A Golgi rhomboid protease Rbd2 recruits Cdc48 to cleave yeast SREBP

Physico-chemical features and functional relevance of tomato rhomboid proteases

In plants, regulated intramembrane proteolysis (RIP) is crucial for proper growth, development, and stress management. Rhomboid proteases (RPs) residing in the membrane play a vital role in orchestrating RIP. Although RPs can be found in most sequenced genomes, tomato rhomboids (SlRPs) have not yet been studied. Using alternative and comprehensive strategies, we found ten SlRPs encoded in the tomato genome. These SlRPs possess signature motifs and transmembrane domains, showing structural similarity to other members of the RP family. Also, SlRPs are genetically related to other known RPs of the Solanaceae family. Seven of the SlRPs retain serine-histidine catalytic dyads, making them proteolytically active, while three iRhoms lack the dyad and other structural motifs. Although SlRPs could have functional redundancy, their distribution and expression pattern indicate tissue specificity and responsiveness to specific external stimuli. The presence of development and stress-response-related cis-elements in the promoters of SlRPs supports this view. Furthermore, our strategically designed substrate-reporter assay shows that SlRPs have proteolytic activity similar to that of known RPs. This study provides a detailed understanding of all SlRPs and their physico-chemical features, shedding light on their involvement in physiological processes.

Proximity Labeling Expansion Microscopy (PL-ExM) resolves structure of the interactome

Elucidating the spatial relationships within the protein interactome is pivotal to understanding the organization and regulation of protein-protein interactions. However, capturing the 3D architecture of the interactome presents a dual challenge: precise interactome labeling and super-resolution imaging. To bridge this gap, we present the Proximity Labeling Expansion Microscopy (PL-ExM). This innovation combines proximity labeling (PL) to spatially biotinylate interacting proteins with expansion microscopy (ExM) to increase imaging resolution by physically enlarging cells. PL-ExM unveils intricate details of the 3D interactome's spatial layout in cells using standard microscopes, including confocal and Airyscan. Multiplexing PL-ExM imaging was achieved by pairing the PL with immunofluorescence staining. These multicolor images directly visualize how interactome structures position specific proteins in the protein-protein interaction network. Furthermore, PL-ExM stands out as an assessment method to gauge the labeling radius and efficiency of different PL techniques. The accuracy of PL-ExM is validated by our proteomic results from PL mass spectrometry. Thus, PL-ExM is an accessible solution for 3D mapping of the interactome structure and an accurate tool to access PL quality.

Rhomboid protease RHBDL4/RHBDD1 cleaves SREBP-1c at endoplasmic reticulum monitoring and regulating fatty acids

The endoplasmic reticulum (ER)-embedded transcription factors, sterol regulatory element-binding proteins (SREBPs), master regulators of lipid biosynthesis, are transported to the Golgi for proteolytic activation to tune cellular cholesterol levels and regulate lipogenesis. However, mechanisms by which the cell responds to the levels of saturated or unsaturated fatty acids remain underexplored. Here, we show that RHBDL4/RHBDD1, a rhomboid family protease, directly cleaves SREBP-1c at the ER. The p97/VCP, AAA-ATPase complex then acts as an auxiliary segregase to extract the remaining ER-embedded fragment of SREBP-1c. Importantly, the enzymatic activity of RHBDL4 is enhanced by saturated fatty acids (SFAs) but inhibited by polyunsaturated fatty acids (PUFAs). Genetic deletion of RHBDL4 in mice fed on a Western diet enriched in SFAs and cholesterol prevented SREBP-1c from inducing genes for lipogenesis, particularly for synthesis and incorporation of PUFAs, and secretion of lipoproteins. The RHBDL4-SREBP-1c pathway reveals a regulatory system for monitoring fatty acid composition and maintaining cellular lipid homeostasis.

