Sharing the wealth: The versatility of proteins targeted to peroxisomes and other organelles

Mpf1 affects the dual distribution of tail-anchored proteins between mitochondria and peroxisomes

Most cellular proteins require targeting to a distinct cellular compartment to function properly. A subset of proteins is distributed to two or more destinations in the cell and little is known about the mechanisms controlling the process of dual/multiple targeting. Here, we provide insight into the mechanism of dual targeting of proteins between mitochondria and peroxisomes. We perform a high throughput microscopy screen in which we visualize the location of the model tail-anchored proteins Fis1 and Gem1 in the background of mutants in virtually all yeast genes. This screen identifies three proteins, whose absence results in a higher portion of the tail-anchored proteins in peroxisomes: the two paralogues Tom70, Tom71, and the uncharacterized gene YNL144C that we rename mitochondria and peroxisomes factor 1 (Mpf1). We characterize Mpf1 to be an unstable protein that associates with the cytosolic face of the mitochondrial outer membrane. Furthermore, our study uncovers a unique contribution of Tom71 to the regulation of dual targeting. Collectively, our study reveals, for the first time, factors that influence the dual targeting of proteins between mitochondria and peroxisomes.

The roles of IDH2 and glutathione metabolism in cetuximab resistance in head and neck squamous cell carcinoma investigated by metabolomics and transcriptomics

Cetuximab resistance is a significant challenge in the treatment of head and neck squamous cell carcinoma (HNSCC). In this study, cetuximab-resistant HNSCC cell lines were established, and untargeted metabolomics was used to detect differences in metabolite profiles between sensitive and resistant cell lines. It was found that glutathione metabolism significantly differed between the sensitive and resistant lines. Combining these findings with transcriptome data, correlation analysis of metabolites revealed that IDH2 regulated glutathione metabolism and contributed to cetuximab resistance in FaDu cells. In vitro experiments showed that IDH2 was highly expressed in FaDu-CR cells, and IDH2 knockdown significantly enhanced the sensitivity of FaDu and FaDu-CR cells to cetuximab. IDH2 knockdown reduced GSH levels and GPX4 expression in FaDu and FaDu-CR cells under cetuximab treatment, while increasing lipid ROS levels. In vivo experiments demonstrated that IDH2 knockdown decreased the tumorigenic ability of FaDu-CR cells in nude mice treated with cetuximab, as well as reduced GPX4 and Ki67 levels in tumor tissues. In conclusion, IDH2 regulated glutathione metabolism and contributed to cetuximab resistance in HNSCC. This study explores strategies to ameliorate cetuximab resistance in HNSCC preclinical models, providing new insights for reversing cetuximab resistance in HNSCC.

Peroxisomal core structures segregate diverse metabolic pathways

Peroxisomes are single membrane-bounded oxidative organelles with various metabolic functions including β-oxidation of fatty acids. Peroxisomes of many species confine certain metabolic enzymes into sub-compartments sometimes visible as electron dense cores. Why these structures form is largely unknown. Here, we report that in the smut fungus Ustilago maydis detergent resistant core structures are enriched for different enzymes excluding several key enzymes of the β-oxidation pathway. This confinement contributes to generation of peroxisome subpopulations that differ in their enzyme content. We identify short amino acid motifs necessary and sufficient for protein self-assembly into aggregates in vitro. The motifs trigger enrichment in cores in vivo and are active in mammalian cells. Perturbation of core assembly via variation of such motifs affects peroxisome function in U. maydis strains challenged with fatty acids. Thus, protein core structures serve to compartmentalize the lumen of peroxisomes thereby preventing interference of biochemical reactions. Metabolic compartmentalization of peroxisomes via assembly of specific proteins may occur in other organisms as well.

A modified procedure for separating yeast peroxisomes from mitochondria

Mitochondria and peroxisomes are mutually dependent organelles that share several membrane proteins that carry out the same function in both organelles. To study the unique features of these dually localized proteins in each of the two organelles, it is essential to separate mitochondria from peroxisomes. Isolating organelles from cells of Baker's yeast, Saccharomyces cerevisiae, is crucial for our understanding of the biogenesis and functions of proteins. Traditionally, subcellular fractionation and isolation of individual organelles by differential centrifugation benefit from the specific and unique density of each organelle. However, when yeast cells are grown under normal conditions, certain organelles like mitochondria and peroxisomes share strikingly similar densities. This similarity challenges the separation of these organelles from one another. In this chapter, we describe an optimized procedure to address this task. We depict growth conditions that would favor stimulation of peroxisomes to increase their number and density, and portray organellar isolation followed by gradient centrifugation, enabling an improved separation of both organelles. Additionally, we illustrate the advantage of the procedure to study the dual localization of the membrane protein Fis1.

