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Elucidating how plants sense and respond to water loss is important for identifying genetic and chemical interventions that may help sustain crop yields in water-limiting environments. Currently, the molecular mechanisms involved in the initial perception and response to dehydration are not well understood. Modern mass spectrometric methods for quantifying changes in the phosphoproteome provide an opportunity to identify key phosphorylation events involved in this process. Here, we have used both untargeted and targeted isotope-assisted mass spectrometric methods of phosphopeptide quantitation to characterize proteins in Arabidopsis (Arabidopsis thaliana) whose degree of phosphorylation is rapidly altered by hyperosmotic treatment. Thus, protein phosphorylation events responsive to 5 min of 0.3 M mannitol treatment were first identified using ¹⁵N metabolic labeling and untargeted mass spectrometry with a high-resolution ion-trap instrument. The results from these discovery experiments were then validated using targeted Selected Reaction Monitoring mass spectrometry with a triple quadrupole. Targeted Selected Reaction Monitoring experiments were conducted with plants treated under nine different environmental perturbations to determine whether the phosphorylation changes were specific for osmosignaling or involved cross talk with other signaling pathways. The results indicate that regulatory proteins such as members of the mitogen-activated protein kinase family are specifically phosphorylated in response to osmotic stress. Proteins involved in 5' messenger RNA decapping and phosphatidylinositol 3,5-bisphosphate synthesis were also identified as targets of dehydration-induced phosphoregulation. The results of these experiments demonstrate the utility of targeted phosphoproteomic analysis in understanding protein regulation networks and provide new insight into cellular processes involved in the osmotic stress response.

Plant cells are immobile; thus, plant growth and development depend on cell expansion rather than cell migration. The molecular mechanism by which the plasma membrane initiates changes in the cell expansion rate remains elusive. We found that a secreted peptide, RALF (rapid alkalinization factor), suppresses cell elongation of the primary root by activating the cell surface receptor FERONIA in Arabidopsis thaliana. A direct peptide-receptor interaction is supported by specific binding of RALF to FERONIA and reduced binding and insensitivity to RALF-induced growth inhibition in feronia mutants. Phosphoproteome measurements demonstrate that the RALF-FERONIA interaction causes phosphorylation of plasma membrane H+–adenosine triphosphatase 2 at Ser899, mediating the inhibition of proton transport. The results reveal a molecular mechanism for RALF-induced extracellular alkalinization and a signaling pathway that regulates cell expansion.

Resistance to immune cell-mediated cytotoxicity poses a significant challenge in cancer therapy, compromising the efficacy of immunotherapeutic approaches such as immune checkpoint blockade (ICB) treatment. To enhance therapy outcomes, it is crucial to identify interventions that can synergize with ICB therapy to overcome tumor resistance. Therefore, we need to define the cellular mechanisms that sensitize tumors to cytotoxic T cells. CD8 T cells rely on cytokines such as TNF to carry out their cytotoxicity against tumors, and recent findings link select tumor mutations in the TNF pathway to increased T cell killing, in a manner dependent on RIPK1 kinase. Here, we demonstrate that sensitized tumor cells fail to initiate inhibitory RIPK1 phosphorylation at site S25 upon T cell attack, thereby foregoing a pro-survival checkpoint early in TNF signal transduction. Consequently, tumor cells experiencing a loss of TNF-induced RIPK1 S25 phosphorylation exhibit increased RIPK1 activation and fail to recruit non-canonical IKK kinases (TBK1 and IKKε) to the TNFR1 complex. Functional knockouts of TBK1 and IKKε in melanoma cells result in heightened sensitivity not only in CD8 T cell but also in Natural Killer cell attacks. Our findings indicate that preventing TBK1 and IKKε recruitment to the TNF signaling complex, thereby blocking RIPK1 pro-survival phosphorylation and promoting direct RIPK1 activation, is a tractable strategy to increase tumor sensitivity to immune cell killing and has the potential to benefit current immunotherapy interventions.

