Plk1 plays a pivotal role in cell proliferation and is considered an attractive target for anti-cancer therapy. The noncatalytic polo-box domain (PBD) of Plk1 forms a phosphoepitope-binding module for protein-protein interaction. Here, we report the identification of minimal phosphopeptides that specifically interacted with the PBD of Plk1, but not the two closely-related Plk2 and Plk3. Comparative binding studies and analyses of crystal structures of the Plk1 PBD in complex with the minimal phosphopeptides revealed that the C-terminal SpT dipeptide functions as a high affinity anchor, whereas the N-terminal residues are critical for providing both specificity and affinity to the interaction. Inhibition of the Plk1 PBD by phospho-Thr mimetic peptides was sufficient to induce mitotic arrest and apoptotic cell death. Thus, the mode of the minimal peptide and PBD interaction may provide a template for designing anti-Plk1 therapeutic agents.

In the current work, unanticipated synthetic byproducts were obtained arising from alkylation of the δ1 nitrogen (N3) of the histidine imidazole ring of the polo-like kinase-1 (Plk1) polo-box domain (PBD)-binding peptide PLHSpT. For the highest affinity byproduct, bearing a C6H5(CH2)8– group, a Plk1 PBD co-crystal structure revealed a new binding channel that had previously been occluded. An N-terminal PEGylated version of this peptide containing a hydrolytically-stable phosphothreonyl residue (pT) bound to the Plk1 PBD with affinity equal to the non-PEGylated parent, yet it exhibited significantly less interaction with the PBDs of the two closely-related Plk2 or Plk3. Treatment of cultured cells with this PEGylated peptide resulted in Plk1 delocalization from centrosomes and kinetochores, and chromosome misalignment that effectively induced mitotic block and apoptotic cell death. This work provides new insights that may advance efforts to develop Plk1 PBD-binding inhibitors as potential Plk1-specific anticancer therapeutic agents.