Figure 7.

Comparison of kinase specificities for BRI1, BAK1, and PEPR1 with lactose operon repressor (lacI) or 30S ribosomal protein S2 (rpsB) as substrate. (A) Abundance of specific phosphopeptides of lacI and (B) ribosomal protein rpsB that were transphosphorylated by the indicated protein kinases expressed in E. coli cells. The relative abundance of each phosphopeptide species was based on the spectral counts in the mass spectrometry identification, and reflects the sum of four independent experiments involving two TiO₂ and two IMAC (Fe²⁺) enrichment steps. Note that there was no evidence for phosphorylation of lacI by PEPR1, or rpsB by PEPR1 or CDPKβ, and that phosphopeptides are identified using the single letter abbreviations for Ser (S) and Thr (T).