DOC_PP2A_B56_1

Protein phosphatase 2A (PP2A) belongs to the superfamily of phosphoprotein phosphatases (PPPs) and catalyzes protein dephosphorylation by hydrolyzing Ser/Thr-linked phosphate ester bonds (Heroes,2013). A heterotrimeric enzyme, PP2A contains a 36 kDa catalytic C subunit, a 65 kDa structural A subunit, and a variable regulatory B subunit. The largest family of cellular regulatory B subunits is B56 (McCright,1996), which contains five isoforms (B56 alpha to epsilon). Unlike some other protein phosphatases, which make use of their catalytic subunits to obtain substrate specificity, PP2A uses the regulatory B subunits for targeting substrates. The B56 interaction is mediated by a conserved motif present in the substrate proteins and a groove formed between B56 HEAT repeats 3 and 4. It is a general motif for B56 interaction and the key interacting residues in B56 are conserved in different B56 isoforms and are expected to have the same mode of interaction (Hertz,2016).
The composition of the motif modulates the affinity for B56, which can in turn determine the phosphorylation status of associated substrates. Phosphorylation of amino acid residues within the motif may strengthen B56 binding, allowing integration of kinase and phosphatase activity. In Repo-Man, phosphorylation of S591 within the B56 docking site promotes the binding of B56 and subsequently aids the dephosphorylation of another phosphorylated residue, S893, within the same protein (Qian,2013). This motif was first identified in BubR1 as a site essential for kinetochore-microtubule attachment and was known as the KARD (Kinetochore Attachment Regulatory Domain) motif, which also binds to the PP2A-B56 complex (Suijkerbuijk,2012). The KARD motif in human BubR1 contains three phosphorylated residues (S670, S676 and T680) which stimulates the interaction of the B56 subunit and BubR1. The binding of BubR1 promotes the localisation of PP2A-B56 to kinetochores where it acts to dephosphorylate the Aurora B substrates and noteworthy does not appear to dephosphorylate the BubR1 S670/676 sites or any nearby phosphorylated sites, as the dephosphorylation of these sites would prevent PP2A from reaching the kinetochore. So binding of a phosphatase to a protein does not always guarantee that it is a substrate of the phosphatase. Thus the B56 subunit also regulates the localization of the PP2A holoenzyme to specific intracellular compartments where the substrates themselves may not contain LxxIxE motifs (Wang,2016; Suijkerbuijk,2012).
The B56 binding LxxIxE motifs have been identified in many important proteins from virus to human but how they affect the biological process are known for only a few examples. In RACGAP1, the mutation of the motif prevented its dephosphorylation and results in cytokinesis defects. For FOXO3, mutation of the motif affects the binding affinity to B56 and altered its localization (7453045). In a viral example, B56 can bind to the integrase and promote δ-retroviral infection (Maertens,2016). Thus the study of the B56 binding motifs may pave the way to discover new PP2A–B56 substrates, with therapeutic potential.