domain of PLK1. This identified a panel of proteins that were either associated with PLK1 in G2 phase and/or mitosis, the strongest interaction being with the MAPK scaffold protein JIP4. PLK1 binding to JIP4 was found in G2 phase and mitosis, and PLK1 binding was self-primed by PLK1 phosphorylation of JIP4. PLK1 binding is required for JIP4-dependent p38MAPK activation in G2 phase during normal cell cycle progression, but not in either G2 phase or mitotic stress response. Finally, JIP4 is a target for caspase-dependent cleavage in mitotically arrested cells. The role for the PLK1–JIP4 regulated p38MAPK activation in G2 phase is unclear, but it does not affect either progression into or through mitosis. [ABSTRACT FROM AUTHOR]