Chemogenetic profiling reveals PP2A-independent cytotoxicity of proposed PP2A activators iHAP1 and DT-061

Parthenolide disrupts mitosis by inhibiting ZNF207/BUGZ-promoted kinetochore-microtubule attachment

Parthenolide is a natural compound that has shown highly promising anticancer activity. Even though its mode of action has been studied for decades, its antimitotic activity has been largely overlooked, limiting the understanding of its full anticancer potential. In this study, we combined click-chemistry with quantitative mass spectrometry and cell biology to elucidate the mechanism of action of parthenolide in mitosis. We show that parthenolide does not act as a microtubule-targeting agent in cells. Instead, it binds to the kinetochore protein ZNF207/BUGZ, preventing the establishment of proper kinetochore-microtubule attachment. Our results show that parthenolide covalently binds to Cys54 of BUGZ via Michael addition to its α-methylene-γ-lactone moiety. Since Cys54 is located within the second zinc-finger domain of the BUGZ microtubule-targeting region, we propose that parthenolide interferes with the microtubule-binding ability of BUGZ, consequently preventing kinetochore-microtubule attachments required for accurate chromosome congression to the spindle equator.

Understanding the regulatory landscape of protein phosphatase 2A (PP2A): Pharmacological modulators and potential therapeutics

Protein phosphatase 2A (PP2A) is a ubiquitously expressed serine/threonine phosphatase with a diverse and integral role in cellular signalling pathways. Consequently, its dysfunction is frequently observed in disease states such as cancer, inflammation and Alzheimer's disease. A growing understanding of both PP2A and its endogenous regulatory proteins has presented numerous targets for therapeutic intervention. This provides important context for the dynamic control and dysregulation of PP2A function in disease states. Understanding the intricate regulation of PP2A signalling in disease has resulted in the development of novel pharmacological agents aimed at restoring cellular homeostasis. Herein we review the structure and function of PP2A together with pharmacological modulators, both endogenous (proteins) and exogenous (small molecules and peptides), with relevance to targeting PP2A as a future pharmacotherapeutic strategy.

STK39 inhibits antiviral immune response by inhibiting DCAF1-mediated PP2A degradation

Evading host immunity killing is a critical step for virus survival. Inhibiting viral immune escape is crucial for the treatment of viral diseases. Serine/threonine kinase 39 (STK39) was reported to play an essential role in ion homeostasis. However, its potential role and mechanism in viral infection remain unknown. In this study, we found that viral infection promoted STK39 expression. Consequently, overexpressed STK39 inhibited the phosphorylation of interferon regulatory factor 3 (IRF3) and the production of type I interferon, which led to viral replication and immune escape. Genetic ablation or pharmacological inhibition of STK39 significantly protected mice from viral infection. Mechanistically, mass spectrometry and immunoprecipitation assays identified that STK39 interacted with PPP2R1A (a scaffold subunit of protein phosphatase 2A (PP2A)) in a kinase activity-dependent manner. This interaction inhibited DDB1 and CUL4 associated factor 1 (DCAF1)-mediated PPP2R1A degradation, maintained the stabilization and phosphatase activity of PP2A, which, in turn, suppressed the phosphorylation of IRF3, decreased the production of type I interferon, and then strengthened viral replication. Thus, our study provides a novel theoretical basis for viral immune escape, and STK39 may be a potential therapeutic target for viral infectious diseases.

The anti-neuroinflammatory effects of cepharanthine in uric acid-induced neuroinflammation

ETHNOPHARMACOLOGICAL RELEVANCE

Cepharanthine (CEP) is an alkaloid extracted from Stephania cephalantha Hayata, a traditional Chinese medicine (TCM) renowned for its heatclearing and dehumidifying properties. For centuries, Stephania cephalantha Hayata has been employed in the treatment of a wide range of diseases, including pain, edema, inflammation, and fever.

AIM OF THE STUDY

Our research aims to investigate the role and mechanism of Cepharanthine in ameliorating uric acid (UA) induced neuroinflammatory responses.