The Role of the Rhomboid Superfamily in ER Protein Quality Control: From Mechanisms and Functions to Diseases

The endoplasmic reticulum (ER) is an essential organelle in eukaryotic cells and is a major site for protein folding, modification, and lipid synthesis. Perturbations within the ER, such as protein misfolding and high demand for protein folding, lead to dysregulation of the ER protein quality control network and ER stress. Recently, the rhomboid superfamily has emerged as a critical player in ER protein quality control because it has diverse cellular functions, including ER-associated degradation (ERAD), endosome Golgi-associated degradation (EGAD), and ER preemptive quality control (ERpQC). This breadth of function both illustrates the importance of the rhomboid superfamily in health and diseases and emphasizes the necessity of understanding their mechanisms of action. Because dysregulation of rhomboid proteins has been implicated in various diseases, such as neurological disorders and cancers, they represent promising potential therapeutic drug targets. This review provides a comprehensive account of the various roles of rhomboid proteins in the context of ER protein quality control and discusses their significance in health and disease.

AAA-ATPase valosin-containing protein (VCP) binds the transcription factor SREBP1 and promotes its proteolytic activation by rhomboid protease RHBDL4

Valosin-containing protein (VCP) is a member of AAA-ATPase superfamily involved in various cellular functions. To investigate the pathophysiological role of VCP in metabolic disorders, we generated knock-in mice bearing an A232E mutation in VCP, a known human VCP pathogenic variant. When heterozygous mutant mice (A232E/+) were fed a high-fat diet, we observed that fatty liver was ameliorated and the proteolytic processing of the transcription factor sterol regulatory element-binding protein 1 (SREBP1) was impaired. Further co-immunoprecipitation analysis in wildtype mice revealed interactions of VCP with SREBP1 and a rhomboid protease, RHBDL4, in the liver, and these interactions were attenuated in A232E/+ mice. Consistent with these results, we show that knockdown or chemical inhibition of VCP or RHBDL4 in human hepatocytes impaired the proteolytic processing of SREBP1. Finally, we found that knockdown of E3 ligases such as glycoprotein 78 and HMG-CoA reductase degradation protein 1 disrupted the interaction of VCP with SREBP1 and impaired the proteolytic processing of SREBP1. These results suggest that VCP recognizes ubiquitinylated SREBP1 and recruits it to RHBDL4 to promote its proteolytic processing. The present study reveals a novel proteolytic processing pathway of SREBP1 and may lead to development of new therapeutic strategies to treat fatty liver diseases.

Maintaining Golgi Homeostasis: A Balancing Act of Two Proteolytic Pathways

The Golgi apparatus is a central hub for cellular protein trafficking and signaling. Golgi structure and function is tightly coupled and undergoes dynamic changes in health and disease. A crucial requirement for maintaining Golgi homeostasis is the ability of the Golgi to target aberrant, misfolded, or otherwise unwanted proteins to degradation. Recent studies have revealed that the Golgi apparatus may degrade such proteins through autophagy, retrograde trafficking to the ER for ER-associated degradation (ERAD), and locally, through Golgi apparatus-related degradation (GARD). Here, we review recent discoveries in these mechanisms, highlighting the role of the Golgi in maintaining cellular homeostasis.

SUBCELLULAR TRANSCRIPTOMICS & PROTEOMICS: A COMPARATIVE METHODS REVIEW

The internal environment of cells is molecularly crowded, which requires spatial organization via subcellular compartmentalization. These compartments harbor specific conditions for molecules to perform their biological functions, such as coordination of the cell cycle, cell survival, and growth. This compartmentalization is also not static, with molecules trafficking between these subcellular neighborhoods to carry out their functions. For example, some biomolecules are multifunctional, requiring an environment with differing conditions or interacting partners, and others traffic to export such molecules. Aberrant localization of proteins or RNA species has been linked to many pathological conditions, such as neurological, cancer, and pulmonary diseases. Differential expression studies in transcriptomics and proteomics are relatively common, but the majority have overlooked the importance of subcellular information. In addition, subcellular transcriptomics and proteomics data do not always colocate because of the biochemical processes that occur during and after translation, highlighting the complementary nature of these fields. In this review, we discuss and directly compare the current methods in spatial proteomics and transcriptomics, which include sequencing- and imaging-based strategies, to give the reader an overview of the current tools available. We also discuss current limitations of these strategies as well as future developments in the field of spatial -omics.

•

Subcellular information of protein and RNA give insights into molecular function.

•

This review discusses strategies available to measure subcellular information.

•

Hybridization of methods shows promise for exploring the composition of organelles.

•

Advances are aiding understanding of the organisation and dynamics of cells.

