Molecular and Cellular Biology

DDI2 and DDI3 (DDI2/3) are duplicated genes in Saccharomyces cerevisiae that exhibit strong induction by a transcription factor Fzf1 in response to chemical treatments like cyanamide (CY) and methyl methanesulfonate (MMS). Although, like DDI2/3, SSU1, YHB1 and YNR064C also contain an Fzf1-binding consensus sequence CS2 and are coordinately regulated by Fzf1, these genes are only modestly induced by CY and MMS. To identify additional cis-acting elements in the DDI2/3 promoter, we made DDI2/3 promoter deletions in a reporter system and identified upstream repressing sequences (URS) spanning 480 nucleotides. To test a hypothesis that the chromatin structure constitutes the URS, we utilized a yeast strain capable of histone H3/H4 depletion by shifting carbon sources. Following histone depletion, DDI2/3 were strongly induced in an Fzf1 dependent manner, while YHB1 was repressed. Interestingly, under histone depletion conditions, CY or MMS treatment further increased expression of all Fzf1-regulated genes to comparable levels in an Fzf1 dependent manner. A genome-wide MNase-seq analysis showed that CY treatment reduced the nucleosome occupancy at the mapped DDI2/3 URS region in wild-type cells, but not in in fzf1{Delta} cells. These findings collectively indicate that Fzf1 plays dual roles in regulating the DDI2/3 response to CY. Firstly, it binds CS2 and serves as a transcription activator. Secondly, it is required for the chromatin remodeling at URS. This two-tier regulation at the DDI2/3 promoter helps to explain why DDI2/3 achieve much higher fold induction by CY and MMS than other Fzf1-regulated genes, suggesting Fzf1 to be a candidate pioneer transcription factor.

Interstrand crosslinks are cytotoxic lesions that inhibit essential processes including replication and transcription. Replication fork reversal occurs in response to interstrand crosslink inducing drug, MMC, but how replication fork reversal promotes repair of interstrand crosslinks is poorly understood. Here, we investigated the role of the RAD54L translocase in interstrand crosslink repair. We found RAD54L is required to promote nascent DNA degradation in FANCD2 and FANCA-depleted cells consistent with a previous study indicating RAD54L promotes replication fork reversal. We further show RAD54L activity is required for formation of radial chromosomes in FANCD2-deficient cells suggesting fork reversal may be required to generate the intermediate undergoing aberrant fusion in FANC-deficient cells. Finally, we demonstrate FANCD2 foci accumulate and DSBs persist in RAD54L-deficient cells indicating RAD54L is required for efficient repair of DSBs. Together, our results indicate RAD54L plays multiple roles in efficient processing and repair of interstrand crosslinks.

Although calmodulin is best known as an intracellular calcium sensor, it also possesses calcium-independent functions in unicellular organisms. This is exemplified by the budding yeast S. cerevisiae calmodulin, which binds its essential targets, the pericentrin-like protein Spc110 and type I and V myosins, without needing calcium. Whether such calcium-independent cellular functions are conserved in other yeasts and vertebrates nevertheless remains an open question. Here, we examined the calcium-independent functions of the fission yeast S. pombe calmodulin Cam1 by measuring its intracellular distribution. Using quantitative fluorescence microscopy, we assessed the intracellular localization of two cam1 mutants, where binding of Ca2+ had been compromised by mutations in their EF hands, compared to the wild type protein. Both Cam1-2V and -3V reduced their localization by 90% to the yeast microtubule-organizing center spindle pole bodies (SPB). In contrast, these two mutants did not affect the myosin-dependent localization to the equatorial division plane and to the cell tips. Replacing the endogenous cam1 with cam1-2V decreased the SPB localization of pericentrin Pcp1 by 69%, without changing the localization of either type V or I myosins. Over-expression of Pcp1 rescued the mitotic defects of cam1-2V cells at the restrictive temperature. Surprisingly, the cytokinesis of this cam1 mutant was largely normal. We concluded that fission yeast calmodulin Cam1 depends on Ca2+to be a component of SPBs, suggesting that calcium plays a critical role in the assembly of SPBs.