MATERIALS AND METHODS

The Connectivity Map (CMap) was utilized to identify the therapeutic drug Cepharanthine, based on the proteomic disturbances associated with uric acid (UA). Limited proteolysis small molecule mapping (LiP-SMap) and thermal proteome profiling (TPP) technologies were used to identify the direct target proteins for UA and Cepharanthine. Additionally, we used the induced-fit docking algorithm integrated within the Schrodinger suite to explore the interactions between Cepharanthine and uric acid targets. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) technology was employed to determine the concentration of Cepharanthine in the mice cerebral cortex. The pro-inflammatory cytokine genes were also quantified by qPCR in U251 cells and in hyperuricemic mice.

RESULTS

The findings indicated that uric acid increased the transcription of pro-inflammatory cytokines and the expression levels of proteins linked to inflammation in U251 cells. PPP2R1A was identified as a potential candidate for direct interaction with uric acid, potentially initiating inflammation. Based on the CMap prediction, Cepharanthine was identified as a candidate drug for interaction with PPP2R1A. TPP analysis indicated that Cepharanthine could reduce the thermal stability of PPP2R1A. Molecular docking confirmed that Cepharanthine could directly bind to PPP2R1A. Furthermore, the detection of Cepharanthine in the cerebral cortex suggested its ability to cross the blood-brain barrier. Proteomic analysis of Cepharanthine-treated mice revealed significant enrichments of differentially expressed proteins (DEPs) in inflammation-related pathways and biological processes. Additionally, Cepharanthine was effective in decreasing the expression of pro-inflammatory cytokine genes induced by uric acid in U251 cells and in hyperuricemic mice.

CONCLUSION

Cepharanthine could effectively alleviate hyperuricemia-induced neuroinflammation via binding to PPP2R1A. This study offers a novel approach for prevention and treatment of neurological diseases caused by hyperuricemia.

A functional map of phosphoprotein phosphatase regulation identifies an evolutionary conserved reductase for the catalytic metal ions

Serine/Threonine phosphoprotein phosphatases (PPPs, PP1-PP7) are conserved metalloenzymes and central to intracellular signaling in eukaryotes, but the details of their regulation is poorly understood. To address this, we performed genome-wide CRISPR knockout and focused base editor screens in PPP perturbed conditions to establish a high-resolution functional map of PPP regulation that pinpoints novel regulatory mechanisms. Through this, we identify the orphan reductase CYB5R4 as an evolutionarily conserved activator of PP4 and PP6, but not the closely related PP2A. Heme binding is essential for CYB5R4 function and mechanistically involves the reduction of the metal ions in the active site. Importantly, CYB5R4-mediated activation of PP4 is critical for cell viability when cells are treated with DNA damage-inducing agents known to cause oxidative stress. The discovery of a dedicated PPP reductase points to shared regulatory principles with protein tyrosine phosphatases, where specific enzymes dictate activity by regulating the active site redox state. In sum, our work provides a resource for understanding PPP function and the regulation of intracellular signaling.

LincR-PPP2R5C regulates the PP2A signaling pathway in the macrophage-myofibroblast transition in a mouse model of epidural fibrosis

Low back pain after spine surgery is a major complication due to excessive epidural fibrosis, which compresses the lumbar nerve. Macrophage-myofibroblast transition (MMT) promoted epidural fibrosis in a mouse laminectomy model. Previously, we demonstrated that LincR-PPP2R5C regulated CD4 + T-cell differentiation. Here, we aimed to explore the roles and mechanisms of LincR-PPP2R5C in macrophages in epidural fibrosis. In M2 macrophages, the level of LincR-PPP2R5C was significantly decreased. Upon overexpression, LincR-PPP2R5C induced M1-macrophage polarization and reduced MMT. In contrast, LincR-PPP2R5C deficiency promoted M2-macrophage polarization and increased MMT. Mechanistically, LincR-PPP2R5C modulated the expression of α-SMA in macrophages via the PP2A signaling pathway. In vivo, LincR-PPP2R5C deficiency aggravated epidural fibrosis by enhancing MMT in a mouse model of laminectomy, and this effect was abolished in mice with macrophage depletion. Our study shed light on the effects of LincR-PPP2R5C on macrophage differentiation and MMT in epidural fibrosis.

