Figure 1.

Comparisons of the mammalian polo-like kinases (PLKs). A, Mammalian PLKs have conserved kinase domains (purple rectangles) and polo box domains (PBD, blue/green squares). PLK4 is the most structurally divergent member of this family, as it contains only one PBD whose sequence is not fully homologous to the other PBDs, and one cryptic polo box (pink half-ovals). In humans, PLK5 contains an in-frame stop codon followed almost immediately by an in-frame start codon, resulting in a truncated kinase domain (purple rectangle). In all other mammals, the full PLK5 gene is encoded (light grey rectangle and line), although it shows no kinase activity. B, Phylogenetic analysis of PLK sequences show PLK4 as the most divergent, as determined using the COBALT Phylogenetic Tree widget (76). C, Alignment of PLK protein sequences using COBALT tool. Red areas are highly conserved, while blue areas are less conserved. Phylogenetic tree and alignment made using NCBI’s Constraint-based Multiple Alignment Tool and the following sequence IDs: NP_001177728.1, NP_001230008.1, NP_005021.2, NP_004064.2, and NP_001239155.1 (76).