<p>A subgroup of serine/threonine protein kinases, Polo or Polo-like kinases play multiple roles during the cell cycle. Polo kinases are required at several key pointsthrough mitosis, starting from control of the G2/M transition through phosphorylation of Cdc25C and mitotic cyclins. Polo kinases are characterised by an amino terminal catalytic domain, and a carboxy terminal non-catalytic domain consisting of three blocks of conservedsequences known as polo boxes which form one single functional domain [<cite idref="PUB00006187"/>]. The domain is named after its founding member encoded by the polo gene of <taxon tax_id="7227">Drosophila melanogaster</taxon> [<cite idref="PUB00006188"/>]. This domain of around 70 amino acids has been found in species ranging from yeast to mammals. Polo boxes appear to mediate interaction with multiple proteins through protein:protein interactions; some but not all of these proteins are substrates for the kinase domain of the molecule [<cite idref="PUB00010650"/>].</p><p> The crystal structure of the polo domain of the murine protein, Sak, is dimeric, consisting of two alpha-helices and two six-stranded beta-sheets [<cite idref="PUB00010649"/>]. The topology of one polypeptide subunit of the dimer consists of, from its N- to C terminus, an extended strand segment, five beta-strands, one alpha-helix (A) and a C-terminal beta-strand. Beta-strands from one subunit form a contiguous antiparallel beta-sheet with beta-strands from the second subunit. The two beta-sheets pack with a crossing angle of 110 degrees, orienting the hydrophobic surfaces inward and the hydrophilic surfaces outward. Helix A, which is colinear with beta-strand 6 of the same polypeptide, buries a large portion of the non-overlapping hydrophobic beta-sheet surfaces. Interactions involving helices A comprise a majority of the hydrophobic core structure and also the dimer interface.</p><p>Point mutations in the Polo box of the budding yeast Cdc5 protein abolish the ability of overexpressed Cdc5 to interact with the spindle poles and to organise cytokinetic structures [<cite idref="PUB00006189"/>].</p> POLO box duplicated domain