Research progress of the relationship between phosphoprotein phosphatases (PPPs) and neurodevelopmental disorders

Reversible protein phosphorylation is a ubiquitous phenomenon essential for eukaryotic cellular processes. Recent advancements in research about neurodevelopmental disorders have prompted investigations into the intricate relationship between protein phosphatases, particularly phosphoprotein phosphatases (PPPs), and neurodevelopment. Notably, variants in 10 coding genes spanning four PPP family members have been implicated in neurodevelopmental disorders. Here, we provide a comprehensive overview of the clinical phenotypes, genotypes, and pathogenic mechanisms observed in affected patients. Our analysis reveals challenges in subsequent statistical analyses due to inconsistent clinical phenotypic descriptions and a lack of large multicenter studies, hampering analysis about genotype-phenotype correlations. The scarcity of follow-up data poses a significant obstacle to prognostic counseling for nearly all rare diseases. Presently, symptomatic treatment strategies are employed for patients with variants, as definitive cures remain elusive. Future research may explore protein phosphatase regulators as potential therapeutic targets. Furthermore, it is imperative not to overlook other members of the protein phosphatase family or coding genes with undiscovered variants. Insights gleaned from the temporal and spatial distribution of proteins, along with observations from animal model phenotypes, may provide valuable directions for uncovering novel pathogenic genes.

Identification of a Novel Protein Phosphatase 2A Activator, PPA24, as a Potential Therapeutic for FOLFOX-Resistant Colorectal Cancer

A series of compounds were designed utilizing molecular modeling and fragment-based design based upon the known protein phosphatase 2A (PP2A) activators, NSC49L and iHAP1, and evaluated for their ability to inhibit the viability of colorectal cancer (CRC) and folinic acid, 5-fluorouracil, and oxaliplatin (FOLFOX)-resistant CRC cells. PPA24 (19a) was identified as the most cytotoxic compound with IC50 values in the range of 2.36-6.75 μM in CRC and FOLFOX-resistant CRC cell lines. It stimulated PP2A activity to a greater extent, displayed lower binding energies through molecular docking, and showed higher binding affinity through surface plasmon resonance for PP2A catalytic subunit α than the known PP2A activators. PPA24 dose-dependently induced apoptosis and oxidative stress, decreased the level of c-Myc expression, and synergistically potentiated cytotoxicity when combined with gemcitabine and cisplatin. Furthermore, a PPA24-encapsulated nanoformulation significantly inhibited the growth of CRC xenografts without systemic toxicities. Together, these results signify the potential of PPA24 as a novel PP2A activator and a prospective therapeutic for CRC and FOLFOX-resistant CRC.

Antipsychotics possess anti-glioblastoma activity by disrupting lysosomal function and inhibiting oncogenic signaling by stabilizing PTEN

The repurposing of medications developed for central nervous system (CNS) disorders, possessing favorable safety profiles and blood-brain barrier permeability, represents a promising strategy for identifying new therapies to combat glioblastoma (GBM). In this study, we investigated the anti-GBM activity of specific antipsychotics and antidepressants in vitro and in vivo. Our results demonstrate that these compounds share a common mechanism of action in GBM, disrupting lysosomal function and subsequently inducing lysosomal membrane rupture and cell death. Notably, PTEN intact GBMs possess an increased sensitivity to these compounds. The inhibition of lysosomal function synergized with inhibitors targeting the EGFR-PI3K-Akt pathway, leading to an energetic and antioxidant collapse. These findings provide a foundation for the potential clinical application of CNS drugs in GBM treatment. Additionally, this work offers critical insights into the mechanisms and determinants of cytotoxicity for drugs currently undergoing clinical trials as repurposing agents for various cancers, including Fluoxetine, Sertraline, Thioridazine, Chlorpromazine, and Fluphenazine.

A strategy to disentangle direct and indirect effects on (de)phosphorylation by chemical modulators of the phosphatase PP1 in complex cellular contexts

Chemical activators and inhibitors are useful probes to identify substrates and downstream effects of enzymes; however, due to the complex signaling environment within cells, it is challenging to distinguish between direct and indirect effects. This is particularly the case for phosphorylation, where a single (de)phosphorylation event can trigger rapid changes in many other phosphorylation sites. An additional complication arises when a single catalytic entity, which acts in the form of many different holoenzymes with different substrates, is activated or inhibited, as it is unclear which holoenzymes are affected, and in turn which of their substrates are (de)phosphorylated. Direct target engaging MS-based technologies to study targets of drugs do not address these challenges. Here, we tackle this by studying the modulation of protein phosphatase-1 (PP1) activity by PP1-disrupting peptides (PDPs), as well as their selectivity toward PP1, by using a combination of mass spectrometry-based experiments. By combining cellular treatment with the PDP with in vitro dephosphorylation by the enzyme, we identify high confidence substrate candidates and begin to separate direct and indirect effects. Together with experiments analyzing which holoenzymes are particularly susceptible to this treatment, we obtain insights into the effect of the modulator on the complex network of protein (de)phosphorylation. This strategy holds promise for enhancing our understanding of PP1 in particular and, due to the broad applicability of the workflow and the MS-based read-out, of chemical modulators with complex mode of action in general.