The yeast secreted proteome plays critical biological roles and influences product and production parameters in industrial fermentation. Systematic profiling of the response of the yeast secretome to intrinsic and extrinsic factors is therefore essential for understanding these functions and for optimizing manufacturing processes. Here, we characterized the yeast secretome under diverse proteosynthetic stress conditions, including glycosylation deficiency, oxidative, reductive, and thermal stresses. The secretome was predominantly composed of conventionally secreted proteins, while a subset of proteins appeared to be secreted via unconventional pathways. Distinct secretome profiles were observed in response to different stressors, driven by a combination of altered intracellular proteomes, altered canonical secretion, and altered cell lysis and unconventional protein secretion, while reflecting the underlying metabolic state of the cells. Heat stress did not impact protein glycosylation but did cause similar protein misfolding stress to N-glycosylation deficiency. Intriguingly, canonically intracellular chaperone BiP was abundant in the secretome in particular stress conditions where its activity would be beneficial. BiP interacted with probable extracellular client proteins in vitro, consistent with it acting as a functional extracellular chaperone/holdase in conditions such as reductive stress in which client proteins could be misfolded outside the cell.

Cells employ a bevy of transcriptional and post-translational stress responses to tolerate the burden of misfolded proteins induced by stress. In particular, the heat shock response facilitates the upregulation of molecular chaperones and protein remodeling factors that mediate proteostasis in response to accumulated misfolded proteins in the nucleus and cytosol. However, in response to stress neurons struggle to induce a canonical heat shock response, highlighting our poor understanding of how neurons maintain proteostasis. Specifically, the ability of post-mitotic respiring cells to regulate the heat shock response in comparison to their rapidly dividing, predominantly glycolytic counterparts has been under-studied. In this study, we employ yeast models that are easily manipulated to generate energy via glycolysis or mitochondrial respiration by changing the carbon source in the media. Using this model, we demonstrate that Hsf1 activity, the heat shock response and proteostasis are impaired in respiring cells. Interestingly, our data show that reduced Hsf1 activity regulates viability of respiring cells, with respiring cells poorly tolerating constitutively activated Hsf1. Finally, we describe alternative post-translational programming of the molecular chaperones Hsp70 and Hsp104 that plausibly enables respiring cells to mediate proteostasis despite a dampened heat shock response. Our findings offer new insights into possible proteostatic strategies employed by cells in different metabolic conditions.

The ISWI chromatin remodeler regulates nucleosome spacing using one of two ATPase subunits Snf2h (Smarca5) and Snf2l (Smarca1). While Snf2h stable knockout (KO) is known to markedly reduce genomic binding of CTCF, an architectural protein organizing the 3D genome, ISWIs role in regulating genomic binding and function of lineage-determining transcription factors (LDTFs) during cell fate transition remains largely unclear. Using conditional KO mice and derived cells, we show Snf2h and Snf2l are partially redundant and are required for embryonic development of muscle and adipose tissue as well as myogenesis and adipogenesis in culture. Stable KO of ISWI impairs LDTF-stimulated cell differentiation and disrupts de novo binding of the myogenic LDTF MyoD and the cBAF chromatin remodeler. Surprisingly, acute depletion of ISWI leaves de novo MyoD binding landscape largely intact while disrupting MyoD-dependent recruitment of cBAF and CTCF, with minimal effects on constitutive genomic binding of cBAF and CTCF. Together, our findings identify ISWI as an important mediator connecting LDTF binding to cBAF recruitment and chromatin organization during cell fate transition. Bullet points- ISWI ATPases Snf2h and Snf2l are partially redundant and essential for muscle and adipose development - ISWI is required for MyoD, C/EBP, and PPAR{gamma}-driven cell fate transition - Stable KO of ISWI disrupts genomic binding of MyoD, while acute depletion does not - Acute ISWI deletion disrupts MyoD-dependent, but not constitutive, genomic binding of cBAF and CTCF

