Figure 2. PP2A^Cdc55 regulates Mob1 phosphorylation.

(A) MS² spectrum of peptide MpSPVLTpTPK, which includes the dually phosphorylated Ser and Thr residues. Peptide sequence and assigned fragment ions y (in blue) and b (in yellow) are indicated (pRS site probabilities = 98.8%; determined by Proteome Discoverer Software v 1.3). (B) Cdc55 counteracts Mob1 phosphorylation. Strains Y1109 (MATa MET-CDC20 CDC14-Pk₉ eGFP-MOB1) and Y1110 (as Y1109, but cdc55Δ) were released into a synchronous anaphase by Cdc20 depletion and reintroduction. Mob1 phosphorylation was analyzed by western blot. (C) Cdc55 and Mob1 interact. Co-immunoprecipitation of Cdc55 and Mob1 was analyzed in protein extracts from Y1106 (MATa Pk₃-CDC55 MOB1-HA₆). Protein extracts from strain Y1104 (MATa MOB1-HA₆) lacking a Pk epitope on Cdc55 served as a control.