The MetaboHealth score is an indicator of physiological frailty in middle aged and older individuals. The aim of the current study was to explore which molecular pathways co-vary with the MetaboHealth score. Using a Luminex cytokine assay and liquid chromatography-mass spectrometry-based proteomics we explored the plasma proteins associating with the difference in 100 extreme scoring individuals selected from two large population cohorts, the Leiden Longevity Study (LLS) and the Rotterdam Study (RS), and discordant monozygotic twin pairs from the Netherlands Twin Register (NTR). In addition, we estimated the heritability of the score using 726 monozygotic (MZ) and 450 dizygotic (DZ) twin pairs. In the contrasting extreme scoring individuals from LLS and RS, we uncovered significant differences in 3 (out of 15) cytokines (GDF15, IL6, and MIG), and 106 (out of 289) plasma proteins. The high, poor health related, score associated with 42 increased inflammatory and immune related protein levels (CRP, LBP, HPT) and lowered levels of 71 HDL remodeling and cholesterol transport related proteins (e.g. APOA1, APOA2, APOA4, and TETN). Using the NTR twins, we subsequently showed that the MetaboHealth score is moderately heritable (h 2  = 0.4). In MZ twins selected for maximal discordance within a pair we found 68 serum proteins associated with the MetaboHealth score indicating that only a minor part of the associations observed in LLS and RS is likely explained by genetic influences. Taken together, our study sheds light on the intricate interplay between the MetaboHealth score, plasma proteins, cytokines, and genetic influences, paving the way for future investigations aimed at optimizing this mortality risk indicator.

γ-Tubulin ring complex (γ-TuRC) is the major microtubule-nucleating factor. After nucleation, microtubules can be released from γ-TuRC and stabilized by other proteins, such as CAMSAPs, but the biochemical cross-talk between minus-end regulation pathways is poorly understood. Here we reconstituted this process in vitro using purified components. We found that all CAMSAPs could bind to the minus ends of γ-TuRC-attached microtubules. CAMSAP2 and CAMSAP3, which decorate and stabilize growing minus ends but not the minus-end tracking protein CAMSAP1, induced microtubule release from γ-TuRC. CDK5RAP2, a γ-TuRC-interactor, and CLASP2, a regulator of microtubule growth, strongly stimulated γ-TuRC-dependent microtubule nucleation, but only CDK5RAP2 suppressed CAMSAP binding to γ-TuRC-anchored minus ends and their release. CDK5RAP2 also improved selectivity of γ-tubulin-containing complexes for 13- rather than 14-protofilament microtubules in microtubule-capping assays. Knockout and overexpression experiments in cells showed that CDK5RAP2 inhibits the formation of CAMSAP2-bound microtubules detached from the microtubule-organizing centre. We conclude that CAMSAPs can release newly nucleated microtubules from γ-TuRC, whereas nucleation-promoting factors can differentially regulate this process. Rai et al. report that CAMSAPs can bind to minus ends of microtubules attached to γ-tubulin ring complex (γ-TuRC) and drive microtubule release. They show that CDK5RAP2, but not CLASP2, inhibits CAMSAP-mediated microtubule release from γ-TuRC.

An Arabidopsis thaliana mitogen-activated protein (MAP) kinase cascade composed of MEKK1, MKK1/MKK2, and MPK4 was previously described as a negative regulator of defense response. MEKK1 encodes a MAP kinase kinase kinase and is a member of a tandemly duplicated gene family with MEKK2 and MEKK3. Using T-DNA insertion lines, we isolated a novel deletion mutant disrupting this gene family and found it to be phenotypically wild-type, in contrast with the mekkl dwarf phenotype. Follow-up genetic analyses indicated that MEKK2 is required for the mekk1, mkk1 mkk2, and mpk4 autoimmune phenotypes. We next analyzed a T-DNA insertion in the MEKK2 promoter region and found that although it does not reduce the basal expression of MEKK2, it does prevent the upregulation of MEKK2 that is observed in mpk4 plants. This mekk2 allele can rescue the mpk4 autoimmune phenotype in a dosage-dependent manner. We also found that expression of constitutively active MPK4 restored MEKK2 abundance to wild-type levels in mekk1 mutant plants. Finally, using mass spectrometry, we showed that MEKK2 protein levels mirror MEKK2 mRNA levels. Taken together, our results indicate that activated MPK4 is responsible for regulating MEKK2 RNA abundance. In turn, the abundance of MEKK2 appears to be under cellular surveillance such that a modest increase can trigger defense response activation.