Cartilage and bone that comprise craniofacial structures as well as neurons and glia of the peripheral nervous system are derived from a multipotent population of cranial neural crest cells, that respond to both cell intrinsic and extrinsic cues to differentiate into precise cell states. Both a genetic and epigenetic regulatory network are required for each step in the differentiation process, involving transcription factors, histone modifiers and chromatin remodelers. Here, we examined the direct transcriptional targets of two histone methyltransferases, Prdm3 and Prdm16 in zebrafish neural crest cells at 48 hours post fertilization in zebrafish. Using CUT&RUN, we examined both direct DNA binding and nucleosome association. At this stage of development, CUT&RUN fragment size analysis indicated that Prdm3 and Prdm16 are largely associated with nucleosomes. We further analyzed these nucleosome peak sets to identify 6 clusters where differential binding of Prdm3 and Prdm16 and differential enrichment of gene ontology terms for target genes was observed. We validated gene expression in each cluster by in situ hybridization chain reaction (HCR) at 48 hpf demonstrating that prdm3 and prdm16 mutants exhibit corresponding changes in gene expression of the putative gene targets identified. Finally, we performed CUT&RUN-qPCR in prdm3 and prdm16 mutant zebrafish embryos and demonstrated reduced binding at putative target loci. Together these data suggest that Prdm3 and Prdm16 regulate their transcriptional targets primarily by binding nucleosomes around their putative target loci to control downstream gene expression. HighlightsPrdm3 and Prdm16 associate with nucleosomes for regulation of gene expression Gene targets are altered in prdm3 and prdm16 mutant zebrafish Reduced binding is observed in respective mutants

Stem cell differentiation depends on transcription factors that are often encoded by mRNAs with highly conserved 3'UTRs. To determine their functional roles, we performed 3'UTR loss-of-function studies. Partial deletion of endogenous 3'UTRs altered stem cell differentiation efficiency in 7/10 cases. As 6/7 3'UTR deletions did not affect expression level of the encoded proteins, we reveal widespread abundance-independent regulatory roles of 3'UTRs. For example, 3'UTR deletion of CTNNB1, an mRNA that encodes the essential Wnt co-activator {beta}-catenin, keeps {beta}-catenin levels unaffected but impairs zebrafish embryogenesis and induction of the Wnt transcriptional program during human stem cell differentiation. We show that long intermolecular 3'UTR-3'UTR interactions between Wnt transcription factor mRNAs and CTNNB1 enable co-translational protein complex assembly of these transcription factors with {beta}-catenin. As antisense oligonucleotide-mediated blocking of 3'UTR interactions impairs Wnt program induction, our findings indicate that transcriptional regulators can form functional units during protein biogenesis to be fully active.

Background & AimsHepatocellular carcinoma (HCC) frequently exhibits resistance to immune checkpoint inhibitors (ICIs), particularly in {beta} -catenin-driven tumors characterized by immune exclusion. While the Unfolded Protein Response (UPR) and the Integrated Stress Responses (ISR) enable tumor adaptation to metabolic stress their role in shaping tumor immunogenicity remains incompletely understood. We investigated whether ATF4, a central effector of the integrated stress response, couples metabolic reprogramming to suppression of anti-tumor immunity in HCC. MethodsWe combined transcriptomic analyses across three independent human HCC cohorts with mechanistic studies using an immunotherapy-resistant MYC/{beta}-catenin-driven murine HCC model. We integrated CRISPR/Cas9-mediated deletion of Atf4 with RNA-sequencing and targeted metabolomics. The impact of tumor-derived metabolites on macrophage differentiation and polarization was evaluated using primary bone marrow-derived cells. Therapeutic responses were evaluated in orthotopic and subcutaneous models treated with anti-PD-1 and anti-VEGFA. ResultsATF4 and XBP1 transcriptional signatures are selectively enriched in human HCC and associate with poor prognosis, vascular invasion, and an immunosuppressive myeloid-enriched tumor microenvironment. Genetic ablation of Atf4 markedly suppressed tumor growth in immunocompetent but not immunodeficient hosts, establishing a requirement for immune-mediated tumor control. Mechanistically, Atf4 loss downregulated Aldh18a1 and disrupted proline biosynthesis, resulting in extracellular proline depletion. This proline-deficient environment abrogated monocyte-to-macrophage differentiation and decreased M2 polarization, thereby reshaping the tumor microenvironment toward enhanced T cell infiltration and activation. Functionally, Atf4-deficient tumors exhibited restored sensitivity to anti-PD-1 monotherapy and showed pronounced responses to combined anti-PD-1/anti-VEGFA treatment in aggressive orthotopic models. ConclusionATF4 programs a proline-dependent metabolic axis that sustains macrophage-mediated immunosuppression and immune evasion in {beta}-catenin-driven HCC. Disruption of this pathway converts immune-excluded tumors into T cell-inflamed states and restores responsiveness to immunotherapy. By governing proline homeostasis and macrophage-mediated immunosuppression, ATF4 is a key metabolic checkpoint for immune evasion, linking stress adaptation to immune escape and a candidate therapeutic target in HCC. Impact and implicationsWe identify ATF4 as a crucial metabolic-immune orchestrator that sustains myeloid-driven immune evasion in {beta}-catenin-dependent HCC through proline-dependent circuitry. Disrupting the ATF4-proline axis converts immune-desert tumors into T cell-inflamed lesions by blocking macrophage differentiation, thereby sensitizing tumors to immune checkpoint therapy. This work positions ATF4 as a tractable therapeutic target to overcome immunotherapy resistance in HCC. Graphical abstract Highlights- ATF4 orchestrates an immunosuppressive tumor microenvironment in HCC by coupling metabolic stress adaptation to immune evasion. - Ablation of ATF4 disrupts proline biosynthesis, leading to a marked depletion of extracellular proline. - Cancer cell-derived proline availability contributes to macrophage differentiation and M2 polarization; its loss restores T cell-mediated anti-tumor surveillance and sensitizes beta-catenin-driven HCC to immune checkpoint blockade.