Imbalanced mitochondrial dynamics contributes to the pathogenesis of X-linked adrenoleukodystrophy

The peroxisomal disease adrenoleukodystrophy (X-ALD) is caused by loss of the transporter of very-long-chain fatty acids (VLCFAs), ABCD1. An excess of VLCFAs disrupts essential homeostatic functions crucial for axonal maintenance, including redox metabolism, glycolysis and mitochondrial respiration. As mitochondrial function and morphology are intertwined, we set out to investigate the role of mitochondrial dynamics in X-ALD models. Using quantitative 3D transmission electron microscopy, we revealed mitochondrial fragmentation in corticospinal axons in Abcd1- mice. In patient fibroblasts, an excess of VLCFAs triggers mitochondrial fragmentation through the redox-dependent phosphorylation of DRP1 (DRP1S616). The blockade of DRP1-driven fission by the peptide P110 effectively preserved mitochondrial morphology. Furthermore, mRNA inhibition of DRP1 not only prevented mitochondrial fragmentation but also protected axonal health in a Caenorhabditis elegans model of X-ALD, underscoring DRP1 as a potential therapeutic target. Elevated levels of circulating cell-free mtDNA in patients' CSF align this leukodystrophy with primary mitochondrial disorders. Our findings underscore the intricate interplay between peroxisomal dysfunction, mitochondrial dynamics and axonal integrity in X-ALD, shedding light on potential avenues for therapeutic intervention.

The Hansenula polymorpha mitochondrial carrier family protein Mir1 is dually localized at peroxisomes and mitochondria

Peroxisomes are ubiquitous cell organelles involved in various metabolic pathways. In order to properly function, several cofactors, substrates and products of peroxisomal enzymes need to pass the organellar membrane. So far only a few transporter proteins have been identified. We analysed peroxisomal membrane fractions purified from the yeast Hansenula polymorpha by untargeted label-free quantitation mass spectrometry. As expected, several known peroxisome-associated proteins were enriched in the peroxisomal membrane fraction. In addition, several other proteins were enriched, including mitochondrial transport proteins. Localization studies revealed that one of them, the mitochondrial phosphate carrier Mir1, has a dual localization on mitochondria and peroxisomes. To better understand the molecular mechanisms of dual sorting, we localized Mir1 in cells lacking Pex3 or Pex19, two peroxins that play a role in targeting of peroxisomal membrane proteins. In these cells Mir1 only localized to mitochondria, indicating that Pex3 and Pex19 are required to sort Mir1 to peroxisomes. Analysis of the localization of truncated versions of Mir1 in wild-type H. polymorpha cells revealed that most of them localized to mitochondria, but only one, consisting of the transmembrane domains 3-6, was peroxisomal. Peroxisomal localization of this construct was lost in a MIR1 deletion strain, indicating that full-length Mir1 was required for the localization of the truncated protein to peroxisomes. Our data suggest that only full-length Mir1 sorts to peroxisomes, while Mir1 contains multiple regions with mitochondrial sorting information. Data are available via ProteomeXchange with identifier PXD050324.

Mitochondria, Peroxisomes and Beyond-How Dual Targeting Regulates Organelle Tethering

Eukaryotic cells feature distinct membrane-enclosed organelles such as mitochondria and peroxisomes, each playing vital roles in cellular function and organization. These organelles are linked at membrane contact sites, facilitating interorganellar molecule and ion exchange. Most contact-forming proteins identified to date are membrane proteins or membrane-associated proteins, which can form very stable contacts. Recent findings suggest additional mechanistically distinct tethering events that arise from dual protein targeting. Proteins bearing targeting signals for multiple organelles, such as an N-terminal signal for mitochondria and a C-terminal signal for peroxisomes, function as tethers, fostering contacts by engaging targeting factors at both organelles. A number of dually targeted membrane proteins can contribute to contact site formation and transit from one organelle to the other as well. These interactions may enable the fine-tuning of organelle proximity, hence, adapting connections to meet varying physiological demands.