How plants perceive and respond to water loss on a molecular level is a fundamental question in plant biology. Understanding the cellular signaling mechanism involved in sensing dehydration and initiating adaptive responses provides a tool for plant biologists to use in engineering crops with enhanced drought tolerance and water use efficiency as well as with developing new chemical approaches towards alleviating the negative effects of drought on crop yields. The increasing frequency and severity of drought, along with the depletion of fresh water resources, fortifies the need to understand the molecular networks that control drought response in plants. Currently the molecular mechanisms involved in the initial perception of dehydration are poorly understood. In this dissertation I present my work towards identifying previously unknown proteins involved in the initial dehydration response, using mass spectrometry based quantitative proteomic and reverse genetic approaches. Employing the model system Arabidopsis thaliana and hyperosmotic stress conditions to simulate water loss, I first identified proteins whose level of phosphorylation changes in response to short-term stress treatments, using quantitative untargeted mass spectrometry based phosphoproteomics technologies. From this work I identified proteins whose phosphorylation state changed within 5 minutes of stress treatment. To further characterize these proteins, I developed targeted proteomic assays to more routinely measure protein phosphorylated across many biotic and abiotic stress conditions. From this analysis I characterized a set of proteins that are uniquely regulated at the posttranslational level by rapid dehydration. These proteins are involved in cellular process such as mRNA degradation, microtubule restructuring, transcription factor activation, phospholipid signaling, and mitogen-activated protein kinase signaling and help elucidate the role that these processes play in the initial dehydration response. This work is presented in Chapter 2 of my dissertation. To validate the biological significance of proteins identified in my phosphoproteome analysis, I used reverse genetics to investigate Vac14, an uncharacterized protein in plants that displayed the largest change in phosphorylation under osmotic stress conditions. Vac14 is a highly conserved protein involved in the biosynthesis of the low abundant phospholipid phosphatidylinositol-3,5 bisphosphate, which regulates vacuole function and endomembrane vesicle transport in yeast and mammals. I demonstrate that Vac14 is an essential gene in plants and is responsible for regulating water homeostasis in cells. Vac14 overexpression mutants have increased drought tolerance and improved ability to germinate under water- limited conditions. This work is presented in Chapter 3 and supports the biological significance of proteins identified in my discovery phosphoproteomic work in Chapter 2. Finally, in Chapter 4 I describe my preliminary results in developing a new method for detecting proteins involved in cell signaling events through direct measurement of protein conformational changes. This method uses thermal denaturation profiling and untargeted quantitative mass spectrometry to identify proteins whose conformation has changed in response to in vitro or in vivo treatment conditions. Together, the data presented in this thesis demonstrate the utility of mass spectrometric based proteomic technologies for discovering previously unidentified proteins involved in cell signaling events and provides valuable insight into pathways activated during the initial few minutes of the osmotic stress response in plants.

Centrioles are microtubule-based organelles required for the formation of centrosomes and cilia. Centriolar microtubules, unlike their cytosolic counterparts, grow very slowly and are very stable. The complex of centriolar proteins CP110 and CEP97 forms a cap that stabilizes the distal centriole end and prevents its over-elongation. Here, we used in vitro reconstitution assays to show that whereas CEP97 does not interact with microtubules directly, CP110 specifically binds microtubule plus ends, potently blocks their growth and induces microtubule pausing. Cryo-electron tomography indicated that CP110 binds to the luminal side of microtubule plus ends and reduces protofilament peeling. Furthermore, CP110 directly interacts with another centriole biogenesis factor, CPAP/SAS-4, which tracks growing microtubule plus ends, slows down their growth and prevents catastrophes. CP110 and CPAP synergize in inhibiting plus-end growth, and this synergy depends on their direct binding. Together, our data reveal a molecular mechanism controlling centriolar microtubule plus-end dynamics and centriole biogenesis. Competing Interest Statement The authors have declared no competing interest. Footnotes * We have updated the data availability section and corrected a small textual mistake in one supplemental figure.

Enhancing the immunogenicity of tumor cells is a major objective in cancer therapy, particularly for tumors with low immune cell infiltration scores. Inducing immunogenic forms of programmed cell death (PCD) offers a promising strategy to strengthen anti-tumor immunity and improve therapeutic outcomes. Necroptosis, a highly inflammatory form of regulated cell death triggered by TNF signaling, can elicit robust immune activation. However, its regulation in tumor cells remains incompletely understood, limiting its therapeutic exploitation. To investigate the protein-protein interactions that govern necroptotic cell death in tumor cells, we established a co-Immunoprecipitation - Mass Spectrometry (coIP-MS) workflow using RIPK3, the central effector kinase driving necroptosis in TNF-induced signaling, as bait. This unbiased proteomic approach enables the identification of candidate regulators directly associated with the necrosome complex components under active necroptotic conditions. Among identified candidates, TCOF1 and GDF15 emerged as previously unrecognized modulators, with functional knockout of either gene markedly enhancing necroptotic cell death in tumor cells. Reciprocal IP experiments confirmed a direct interaction between GDF15 and RIPK3, supporting its mechanistic role as a negative regulator that suppresses necroptotic signaling. Thus, our findings extend the function of GDF15 beyond its established role in inflammation, uncovering an additional layer of regulation at the level of cell-intrinsic death signaling. Collectively, our findings position GDF15 as a RIPK3-interacting “brake” on necroptotic cell death and highlight TCOF1 as an additional inhibitory node. Our study underscores the potential of targeting necroptosis-suppressive mechanisms to influence PCD outcomes in tumors and demonstrates the power of coIP-MS for mapping TNF-induced interactions to reveal actionable molecular targets for tumor sensitization.