Fig. 3.

Substrate motif discovery. We extracted phospho-serine motifs from mitotically regulated peptides using Motif-X (9). Only sites with Ascore ≥ 13 and median abundance ratio (L:H) ≥ 4 (n = 2,949) were included. (A) More than half of these sites, 1,670, lie in pS-P motifs. (B and C) We identified two motifs similar to those for the mitotic kinases (B) Aurora kinase A and (C) Polo-like kinase 1, whose consensus substrate sequences are [K/N/R]-R-X-[pS/pT]-Φ and [D/E]-X-[pS/pT]-Φ-X-[D/E], respectively, where Φ denotes any hydrophobic residue. Note the bias for leucine in the +1 position for both. (D and E) We also found two notable motifs that included a basic residue at +3 but that lacked the +1 proline.