A subgroup of serine/threonine protein kinases (see <PDOC00100>) playing multiple role during cell cycle, especially in M phase progression and cytokinesis, contain a duplicated domain in their C-terminal part, the POLO box [1]. It is named after its founding member encoded by the polo gene of Drosophila [2]. This domain of around 70 amino acids has been found in species ranging from yeast to mammals.

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 organize cytokinetic structures [3].

Some of the proteins that contain a POLO box are listed below:

• Mammalian serine/threonine-protein kinase SNK. May play a role in the division of cell types, such as fibroblasts and could function in embryogenesis.
• Mammmalian serine/threonine-protein kinase PLK. PLK is required for cell division with a role during G1 or S phase.
• Drosophila protein kinase Polo. May play a role in regulating both nuclear and cytoplasmic aspects of the mitotic cycle.
• Fission yeast serine/threonine-protein kinase PLO1. Required to form a bipolar spindle, the actin ring and septum.
• Yeast cell cycle protein kinase CDC5/MSD2. Protein kinase required for cell cycle progression.