Spontaneous and chaperone-assisted metal loading in the active site of protein phosphatase-1

Protein phosphatase PP1 has two active-site metals (Zn2+/Fe2+) that are essential for catalysis. However, when expressed in bacteria, PP1 has two Mn2+-ions in its active site, indicating that the incorporation of Zn2+/Fe2+ depends on additional eukaryotic component(s). Here, we used purified, metal-deficient PP1 to study metal incorporation. Fe2+ was incorporated spontaneously, but Zn2+ was not. Mn2+-incorporation at physiological pH depended on the co-expression of PP1 with PPP1R2 (Inhibitor-2) or PPP1R11 (Inhibitor-3), or a pre-incubation of PP1 at pH 4. We also demonstrate that PPP1R2 and PPP1R11 are Zn2+-binding proteins but are, by themselves, not able to load PP1 with Zn2+. Our data suggest that PPP1R2 and PPP1R11 function as metal chaperones for PP1 but depend on co-chaperone(s) and/or specific modification(s) for the transfer of associated Zn2+ to PP1.

PPP1R2 stimulates protein phosphatase-1 through stabilisation of dynamic subunit interactions

Protein Ser/Thr phosphatase PP1 is always associated with one or two regulatory subunits or RIPPOs. One of the earliest evolved RIPPOs is PPP1R2, also known as Inhibitor-2. Since its discovery nearly 5 decades ago, PPP1R2 has been variously described as an inhibitor, activator or (metal) chaperone of PP1, but it is still unknown how PPP1R2 affects the function of PP1 in intact cells. Here, using specific research tools, we demonstrate that PPP1R2 stabilises a subgroup of PP1 holoenzymes, exemplified by PP1:RepoMan, thereby promoting the dephosphorylation of their substrates. Mechanistically, the recruitment of PPP1R2 disrupts an inhibitory, fuzzy interaction between the C-terminal tail and catalytic domain of PP1, and generates an additional C-terminal RepoMan-interaction site. The resulting holoenzyme is further stabilized by a direct PPP1R2:RepoMan interaction, which renders it refractory to competitive disruption by RIPPOs that do not interact with PPP1R2. Our data demonstrate that PPP1R2 modulates the function of PP1 by altering the balance between holoenzymes through stabilisation of specific subunit interactions.

Engineered targeting OIP5 sensitizes bladder cancer to chemotherapy resistance via TRIP12-PPP1CB-YBX1 axis

Chemoresistance is an important cause of treatment failure in bladder cancer, and identifying genes that confer drug resistance is an important step toward developing new therapeutic strategies to improve treatment outcomes. In the present study, we show that gemcitabine plus cisplatin (GEM/DDP) therapy induces NF-κB signaling, which promotes p65-mediated transcriptional activation of OIP5. OIP5 recruits the E3 ubiquitin ligase TRIP12 to bind to and degrade the phosphatase PPP1CB, thereby enhancing the transcription factor activity of YBX1. This in turn upregulates drug-resistance-related genes under the transcriptional control of YBX1, leading to chemoresistance. Moreover, PPP1CB degradation can enhance the phosphorylation activity of IKKβ, triggering the NF-κB signaling cascade, which further stimulates OIP5 gene expression, thus forming a negative feedback regulatory loop. Consistently, elevated OIP5 expression was associated with chemoresistance and poor prognosis in patients with bladder cancer. Furthermore, we used a CRISPR/Cas9-based engineered gene circuit, which can monitor the progression of chemoresistance in real-time, to induce OIP5 knockout upon detection of increased NF-κB signaling. The gene circuit significantly inhibited tumor cell growth in vivo, underscoring the potential for synergy between gene therapy and chemotherapy in the treatment of cancer.

Substrate and phosphorylation site selection by phosphoprotein phosphatases

Dynamic protein phosphorylation and dephosphorylation are essential regulatory mechanisms that ensure proper cellular signaling and biological functions. Deregulation of either reaction has been implicated in several human diseases. Here, we focus on the mechanisms that govern the specificity of the dephosphorylation reaction. Most cellular serine/threonine dephosphorylation is catalyzed by 13 highly conserved phosphoprotein phosphatase (PPP) catalytic subunits, which form hundreds of holoenzymes by binding to regulatory and scaffolding subunits. PPP holoenzymes recognize phosphorylation site consensus motifs and interact with short linear motifs (SLiMs) or structural elements distal to the phosphorylation site. We review recent advances in understanding the mechanisms of PPP site-specific dephosphorylation preference and substrate recruitment and highlight examples of their interplay in the regulation of cell division.

Regulation of PP1 interaction with I-2, neurabin, and F-actin

Reversible phosphorylation is a fundamental regulatory mechanism required for many biological processes and is coordinated by the opposing actions of protein kinases and phosphatases. Protein phosphatase 1 (PP1) is a major protein phosphatase that plays an important role in many fundamental physiological processes including synaptic transmission and memory formation. Here we investigate the regulation of PP1 by prominent signaling proteins and synaptic scaffolds including GSK3β, inhibitor-2 (I-2), neurabin (Nrb), and actin. While GSK3β is known to regulate PP1 via phosphorylation of the PP1-binding protein I-2, we found that GSK3β directly regulates PP1 via inhibitory phosphorylation in neurons. Additionally, using bioluminescence resonance energy transfer (BRET), we found that GSK3β alters PP1-I-2 interaction in living cells. The effect of GSK3β on PP1-I-2 interaction is independent of the PP1 C-terminal tail, contrary to predictions based on previous findings from purified proteins. I-2 has been shown to form a trimeric complex with PP1 and Nrb, a major synaptic scaffold for promoting PP1 localization to the actin cytoskeleton. Utilizing BRET, we found that Nrb promotes PP1-actin interaction, however no BRET was detected between I-2 and F-actin. Finally, we found that stabilizing F-actin promotes Nrb-PP1 binding and may also lead to conformational changes between Nrb-I-2 and Nrb-F-actin complexes. Overall, our findings elaborate the dynamic regulation of PP1 complexes by GSK3β, targeting proteins, and actin polymerization.

