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Substrate recognition principles for PP2A-B55

ABSTRACT: The trimeric PP2A-B55 phosphoprotein phosphatase regulates a plethora of fundamental signaling pathways throughout eukaryotes. How PP2A-B55 selects its substrates presents a severe knowledge gap in our understanding of phospho-dependent signaling. By integrating AlphaFold modelling with comprehensive experimental high-resolution mutational scanning, we show that alpha-helices in interactors bind a conserved platform on the B55 regulatory subunit. Despite limited sequence similarity, these alpha-helices share key amino acid determinants that engage hydrophobic and electrostatic patches on B55 in a remarkably similar way. We characterize a total of 15 instances in yeast, humans, and viruses, suggesting an evolutionarily conserved mechanism. Using this insight and deep learning protein design, we generate a specific and highly potent competitive peptide inhibitor that binds to B55 with low nanomolar affinity. With this inhibitor, we uncover that PP2A-B55 regulates the nuclear exosome targeting (NEXT) complex by binding to an alpha-helical recruitment module in one of its core components, RBM7. Collectively, our findings provide a framework for the understanding and interrogation of PP2A-B55 in health and disease.

INSTRUMENT(S): Orbitrap Fusion Lumos, Orbitrap Fusion, Q Exactive Plus

ORGANISM(S): Homo Sapiens (ncbitaxon:9606)

SUBMITTER: Arminja Kettenbach

PROVIDER: MSV000094053 | MassIVE | Fri Feb 09 12:17:00 GMT 2024

SECONDARY ACCESSION(S): PXD049307

REPOSITORIES: MassIVE

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Project description:The Ser/Thr protein phosphatase 2A (PP2A) is a highly conserved collection of heterotrimeric holoenzymes responsible for the dephosphorylation of many regulated phosphoproteins. Substrate recognition and the integration of regulatory cues are mediated by B regulatory subunits that are complexed to the catalytic subunit (C) by a scaffold protein (A). PP2A/B55 substrate recruitment was thought to be mediated by charge-charge interactions between the surface of B55a and its substrates. Challenging this view, we recently discovered a conserved SLiM [RK]-V-x-x-[VI]-R in a range of proteins, including substrates such as the retinoblastoma-related protein p107 and TAU (Fowle et al. eLife 2021;10:e63181). Here we report the identification of this SLiM in FAM122A, an inhibitor of B55a/PP2A. This conserved SLiM is necessary for FAM122A binding to B55a in vitro and in cells. Computational structure prediction with AlphaFold-Multimer predicts an interaction consistent with the mutational and biochemical data and supports a mechanism whereby FAM122A uses the 'SLiM' in the form of a short a-helix (helix 1) to dock to the B55a top groove, thereby blocking substrate binding to the same site. In this model, FAM122A also spatially constrains substrate access by occluding the catalytic subunit with a second a-helix immediately adjacent to helix 1. Consistently, FAM122A functions as a competitive inhibitor as it prevents the binding of substrates in in vitro competition assays and the dephosphorylation of CDK substrates by B55a/PP2A in cell lysates. The ablation of FAM122A in human cell lines reduces the rate of proliferation, progression through cell cycle transitions, and abrogates G1/S and intra-S phase cell cycle checkpoints. FAM122A-KO in HEK293 cells results in attenuation of CHK1 and CHK2 activation in response to replication stress. Overall, these data strongly suggest that FAM122A is a short helical motif (SHeM)-dependent, substrate-competitive inhibitor of B55a/PP2A that suppresses multiple functions of B55a in the DNA damage response and in timely progression through the cell cycle interphase.

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Substrate recognition principles for PP2A-B55

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