Phosphorylation of PP1 Regulator Sds22 by PLK1 Ensures Accurate Chromosome Segregation

Lasiokaurin Regulates PLK1 to Induce Breast Cancer Cell G2/M Phase Block and Apoptosis

Aim of the study: To investigate the anti-tumor effects of Lasiokaurin on breast cancer and explore its underlying molecular mechanism. Materials and methods: In this study, MTT assay, plate colony formation assays, soft agar assay, and EdU assay were employed to evaluate the anti-proliferation effects of LAS. Apoptosis and cell cycle distribution were detected by flow cytometry. The molecular mechanism was predicted by performing RNA sequencing and verified by using immunoblotting assays. Breast cancer organiods derived from patient-derived xenografts model and MDA-MB-231 xenograft mouse model were established to assess the effect of LAS. Results: Our study showed that LAS treatment significantly suppressed cell viability of 5 breast cancer cell lines, with the IC50 value of approximately 1-5 μM. LAS also inhibitied the clonogenic ability and DNA synthesis of breast cancer cells, Moreover, LAS induced apoptosis and G2/M cell cycle arrest in SK-BR-3 and MDA-MB-231 cells. Notably, transcriptomic analysis predicted the mechanistic involvement of PLK1 in LAS-suppressed breast cancer progression. Our experiment data further verified that LAS reduced PLK1 mRNA and protein expression in breast cancer, accompanied by downregulating CDC25C and AKT phosphorylation. Ultimately, we confirmed that LAS inhibit breast cancer growth via inhibiting PLK1 pathway in vivo. Conclusions: Collectively, our findings revealed that LAS inhibits breast cancer progression via regulating PLK1 pathway, which provids scientific evidence for the use of traditional Chinese medicine in cancer therapy.

Plasmodium berghei leucine-rich repeat protein 1 downregulates protein phosphatase 1 activity and is required for efficient oocyst development

Protein phosphatase 1 (PP1) is a key enzyme for Plasmodium development. However, the detailed mechanisms underlying its regulation remain to be deciphered. Here, we report the functional characterization of the Plasmodium berghei leucine-rich repeat protein 1 (PbLRR1), an orthologue of SDS22, one of the most ancient and conserved PP1 interactors. Our study shows that PbLRR1 is expressed during intra-erythrocytic development of the parasite, and up to the zygote stage in mosquitoes. PbLRR1 can be found in complex with PbPP1 in both asexual and sexual stages and inhibits its phosphatase activity. Genetic analysis demonstrates that PbLRR1 depletion adversely affects the development of oocysts. PbLRR1 interactome analysis associated with phospho-proteomics studies identifies several novel putative PbLRR1/PbPP1 partners. Some of these partners have previously been characterized as essential for the parasite sexual development. Interestingly, and for the first time, Inhibitor 3 (I3), a well-known and direct interactant of Plasmodium PP1, was found to be drastically hypophosphorylated in PbLRR1-depleted parasites. These data, along with the detection of I3 with PP1 in the LRR1 interactome, strongly suggest that the phosphorylation status of PbI3 is under the control of the PP1-LRR1 complex and could contribute (in)directly to oocyst development. This study provides new insights into previously unrecognized PbPP1 fine regulation of Plasmodium oocyst development through its interaction with PbLRR1.

PPP1R7 Is a Novel Translocation Partner of CBFB via t(2;16)(q37;q22) in Acute Myeloid Leukemia

In a subset of acute myeloid leukemia (AML) cases, the core binding factor beta subunit gene (CBFB) was rearranged via inv(16)(p13.1q22) or t(16;16)(p13.1;q22), in which the smooth muscle myosin heavy chain 11 gene (MYH11) was the partner (CBFB::MYH11). Rare variants of CBFB rearrangement occurring via non-classic chromosomal aberrations have been reported, such as t(1;16), t(2;16), t(3;16), t(5;16), and t(16;19), but the partners of CBFB have not been characterized. We report a case of AML with a complex karyotype, including t(2;16)(q37;q22), in which the protein phosphatase 1 regulatory subunit 7 gene (PPP1R7) at chromosome 2q37 was rearranged with CBFB (CBFB::PPP1R7). This abnormality was inconspicuous by conventional karyotype and interphase fluorescence in situ hybridization (FISH), thus leading to an initial interpretation of inv(16)(p13.1q22); however, metaphase FISH showed that the CBFB rearrangement involved chromosome 2. Using whole genome and Sanger sequencing, the breakpoints were identified as being located in intron 5 of CBFB and intron 7 of PPP1R7. A microhomology of CAG was found in the break and reconnection sites of CBFB and PPP1R7, thus supporting the formation of CBFB::PPP1R7 by microhomology-mediated end joining.