Protein phosphatase-1 inhibitor-2 promotes PP1γ positive regulation of synaptic transmission

Inhibitor-2 (I-2) is a prototypic inhibitor of protein phosphatase-1 (PP1), a major serine-threonine phosphatase that regulates synaptic plasticity and learning and memory. Although I-2 is a potent inhibitor of PP1 in vitro, our previous work has elucidated that, in vivo, I-2 may act as a positive regulator of PP1. Here we show that I-2 and PP1γ, but not PP1α, positively regulate synaptic transmission in hippocampal neurons. Moreover, we demonstrated that I-2 enhanced PP1γ interaction with its major synaptic scaffold, neurabin, by Förster resonance energy transfer (FRET)/Fluorescence lifetime imaging microscopy (FLIM) studies, while having a limited effect on PP1 auto-inhibitory phosphorylation. Furthermore, our study indicates that the effect of I-2 on PP1 activity in vivo is dictated by I-2 threonine-72 phosphorylation. Our work thus demonstrates a molecular mechanism by which I-2 positively regulates PP1 function in synaptic transmission.

Exploring prognostic value and regulation network of PPP1R1A in hepatocellular carcinoma

Novel and accurate biomarkers are needed for early detection and progression evaluation of hepatocellular carcinoma (HCC). Protein phosphatase 1 regulatory subunit 1A (PPP1R1A) has been studied in cancer biology; however, the expression pattern and biological function of PPP1R1A in HCC are unclear. The differentially expressed genes (DEGs) in HCC were screened by The Cancer Genome Atlas (TCGA) database. Real-time PCR and immunohistochemistry (IHC) assay were used to detect the expression of PPP1R1A in BALB/c mice, human normal tissues and corresponding tumor tissues, especially HCC. Then, Kaplan-Meier analysis of patients with HCC was performed to evaluate the relationship between PPP1R1A expression and prognosis. The transcriptional regulatory network of PPP1R1A was constructed based on the differentially expressed mRNAs, microRNAs and transcription factors (TFs). To explore the downstream regulation of PPP1R1A, the Gene Ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG) functional enrichment analysis and immune infiltration score were performed. A total of 4 DEGs were screened out. PPP1R1A was differentially distributed and expressed in BALB/c mice and human tissues. PPP1R1A expression was higher in normal tissues than that in tumor tissues, and patients with higher PPP1R1A expression had better clinical outcome in HCC. In addition, we constructed miR-21-3p/TAL1/PPP1R1A transcriptional network. Furthermore, PPP1R1A may modulate the activation of PI3K-Akt pathway, cell cycle, glycogen metabolism and the recruitment of M2 macrophage in HCC. This study may help to clarify the function and mechanism of PPP1R1A in HCC and provide a potential biomarker for tumor prevention and treatment.

Protein phosphatase 1: life-course regulation by SDS22 and Inhibitor-3

Protein phosphatase 1 (PP1) is expressed in all eukaryotic cells and catalyzes a sizable fraction of protein Ser/Thr dephosphorylation events. It is tightly regulated in space and time through association with a wide array of regulatory interactors of protein phosphatase one (RIPPOs). Suppressor-of-Dis2-number 2 (SDS22) and Inhibitor-3 (I3), which form a ternary complex with PP1, are the first two evolved and most widely expressed RIPPOs. Their deletion causes mitotic-arrest phenotypes and is lethal in some organisms. The role of SDS22 and I3 in PP1 regulation has been a mystery for decades as they were independently identified as both activators and inhibitors of PP1. This conundrum has largely been solved by recent reports showing that SDS22 and I3 control multiple steps of the life course of PP1. Indeed, they contribute to (a) the stabilization and activation of newly translated PP1, (b) the translocation of PP1 to the nucleus, and (c) the storage of PP1 as a reserve for holoenzyme assembly. Preliminary evidence suggests that SDS22 and I3 may also function as scavengers of released or aged PP1 for re-use in holoenzyme assembly or proteolytical degradation, respectively. Hence, SDS22 and I3 are emerging as master regulators of the life course of PP1.

Regulation of Synaptic Transmission and Plasticity by Protein Phosphatase 1

Protein phosphatases, by counteracting protein kinases, regulate the reversible phosphorylation of many substrates involved in synaptic plasticity, a cellular model for learning and memory. A prominent phosphatase regulating synaptic plasticity and neurologic disorders is the serine/threonine protein phosphatase 1 (PP1). PP1 has three isoforms (α, β, and γ, encoded by three different genes), which are regulated by a vast number of interacting subunits that define their enzymatic substrate specificity. In this review, we discuss evidence showing that PP1 regulates synaptic transmission and plasticity, as well as presenting novel models of PP1 regulation suggested by recent experimental evidence. We also outline the required targeting of PP1 by neurabin and spinophilin to achieve substrate specificity at the synapse to regulate AMPAR and NMDAR function. We then highlight the role of inhibitor-2 in regulating PP1 function in plasticity, including its positive regulation of PP1 function in vivo in memory formation. We also discuss the distinct function of the three PP1 isoforms in synaptic plasticity and brain function, as well as briefly discuss the role of inhibitory phosphorylation of PP1, which has received recent emphasis in the regulation of PP1 activity in neurons.