A growing body of evidence suggests that polo kinase plays a critical role for normal mitotic entry and exit in eucaryotic organisms. Constitutive expression of mammalian polo kinase Plk1 can induce oncogenic focus formation in NIH3T3 cells and tumors in nude mice, suggesting that uncontrolled mitotic activity leads to the development of cancers in humans. Since inhibition of Plk1 in cultured mammalian cells leads to a mitotic catastrophe and cell death, developing anti-polo agents may likely provide means to control highly proliferative cancer cells. We found that PBIP1 is critical for proper localization of Plk1 to the kinetochores at an early stage of the cell cycle before it is degraded by the Plk1-dependent phosphorylation. In vitro biochemical analyses revealed that Plk1 phosphorylates PBIP1 at multiple sites. Co-immunoprecipitation studies revealed that phosphorylation at one of these sites, Thr78, appeared to be critical for the interaction between PBIP1 and the polo-box domain (PBD) of Plk1, suggesting that Plk1 creates a self-docking site at Thr78 for a stable PBD interaction. In line with this observation, provision of phospho-Thr78 peptide (5-mer bearing the Thr78 residue), but not the unphosphorylated corresponding peptide interfered with the PBIP1-Plk1 interaction. Since the polo-box is required for the normal localization and mitotic functions of Plk1, the phospho-Thr78 peptide can be developed as an anti-PBD agent. To test this possibility, we plan to transduce optimized phospho-Thr78 peptides peptides into cultured mammalian cells and examine their effects on the localization of Plk1, Plk1-PBIP1 interaction, and cell growth. To prevent dephosphorylation of the peptides, chemically modified stable phosphate group will be engineered. If the initial studies are promising, then we will examine the effect of optimized peptides in various cancer cells.