POLO-Box Protein Domain

The PLK1 PBD bound to phospho-threonine peptide (red).

Domain Binding and Function

The POLO-Box domain is exclusively found in the family of Polo-like kinases that regulate mitotic entry, spindle assembly, centrosome maturation, chromosome segregation, cytokinesis and cell cycle arrest. The approximately 200 amino acid POLO-Box domain consists of two highly conserved Polo Box motifs of ~70-80 amino acids each, and is positioned C-terminal to an N-terminal Ser/Thr kinase domain. The POLO-Box domain performs dual roles in determining subcellular localization and autoinhibitory regulation of the kinase domain. The POLO-Box module consists of two POLO-Box motifs, the region between them, and a portion of the linker between the end of the kinase domain and the first POLO-Box. This module recognizes phosphopeptides with the core consensus motif Ser-pThr/pSer-Pro/X. Furthermore, PLK1 is overexpressed in a broad range of human tumors suggesting a role in carcinogenesis.

Structure

The POLO-Box domain consists of two β₆α motifs that comprise of two POLO-Box regions (PB1 and 2). The two POLO-Box motifs form a 12-stranded β sandwich flanked by three α-helical segments. Though both POLO-Boxes comprise of a six-stranded antiparallel β-sheet shielded by one α-helix, the two POLO-Boxes exhibit only 12% sequence identity. The phosphopeptide interacts with β1 from PB1, the N-terminal end of L2 and β8/9 from PB2. The only residues that interact with the phosphate group are His-538 and Lys-540 from PB2. These phosphopeptide interacting residues are highly conserved in the POLO like kinases, suggesting a conserved ability to bind phosphopeptide motifs across all family members.

Structure Reference

1. Cheng, K.Y. et al. (2003) EMBO 22(21), 5757–5768.
2. Elia, A. et al. (2003) Cell 115(1), 83–95.

Examples of Domain Proteins

Binding Examples

Consensus Binding Sites

[Pro/Phe]-[φ/Pro]-[φ/Ala/Gln]-[Thr/Gln/His/Met]-Ser-[pThr/pSer]-[Pro/X]