Protein phosphatase 2A (PP2A) is a major Ser/Thr phosphatase; it forms diverse heterotrimeric holoenzymes that counteract kinase actions. Using a peptidome that tiles the disordered regions of the human proteome， we identified proteins containing [LMFI]xx[ILV]xEx motifs that serve as interaction sites for B'-family PP2A regulatory subunits and holoenzymes. The B'-binding motifs have important roles in substrate recognition and in competitive inhibition of substrate binding. With more than 100 novel ligands identified， we confirmed that the recently identified LxxIxEx B'α-binding motifs serve as common binding sites for B' subunits with minor variations， and that S/T phosphorylation or D/E residues at pos... More
Protein phosphatase 2A (PP2A) is a major Ser/Thr phosphatase; it forms diverse heterotrimeric holoenzymes that counteract kinase actions. Using a peptidome that tiles the disordered regions of the human proteome， we identified proteins containing [LMFI]xx[ILV]xEx motifs that serve as interaction sites for B'-family PP2A regulatory subunits and holoenzymes. The B'-binding motifs have important roles in substrate recognition and in competitive inhibition of substrate binding. With more than 100 novel ligands identified， we confirmed that the recently identified LxxIxEx B'α-binding motifs serve as common binding sites for B' subunits with minor variations， and that S/T phosphorylation or D/E residues at positions 2， 7， 8 and 9 of the motifs reinforce interactions. Hundreds of proteins in the human proteome harbor intrinsic or phosphorylation-responsive B'-interaction motifs， and localize at distinct cellular organelles， such as midbody， predicting kinase-facilitated recruitment of PP2A-B' holoenzymes for tight spatiotemporal control of phosphorylation at mitosis and cytokinesis. Moroever， Polo-like kinase 1-mediated phosphorylation of Cyk4/RACGAP1， a centralspindlin component at the midbody， facilitates binding of both RhoA guanine nucleotide exchange factor (epithelial cell transforming sequence 2 (Ect2)) and PP2A-B' that in turn dephosphorylates Cyk4 and disrupts Ect2 binding. This feedback signaling loop precisely controls RhoA activation and specifies a restricted region for cleavage furrow ingression. Our results provide a framework for further investigation of diverse signaling circuits formed by PP2A-B' holoenzymes in various cellular processes.