PP2A complex disruptor SET prompts widespread hypertranscription of growth-essential genes in the pancreatic cancer cells

Hyperactivation of the oncogenic transcription reflects the epigenetic plasticity of the cancer cells. Su(var)3-9, enhancer of zeste, Trithorax (SET) was described as a nuclear factor that stimulated transcription from the chromatin template. However, the mechanisms of SET-dependent transcription are unknown. Here, we found that overexpression of SET and CDK9 induced very similar transcriptome signatures in multiple cancer cell lines. SET localized in the transcription start site (TSS)-proximal regions and supported the RNA transcription. SET specifically bound the PP2A-C subunit and induced PP2A-A subunit repulsion from the C subunit, which indicated the role of SET as a PP2A-A/C complex disruptor in the TSS-proximal regions. Through blocking PP2A activity, SET assisted CDK9 to maintain Pol II CTD phosphorylation and activated mRNA transcription. Our findings position SET as a key factor that modulates chromatin PP2A activity, promoting the oncogenic transcription in the pancreatic cancer.

Comprehensive review of CRISPR-based gene editing: mechanisms, challenges, and applications in cancer therapy

The CRISPR system is a revolutionary genome editing tool that has the potential to revolutionize the field of cancer research and therapy. The ability to precisely target and edit specific genetic mutations that drive the growth and spread of tumors has opened up new possibilities for the development of more effective and personalized cancer treatments. In this review, we will discuss the different CRISPR-based strategies that have been proposed for cancer therapy, including inactivating genes that drive tumor growth, enhancing the immune response to cancer cells, repairing genetic mutations that cause cancer, and delivering cancer-killing molecules directly to tumor cells. We will also summarize the current state of preclinical studies and clinical trials of CRISPR-based cancer therapy, highlighting the most promising results and the challenges that still need to be overcome. Safety and delivery are also important challenges for CRISPR-based cancer therapy to become a viable clinical option. We will discuss the challenges and limitations that need to be overcome, such as off-target effects, safety, and delivery to the tumor site. Finally, we will provide an overview of the current challenges and opportunities in the field of CRISPR-based cancer therapy and discuss future directions for research and development. The CRISPR system has the potential to change the landscape of cancer research, and this review aims to provide an overview of the current state of the field and the challenges that need to be overcome to realize this potential.

Altering phosphorylation in cancer through PP2A modifiers

Protein phosphatase 2A (PP2A) is a serine/threonine phosphatase integral to the regulation of many cellular processes. Due to the deregulation of PP2A in cancer, many of these processes are turned toward promoting tumor progression. Considerable research has been undertaken to discover molecules capable of modulating PP2A activity in cancer. Because PP2A is capable of immense substrate specificity across many cellular processes, the therapeutic targeting of PP2A in cancer can be completed through either enzyme inhibitors or activators. PP2A modulators likewise tend to be effective in drug-resistant cancers and work synergistically with other known cancer therapeutics. In this review, we will discuss the patterns of PP2A deregulation in cancer, and its known downstream signaling pathways important for cancer regulation, along with many activators and inhibitors of PP2A known to inhibit cancer progression.