Feedback control of PLK1 by Apolo1 ensures accurate chromosome segregation

Stable transmission of genetic material during cell division requires accurate chromosome segregation. PLK1 dynamics at kinetochores control establishment of correct kinetochore-microtubule attachments and subsequent silencing of the spindle checkpoint. However, the regulatory mechanism responsible for PLK1 activity in prometaphase has not yet been affirmatively identified. Here we identify Apolo1, which tunes PLK1 activity for accurate kinetochore-microtubule attachments. Apolo1 localizes to kinetochores during early mitosis, and suppression of Apolo1 results in misaligned chromosomes. Using the fluorescence resonance energy transfer (FRET)-based PLK1 activity reporter, we found that Apolo1 sustains PLK1 kinase activity at kinetochores for accurate attachment during prometaphase. Apolo1 is a cognate substrate of PLK1, and the phosphorylation enables PP1γ to inactivate PLK1 by dephosphorylation. Mechanistically, Apolo1 constitutes a bridge between kinase and phosphatase, which governs PLK1 activity in prometaphase. These findings define a previously uncharacterized feedback loop by which Apolo1 provides fine-tuning for PLK1 to guide chromosome segregation in mitosis.

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.

The Greatwall kinase safeguards the genome integrity by affecting the kinome activity in mitosis

Progression through mitosis is balanced by the timely regulation of phosphorylation and dephosphorylation events ensuring the correct segregation of chromosomes before cytokinesis. This balance is regulated by the opposing actions of CDK1 and PP2A, as well as the Greatwall kinase/MASTL. MASTL is commonly overexpressed in cancer, which makes it a potential therapeutic anticancer target. Loss of Mastl induces multiple chromosomal errors that lead to the accumulation of micronuclei and multilobulated cells in mitosis. Our analyses revealed that loss of Mastl leads to chromosome breaks and abnormalities impairing correct segregation. Phospho-proteomic data for Mastl knockout cells revealed alterations in proteins implicated in multiple processes during mitosis including double-strand DNA damage repair. In silico prediction of the kinases with affected activity unveiled NEK2 to be regulated in the absence of Mastl. We uncovered that, RAD51AP1, involved in regulation of homologous recombination, is phosphorylated by NEK2 and CDK1 but also efficiently dephosphorylated by PP2A/B55. Our results suggest that MastlKO disturbs the equilibrium of the mitotic phosphoproteome that leads to the disruption of DNA damage repair and triggers an accumulation of chromosome breaks even in noncancerous cells.

Methylation of PLK1 by SET7/9 ensures accurate kinetochore-microtubule dynamics

Faithful segregation of mitotic chromosomes requires bi-orientation of sister chromatids, which relies on the sensing of correct attachments between spindle microtubules and kinetochores. Although the mechanisms underlying PLK1 activation have been extensively studied, the regulatory mechanisms that couple PLK1 activity to accurate chromosome segregation are not well understood. In particular, PLK1 is implicated in stabilizing kinetochore-microtubule attachments, but how kinetochore PLK1 activity is regulated to avoid hyperstabilized kinetochore-microtubules in mitosis remains elusive. Here, we show that kinetochore PLK1 kinase activity is modulated by SET7/9 via lysine methylation during early mitosis. The SET7/9-elicited dimethylation occurs at the Lys191 of PLK1, which tunes down its activity by limiting ATP utilization. Overexpression of the non-methylatable PLK1 mutant or chemical inhibition of SET7/9 methyltransferase activity resulted in mitotic arrest due to destabilized kinetochore-microtubule attachments. These data suggest that kinetochore PLK1 is essential for stable kinetochore-microtubule attachments and methylation by SET7/9 promotes dynamic kinetochore-microtubule attachments for accurate error correction. Our findings define a novel homeostatic regulation at the kinetochore that integrates protein phosphorylation and methylation with accurate chromosome segregation for maintenance of genomic stability.

SDS22 selectively recognizes and traps metal-deficient inactive PP1

The metalloenzyme protein phosphatase 1 (PP1), which is responsible for ≥50% of all dephosphorylation reactions, is regulated by scores of regulatory proteins, including the highly conserved SDS22 protein. SDS22 has numerous diverse functions, surprisingly acting as both a PP1 inhibitor and as an activator. Here, we integrate cellular, biophysical, and crystallographic studies to address this conundrum. We discovered that SDS22 selectively binds a unique conformation of PP1 that contains a single metal (M2) at its active site, i.e., SDS22 traps metal-deficient inactive PP1. Furthermore, we showed that SDS22 dissociation is accompanied by a second metal (M1) being loaded into PP1, as free metal cannot dissociate the complex and M1-deficient mutants remain constitutively trapped by SDS22. Together, our findings reveal that M1 metal loading and loss are essential for PP1 regulation in cells, which has broad implications for PP1 maturation, activity, and holoenzyme subunit exchange.