Phosphatidylinositol (4,5)-bisphosphate (PIP2), the most abundant cellular poly-phosphoinositide (PPI) class of phospholipid, is a central plasma membrane (PM)-associated signaling hub that controls many cellular processes. In this study, we demonstrate that either deletion of the gene encoding actin-binding protein profilin1 (Pfn1) or disruption of Pfn1-actin interaction leads to downregulation of PM PIP2 content in cells. This is also phenocopied when F-actin is depolymerized implying that Pfn1-dependent PIP2 alteration is related to its actin-regulatory function. Phospholipase C (PLC) activity is critical for Pfn1-deficient cells to exhibit the PIP2-related phenotype. These findings, taken together with biochemical signatures of elevated PIP2 hydrolysis (higher baseline PM diacylglycerol-to PIP2 ratio and protein kinase C activity) exhibited by Pfn1-deficient cells, imply that PLC-mediated PIP2 hydrolysis plays a role in Pfn1-dependent regulation of PM PIP2. Furthermore, we unexpectedly found that Pfn1 loss leads to dramatic alterations in several other important forms of lipids, revealing a previously unrecognized role of Pfn1 as a broad regulator of cellular lipid environment that extends beyond PPI control. In conclusion, our study establishes Pfn1 as an important regulator of cellular lipid homeostasis. SUMMARY STATEMENTThis study uncovers a mechanism of how functional loss of Profilin1, a key regulator of actin cytoskeleton, can trigger downregulation of plasma membrane content of PIP2, an important class of phospholipid, in cells.

Osteocytes and adipocytes represent cells with disparate functions. Osteocytes regulate bone metabolism (remodeling) and bone homeostasis, while adipocytes regulate energy metabolism and energy storage. Here, we demonstrate that osteocyte phenotype consists of adipocytic features which are under control of peroxisome proliferator-activated receptor gamma (PPARG), a master regulator of adipocyte differentiation and function. Using a mouse model with osteocyte-specific deletion of PPARG (OT{gamma}KO) and osteocyte cellular model of MLO-Y4 cells edited with CRISPR/Cas9 for PPARG deficiency, we are demonstrating that under PPARG control osteocytes produce and secrete adiponectin (ADIPOQ), and they are equipped in adipocyte-specific mechanisms for lipid-storage and their metabolism. Under PPARG, osteocytes accumulate lipid droplets which correlate with their capability to cover up to 20% of energy requirements from fatty acids metabolism. Although osteocytes like osteoblasts mainly express perilipin 2 (Plin2), however similarly to adipocytes, lipid droplets accumulation is associated with expression of perilipin 1 (Plin1) under PPARG control. Similarly, lipids accumulation and metabolism involve adipocyte-specific activities including fatty acids binding protein 4 (Fabp4), hormone-specific lipase (Hsl) and adipocyte-specific triglyceride lipase (Atgl), which expression are under PPARG control. These studies provide a new understanding of osteocyte biology which include adipocyte-like endocrine and lipid metabolism features probably reflecting an adaptation to their unique localization and a need for a maintenance of functional fitness in these conditions. They deepen our comprehension of the crossroads of osteocyte and adipocyte function and underscore the therapeutic potential of targeting common molecular pathways in both cell types for managing metabolic disorders and skeletal diseases.