Small-Molecule Activation of Protein Phosphatase 2A Counters Bleomycin-Induced Fibrosis in Mice

The activity of protein phosphatase 2A (PP2A), a serine-threonine phosphatase, is reduced in the lung fibroblasts of idiopathic pulmonary fibrosis (IPF) patients. The objective of this study was to determine whether the reactivation of PP2A could reduce fibrosis and preserve the pulmonary function in a bleomycin (BLM) mouse model. Here, we present a new class of direct small-molecule PP2A activators, diarylmethyl-pyran-sulfonamide, exemplified by ATUX-1215. ATUX-1215 has improved metabolic stability and bioavailability compared to our previously described PP2A activators. Primary human lung fibroblasts were exposed to ATUX-1215 and an older generation PP2A activator in combination with TGFβ. ATUX-1215 treatment enhanced the PP2A activity, reduced the phosphorylation of ERK and JNK, and reduced the TGFβ-induced expression of ACTA2, FN1, COL1A1, and COL3A1. C57BL/6J mice were administered 5 mg/kg ATUX-1215 daily following intratracheal instillation of BLM. Three weeks later, forced oscillation and expiratory measurements were performed using the Scireq Flexivent System. ATUX-1215 prevented BLM-induced lung physiology changes, including the preservation of normal PV loop, compliance, tissue elastance, and forced vital capacity. PP2A activity was enhanced with ATUX-1215 and reduced collagen deposition within the lungs. ATUX-1215 also prevented the BLM induction of Acta2, Ccn2, and Fn1 gene expression. Treatment with ATUX-1215 reduced the phosphorylation of ERK, p38, JNK, and Akt and the secretion of IL-12p70, GM-CSF, and IL1α in BLM-treated animals. Delayed treatment with ATUX-1215 was also observed to slow the progression of lung fibrosis. In conclusion, our study indicates that the decrease in PP2A activity, which occurs in fibroblasts from the lungs of IPF subjects, could be restored with ATUX-1215 administration as an antifibrotic agent.

PP2A-based triple-strike therapy overcomes mitochondrial apoptosis resistance in brain cancer cells

Mitochondrial glycolysis and hyperactivity of the phosphatidylinositol 3-kinase-protein kinase B (AKT) pathway are hallmarks of malignant brain tumors. However, kinase inhibitors targeting AKT (AKTi) or the glycolysis master regulator pyruvate dehydrogenase kinase (PDKi) have failed to provide clinical benefits for brain tumor patients. Here, we demonstrate that heterogeneous glioblastoma (GB) and medulloblastoma (MB) cell lines display only cytostatic responses to combined AKT and PDK targeting. Biochemically, the combined AKT and PDK inhibition resulted in the shutdown of both target pathways and priming to mitochondrial apoptosis but failed to induce apoptosis. In contrast, all tested brain tumor cell models were sensitive to a triplet therapy, in which AKT and PDK inhibition was combined with the pharmacological reactivation of protein phosphatase 2A (PP2A) by NZ-8-061 (also known as DT-061), DBK-1154, and DBK-1160. We also provide proof-of-principle evidence for in vivo efficacy in the intracranial GB and MB models by the brain-penetrant triplet therapy (AKTi + PDKi + PP2A reactivator). Mechanistically, PP2A reactivation converted the cytostatic AKTi + PDKi response to cytotoxic apoptosis, through PP2A-elicited shutdown of compensatory mitochondrial oxidative phosphorylation and by increased proton leakage. These results encourage the development of triple-strike strategies targeting mitochondrial metabolism to overcome therapy tolerance in brain tumors.

RAS and PP2A activities converge on epigenetic gene regulation

RAS-mediated human cell transformation requires inhibition of the tumor suppressor protein phosphatase 2A (PP2A). However, the phosphoprotein targets and cellular processes in which RAS and PP2A activities converge in human cancers have not been systematically analyzed. Here, we discover that phosphosites co-regulated by RAS and PP2A are enriched on proteins involved in epigenetic gene regulation. As examples, RAS and PP2A co-regulate the same phosphorylation sites on HDAC1/2, KDM1A, MTA1/2, RNF168, and TP53BP1. We validate RAS- and PP2A-elicited regulation of HDAC1/2 chromatin recruitment, of RNF168-TP53BP1 interaction, and of gene expression. Consistent with their known synergistic effects in cancer, RAS activation and PP2A inhibition resulted in epigenetic reporter derepression and activation of oncogenic transcription. Transcriptional derepression by PP2A inhibition was associated with an increase in euchromatin and a decrease in global DNA methylation. Collectively, the results indicate that epigenetic protein complexes constitute a significant point of convergence for RAS hyperactivity and PP2A inhibition in cancer. Furthermore, the work provides an important resource for future studies focusing on phosphoregulation of epigenetic gene regulation in cancer and in other RAS/PP2A-regulated cellular processes.

