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Table 1 PLK4 phosphorylation motifs

From: Polo-like kinase 4: the odd one out of the family

  -3 -2 -1 S/T 1 2 3 4
Leung et al. (2007) - Charged I, L and V unfavoured   Hydrophobic (large) Hydrophobic (large) X Charged or P
Johnson et al. (2007) R/K E/D X   Hydrophobic/Y Hydrophobic/Y X S/T/A
Sillibourne et al. (2010) Aliphatic, hydrophobic or basic (small to medium) X Large residues unfavoured   Aliphatic (charged residues unfavoured) Aromatic or aliphatic (large) X -
  1. Three different groups have derived a PLK4 phosphorylation motif, while differences exist there are some common elements in all three. PLK4 has a preference for small to medium aliphatic or basic residues at the -3 position, aliphatic or charged residues at the +1 position and aromatic or large hydrophobic residues at the +2 position. The studies of Leung et al (2007) and Sillibourne et al (2010) show that large amino acids, in particular I, L and V, are unfavoured at the -1 position. There is a preference for a charged residue at the -2 position which is influenced by the residue in the +4 position and those outside of the motif. The interdependency between residues in the -2 and +4 positions, coupled with the influence of residues surrounding the phosphorylation motif, indicates that PLK4 is a context dependent kinase and renders the prediction of phosphorylation sites more difficult. Supporting this, it has been shown that a large number of predicted sites present in candidate PLK4 substrates, which fit the consensus phosphorylation motif well, are not phosphorylated by the kinase. All in all, this means that the identification of PLK4 substrates will remain a significant challenge for the future.