How enhancers transmit their activation signals to target promoters is a fundamental unanswered question. Gene activation by distal enhancers is affected by the three-dimensional organisation of chromatin driven by cohesin-mediated loop extrusion, but how the dynamic loop extrusion cycle shapes transcriptional outcomes is not fully understood. In addition, genes are typically regulated by multiple enhancers and cases of cooperativity between enhancers have been observed that depend on the relative position of enhancers and gene. The reason for such context-dependent cooperativity is unknown, and whether it is linked to loop extrusion has not been explored. Here, we use synthetic gene activation to show that NIPBL depletion, which decreases loop extrusion rate, reduces long-range activation in a dose-dependent manner. In contrast, WAPL depletion, which reduces cohesin turnover, can both increase and decrease distal gene activation depending on the presence of adjacent CTCF binding sites. These effects on gene activation correlate with the impact on chromatin contacts. Depletion of the co-activator BRD4 also affects long-range activation, but not by changing chromatin contacts. Synthetic activation from two locations results in super-additive responses, which can be predicted by an independently determined response model. These predictions hold for dual activation from proximal and/or distal sites, and with perturbed NIPBL, WAPL, or BRD4. Distal activation therefore appears to be equivalent in nature to proximal activation, and context-dependent cooperativity can arise simply from different level of activation inputs operating on a non-linear response function.

Polycomb Repressive Complex 2 (PRC2) maintains repression of genes specific for other cell differentiation states. PRC2 binds RNA in vitro with a preference for G-rich sequences. UV-based crosslinking coupled with immunoprecipitation (CLIP) experiments have shown that PRC2 also binds RNA in cells. Recently, Guo et al reported that a stringent denaturing variant of CLIP called CLAP did not detect PRC2 RNA binding in cells. We present a reanalysis of CLAP data that supports direct interaction of PRC2 with RNA in cells. CLAP for Halo-tagged PRC2 subunits from mixed populations of human and mouse cells specifically enriched for RNA from the species in which the proteins were tagged. The lack of apparent PRC2 RNA binding in Guo and colleagues analysis stems from a scaling step that deflates enrichment scores for low-complexity CLAP samples. Our findings pave the way for studies seeking to determine the physiological roles of PRC2 RNA binding activity.

The tight regulation of iron homeostasis is of great importance for cellular health. An increase in intracellular iron levels results in the formation of free radicals, which damages macromolecules and membranes, eventually resulting in cell death by Ferroptosis. Recently, we showed that patients with mutations in VPS41 display a severe neurodegenerative phenotype with iron deposition in the brain. VPS41 is well known as subunit of the HOPS complex required for fusion of late endosomes and autophagosomes with lysosomes. However, VPS41 has also been identified as inhibitor of Ferroptosis and regulator of redox homeostasis. How VPS41 exerts these functions and if these are dependent on the HOPS complex is unknown. Here we show that depletion of VPS41 results in increased intracellular iron levels, ROS formation and mitochondrial fission. Our findings indicate an important role for VPS41 in the regulation of iron homeostasis and mitochondrial fission and suggest Ferroptosis as a possible cause for neurodegeneration in VPS41 patients.