Non-catalytic allostery in α-TAT1 by a phospho-switch drives dynamic microtubule acetylation

Spatiotemporally dynamic microtubule acetylation underlies diverse physiological and pathological events. Despite its ubiquity, the molecular mechanisms that regulate the sole microtubule acetylating agent, α-tubulin-N-acetyltransferase-1 (α-TAT1), remain obscure. Here, we report that dynamic intracellular localization of α-TAT1 along with its catalytic activity determines efficiency of microtubule acetylation. Specifically, we newly identified a conserved signal motif in the intrinsically disordered C-terminus of α-TAT1, consisting of three competing regulatory elements-nuclear export, nuclear import, and cytosolic retention. Their balance is tuned via phosphorylation by CDK1, PKA, and CK2, and dephosphorylation by PP2A. While the unphosphorylated form binds to importins and resides both in cytosol and nucleus, the phosphorylated form binds to specific 14-3-3 adapters and accumulates in the cytosol for maximal substrate access. Unlike other molecules with a similar phospho-regulated signal motif, α-TAT1 uniquely uses the nucleus as a hideout. This allosteric spatial regulation of α-TAT1 function may help uncover a spatiotemporal code of microtubule acetylation in normal and aberrant cell behavior.

The Pursuit of Enzyme Activation: A Snapshot of the Gold Rush

A range of enzymes drive human physiology, and their activities are tightly regulated through numerous signaling pathways. Depending on the context, these pathways may activate or inhibit an enzyme as a way to ensure proper execution of cellular functions. From a drug discovery and development perspective, pharmacological inhibition of enzymes has been a focus of interest, as many diseases are associated with the upregulation of enzyme function. On the other hand, however, pharmacological activation of enzymes such as kinases and phosphatases has been of increasing interest. In this review, we discuss seven case studies that highlight pharmacological activation strategy, describe the binding modes and pharmacology of the activators, and comment on how this on-demand activation strategy complements the commonly pursued inhibition strategy, thus jointly enabling bidirectional modulation of specific target of interest. Going forward, we expect activators to play important roles as chemical probes and drug leads.

The role of serine/threonine phosphatases in human development: Evidence from congenital disorders

Reversible protein phosphorylation is a fundamental regulation mechanism in eukaryotic cell and organismal physiology, and in human health and disease. Until recently, and unlike protein kinases, mutations in serine/threonine protein phosphatases (PSP) had not been commonly associated with disorders of human development. Here, we have summarized the current knowledge on congenital diseases caused by mutations, inherited or de novo, in one of 38 human PSP genes, encoding a monomeric phosphatase or a catalytic subunit of a multimeric phosphatase. In addition, we highlight similar pathogenic mutations in genes encoding a specific regulatory subunit of a multimeric PSP. Overall, we describe 19 affected genes, and find that most pathogenic variants are loss-of-function, with just a few examples of gain-of-function alterations. Moreover, despite their widespread tissue expression, the large majority of congenital PSP disorders are characterised by brain-specific abnormalities, suggesting a generalized, major role for PSPs in brain development and function. However, even if the pathogenic mechanisms are relatively well understood for a small number of PSP disorders, this knowledge is still incomplete for most of them, and the further identification of downstream targets and effectors of the affected PSPs is eagerly awaited through studies in appropriate in vitro and in vivo disease models. Such lacking studies could elucidate the exact mechanisms through which these diseases act, and possibly open up new therapeutic avenues.

Emerging insights into serine/threonine-specific phosphoprotein phosphatase function and selectivity

Protein phosphorylation on serine and threonine residues is a widely distributed post-translational modification on proteins that acts to regulate their function. Phosphoprotein phosphatases (PPPs) contribute significantly to a plethora of cellular functions through the accurate dephosphorylation of phosphorylated residues. Most PPPs accomplish their purpose through the formation of complex holoenzymes composed of a catalytic subunit with various regulatory subunits. PPP holoenzymes then bind and dephosphorylate substrates in a highly specific manner. Despite the high prevalence of PPPs and their important role for cellular function, their mechanisms of action in the cell are still not well understood. Nevertheless, substantial experimental advancements in (phospho-)proteomics, structural and computational biology have contributed significantly to a better understanding of PPP biology in recent years. This Review focuses on recent approaches and provides an overview of substantial new insights into the complex mechanism of PPP holoenzyme regulation and substrate selectivity.