Protein Serine/Threonine Phosphatases: Keys to Unlocking Regulators and Substrates

Protein serine/threonine phosphatases (PPPs) are ancient enzymes, with distinct types conserved across eukaryotic evolution. PPPs are segregated into types primarily on the basis of the unique interactions of PPP catalytic subunits with regulatory proteins. The resulting holoenzymes dock substrates distal to the active site to enhance specificity. This review focuses on the subunit and substrate interactions for PPP that depend on short linear motifs. Insights about these motifs from structures of holoenzymes open new opportunities for computational biology approaches to elucidate PPP networks. There is an expanding knowledge base of posttranslational modifications of PPP catalytic and regulatory subunits, as well as of their substrates, including phosphorylation, acetylation, and ubiquitination. Cross talk between these posttranslational modifications creates PPP-based signaling. Knowledge of PPP complexes, signaling clusters, as well as how PPPs communicate with each other in response to cellular signals should unlock the doors to PPP networks and signaling "clouds" that orchestrate and coordinate different aspects of cell physiology.

A Kinase-Phosphatase Network that Regulates Kinetochore-Microtubule Attachments and the SAC

The KMN network (for KNL1, MIS12 and NDC80 complexes) is a hub for signalling at the outer kinetochore. It integrates the activities of two kinases (MPS1 and Aurora B) and two phosphatases (PP1 and PP2A-B56) to regulate kinetochore-microtubule attachments and the spindle assembly checkpoint (SAC). We will first discuss each of these enzymes separately, to describe how they are regulated at kinetochores and why this is important for their primary function in controlling either microtubule attachments or the SAC. We will then discuss why inhibiting any one of them individually produces secondary effects on all the others. This cross-talk may help to explain why all enzymes have been linked to both processes, even though the direct evidence suggests they each control only one. This chapter therefore describes how a network of kinases and phosphatases work together to regulate two key mitotic processes.

Structure-Guided Exploration of SDS22 Interactions with Protein Phosphatase PP1 and the Splicing Factor BCLAF1

SDS22 is an ancient regulator of protein phosphatase-1 (PP1). Our crystal structure of SDS22 shows that its twelve leucine-rich repeats adopt a banana-shaped fold that is shielded from solvent by capping domains at its extremities. Subsequent modeling and biochemical studies revealed that the concave side of SDS22 likely interacts with PP1 helices α5 and α6, which are distal from the binding sites of many previously described PP1 interactors. Accordingly, we found that SDS22 acts as a "third" subunit of multiple PP1 holoenzymes. The crystal structure of SDS22 also revealed a large basic surface patch that enables binding of a phosphorylated form of splicing factor BCLAF1. Taken together, our data provide insights into the formation of PP1:SDS22 and the recruitment of additional interaction proteins, such as BCLAF1.

Kinase and Phosphatase Cross-Talk at the Kinetochore

Multiple kinases and phosphatases act on the kinetochore to control chromosome segregation: Aurora B, Mps1, Bub1, Plk1, Cdk1, PP1, and PP2A-B56, have all been shown to regulate both kinetochore-microtubule attachments and the spindle assembly checkpoint. Given that so many kinases and phosphatases converge onto two key mitotic processes, it is perhaps not surprising to learn that they are, quite literally, entangled in cross-talk. Inhibition of any one of these enzymes produces secondary effects on all the others, which results in a complicated picture that is very difficult to interpret. This review aims to clarify this picture by first collating the direct effects of each enzyme into one overarching schematic of regulation at the Knl1/Mis12/Ndc80 (KMN) network (a major signaling hub at the outer kinetochore). This schematic will then be used to discuss the implications of the cross-talk that connects these enzymes; both in terms of why it may be needed to produce the right type of kinetochore signals and why it nevertheless complicates our interpretations about which enzymes control what processes. Finally, some general experimental approaches will be discussed that could help to characterize kinetochore signaling by dissociating the direct from indirect effect of kinase or phosphatase inhibition in vivo. Together, this review should provide a framework to help understand how a network of kinases and phosphatases cooperate to regulate two key mitotic processes.