The RNA polymerase-associated factor 1 complex (PAF1C) is an evolutionarily conserved transcription elongation complex that regulates RNA polymerase II-mediated transcription and chromatin modification. LEO1, a core subunit of PAF1C, has been implicated in developmental gene regulation, WNT signaling, and leukemogenesis; however, its role in solid tumor progression remains poorly understood. In this study, we found that although LEO1 expression is generally elevated in colorectal cancer (CRC), its expression is reduced in stage IV tumors and is associated with poor clinical outcomes. To investigate its function, we established LEO1 -deficient HCT116 cell line and performed transcriptomic analyses. Loss of LEO1 suppressed epithelial differentiation and developmental gene programs while inducing cell cycle delay. Despite these changes, LEO1-deficient cells exhibited aggressive phenotypes, including enlarged nuclei and increased expression of migration-associated genes, which were further enhanced under glucose deprivation. Motif analysis identified FOXM1 as a key regulator of these migration-related genes. Mechanistically, LEO1 deficiency promoted accelerated transcriptional activation of GRP78, a central regulator of endoplasmic reticulum (ER) stress adaptation. GRP78 was required for survival under ER stress conditions, and its inhibition suppressed both migration and migration-associated gene expression. In addition, transcriptomic analyses revealed upregulation of cholesterol metabolism-related genes in LEO1-deficient cells. Consistently, treatment with the HMG-CoA reductase inhibitor atorvastatin selectively impaired their survival, indicating cholesterol metabolic dependency. Collectively, these findings demonstrate that LEO1 loss promotes ER stress-adapted migration and cholesterol metabolic dependency in CRC, suggesting that these pathways may represent therapeutic vulnerabilities in metastatic LEO1-low CRC.

Cadmium, being a highly toxic metal, perturbs cellular homeostasis by forming stable complexes with numerous thiol-active proteins, ultimately leading to severe liver and lung damage. Despite its well-documented toxicity, the molecular mechanisms governing cadmium export remain poorly understood. Given the chemical similarity between cadmium and copper, we investigated whether the canonical copper-exporting ATPases, ATP7A and ATP7B participate in cadmium handling. Upon Cd treatment in hepatocytes, ATP7B undergoes trafficking to lysosomes via the retromer complex, as also observed in the case of elevated copper, accompanied by the upregulation of acidic lysosomal populations. In contrast, ATP7A expressed in lung adenocarcinoma cells, though exhibit vesicular redistribution upon Cd exposure, does not mediate lysosomal sequestration, suggesting distinct deployment of late secretory pathways by the two copper ATPases in response to cadmium. We have also observed that ATP7B-/- hepatocytes exhibit increased sensitivity to Cd exposure compared to wild-type cells. Whereas, overexpressing the ATP7B amino-terminal copper-binding domain in bacteria alleviates cadmium-induced stress, indicating its capacity to sequester Cd. Caenorhabditis elegans lacking copper-ATPase cua-1, displayed increased Cd sensitivity, while mutants (glo-1-/-), deficient in lysosome-related organelles (LRO), and (lmp-1-/-), deficient in lysosomal membrane glycoprotein, showed reduced resistance to cadmium toxicity. Treatment of the worm with cadmium increases the abundance of lysosomes marked by elevation in lysosomal biogenesis and functional genes, reinforcing the importance of lysosomal pathways in cadmium detoxification. To summarise, we delineated the non-canonical role of copper ATPases and lysosomes in cadmium-induced cellular toxicity.

While many studies of developmental control have focused on gene activation, less is known about the extent to which regulatory programs are actively repressed in progenitor cells. We previously showed that trimethylation of histone H3 at lysine 9 (H3K9me3) is a repressive mark that is remodeled on protein-coding genes when endodermal progenitors transition to liver and pancreatic {beta} cell fates. Yet whether H3K9me3 is dynamic at promoters and enhancers has not been determined. Here we find that promoters of liver-specific genes are strongly enriched for H3K9me3 in undifferentiated progenitors, whereas such enrichment is not observed at promoters of more broadly expressed liver genes. We further show that enhancers specific to differentiated tissues--including liver, islet, and cerebral cortex--are strongly enriched for H3K9me3 in their corresponding tissue stem and progenitor cells. In hepatoblasts, H3K9me3 contributes to maintaining the undifferentiated state by restricting FOXA2 and HNF4 from binding to most enhancers, while there remain thousands of H3K9me3-marked enhancers where the factors are not restricted from binding. Our findings illustrate how H3K9me3-mediated heterochromatinization can restrict transcription factor engagement in progenitor cells to prevent inappropriate activation during early development. H3K9me3 at enhancers that allow transcription factor binding may reflect developmental competence.

Conflicting roles have been proposed for the E. coli protein RatA. Initially described as a ribosome targeting toxin, a later report pronounced it the bacterial homologue to the inner mitochondrial membrane protein Coq10. Coq10 proteins are conserved from prokaryotes to human and implicated to serve a lipid chaperone role in the biosynthesis of ubiquinone, a crucial electron carrier during aerobic respiration. We recently identified that the contradictory results published for RatA can be attributed to a mis-annotation of the gene in the reference genome. Here, we further elucidate the molecular function of RatA. We clarify that RatA is not a toxin but serves as a lipid shuttle for ubiquinone from its cytosolic biosynthesis complex to the inner membrane. Furthermore, we show that the loss of RatA results in an impaired, but not abolished electron transport chain and demonstrate broad metabolic adaptations of the cells as a consequence. Therefore, we propose to rename RatA to UbiM to reflect its function and to be in accordance with the naming convention of other ubiquinone biosynthesis proteins.

Gene-by-environment (GxE) interactions play a major role in shaping both phenotypic and molecular variation, with important implications for human health and disease. In this study, we used the Doxycycline (Dox) regulated, tetracycline-responsive (Tet-Off) promoter system to sequentially reduce or titrate gene expression levels of the essential yeast transcription factor Repressor Activator Protein 1 (RAP1) similar to a hypomorph allele series, across three distinct environments: Yeast Peptone Dextrose (YPD) media, YPD media with Heat Shock (HS), and Yeast Peptone Acetate (YPAC) media. We then performed RNA sequencing (RNA Seq) to assess global transcriptional responses to RAP1 reduction in these different growth environments. Our analysis first focused on the independent effects of varying RAP1 expression levels within and across environments. We then explored GxE interactions, revealing a subset of genes with significant consequences of reduced levels of RAP1 and environment-specific expression patterns. Notably, many genes exhibited opposite effects of RAP1 titration on gene expression when yeast were grown in YPAC media compared to YPD media and/or HS, suggesting environment-dependent regulatory architecture. This design reveals how cells integrate internal transcriptional and regulatory changes with external environmental cues, providing a deeper view of GxE architecture. Using Weighted Gene Co-expression Network Analysis (WGCNA), we identified co-regulated gene modules, and by combining this with transcription factor motif enrichment tests, our study identified candidate regulators driving their dynamics. Our findings demonstrate that gene regulatory networks can vary dramatically depending on the environmental context an organism experiences, which can then influence the specific phenotypes produced by a particular genetic perturbation. This illustrates the complexity of genotype-environment interactions and the importance of studying gene function in multiple environments to gain a truly comprehensive understanding of a genes sometimes numerous and diverse functions.

Centrosome dysfunction is linked to developmental disorders affecting brain and body size, including microcephaly and primordial dwarfism. However, the cellular mechanisms underlying these rare conditions remain poorly understood. In this study, we investigate a rare variant of the centrosome-associated protein Pericentrin, which was discovered in a single family with Majewski/microcephalic osteodysplastic primordial dwarfism type II (MOPD II). Unlike the majority of pathogenic PCNT variants that cause severe protein truncation, the p.Lys3154del variant ({Delta}K3154) involves a single amino acid deletion in the proteins only conserved functional domain, providing a unique opportunity to explore PCNT function in MOPD II. To model PCNT{Delta}K3154, we examined the effects of Drosophila Pericentrin-like protein (PLP) carrying an orthologous deletion (Plp{Delta}R). Our results show that plp{Delta}R animals exhibit smaller tissues that recapitulate MOPD II phenotypes. Behavioral assays revealed defects in climbing and mechanosensation, suggesting impaired sensory cilia function. We also found that Plp{Delta}R cells exhibit accelerated mitosis, increased apoptosis, and reduced pericentriolar material recruitment. In silico structural modeling, yeast two-hybrid, and co-immunoprecipitation experiments show that Plp{Delta}R produces a protein that disrupts PLP dimerization and PLP interaction with Asterless, another centrosome protein. Overall, modeling the human MOPD II patient variant PCNT{Delta}K3154 in Drosophila reveals how a single amino acid deletion affects biological processes from the molecular level to the organismal level. Our work offers new insights into the defective cellular mechanisms underlying MOPD II in patients with the PCNT{Delta}K3154 variant, potentially linking the etiology of the disease in these individuals to the loss of a single protein-protein interaction.