Proteins associated with the Arabidopsis thaliana plastid rhomboid-like protein RBL10

Rhomboid-like proteins are intramembrane proteases with a variety of regulatory roles in cells. Though many rhomboid-like proteins are predicted in plants, their detailed molecular mechanisms or cellular functions are not yet known. Of the 13 predicted rhomboids in Arabidopsis thaliana, one, RBL10, affects lipid metabolism in the chloroplast, because in the respective rbl10 mutant the transfer of phosphatidic acid through the inner envelope membrane is disrupted. Here we show that RBL10 is part of a high-molecular-weight complex of 250 kDa or greater in size. Nine likely components of this complex are identified by two independent methods and include Acyl Carrier Protein 4 (ACP4) and Carboxyltransferase Interactor1 (CTI1), which have known roles in chloroplast lipid metabolism. The acp4 mutant has decreased C16:3 fatty acid content of monogalactosyldiacylglycerol, similar to the rbl10 mutant, prompting us to offer a mechanistic model of how an interaction between ACP4 and RBL10 might affect chloroplast lipid assembly. We also demonstrate the presence of a seventh transmembrane domain in RBL10, refining the currently accepted topology of this protein. Taken together, the identity of possible RBL10 complex components as well as insights into RBL10 topology and distribution in the membrane provide a stepping-stone towards a deeper understanding of RBL10 function in Arabidopsis lipid metabolism.

The SREBP (Sterol Regulatory Element-Binding Protein) pathway: a regulatory bridge between carotenogenesis and sterol biosynthesis in the carotenogenic yeast Xanthophyllomyces dendrorhous

Xanthophyllomyces dendrorhous is a basidiomycete yeast that naturally produces the red–orange carotenoid astaxanthin, which has remarkable antioxidant properties. The biosynthesis of carotenoids and sterols share some common elements that have been studied in X. dendrorhous. For example, their synthesis requires metabolites derived from the mevalonate pathway and in both specific pathways, cytochrome P450 enzymes are involved that share a single cytochrome P450 reductase, CrtR, which is essential for astaxanthin biosynthesis, but is replaceable for ergosterol biosynthesis. Research on the regulation of carotenoid biosynthesis is still limited in X. dendrorhous; however, it is known that the Sterol Regulatory Element-Binding Protein (SREBP) pathway, which is a conserved regulatory pathway involved in the control of lipid metabolism, also regulates carotenoid production in X. dendrorhous. This review addresses the similarities and differences that have been observed between mammal and fungal SREBP pathways and what it is known about this pathway regarding the regulation of the production of carotenoids and sterols in X. dendrorhous.

Derlin rhomboid pseudoproteases employ substrate engagement and lipid distortion to enable the retrotranslocation of ERAD membrane substrates

Nearly one-third of proteins are initially targeted to the endoplasmic reticulum (ER) membrane, where they are correctly folded and then delivered to their final cellular destinations. To prevent the accumulation of misfolded membrane proteins, ER-associated degradation (ERAD) moves these clients from the ER membrane to the cytosol, a process known as retrotranslocation. Our recent work in Saccharomyces cerevisiae reveals a derlin rhomboid pseudoprotease, Dfm1, is involved in the retrotranslocation of ubiquitinated ERAD membrane substrates. In this study, we identify conserved residues of Dfm1 that are critical for retrotranslocation. We find several retrotranslocation-deficient Loop 1 mutants that display impaired binding to membrane substrates. Furthermore, Dfm1 possesses lipid thinning function to facilitate in the removal of ER membrane substrates, and this feature is conserved in its human homolog, Derlin-1, further implicating that derlin-mediated retrotranslocation is a well-conserved process.

Maintenance of organellar protein homeostasis by ER-associated degradation and related mechanisms

Protein homeostasis mechanisms are fundamentally important to match cellular needs and to counteract stress conditions. A fundamental challenge is to understand how defective proteins are recognized and extracted from cellular organelles to be degraded in the cytoplasm. The endoplasmic reticulum (ER)-associated degradation (ERAD) pathway is the best-understood organellar protein quality control system. Here, we review new insights into the mechanism of recognition and retrotranslocation of client proteins in ERAD. In addition to the membrane-integral ERAD E3 ubiquitin ligases, we highlight one protein family that is remarkably often involved in various aspects of membrane protein quality control and protein dislocation: the rhomboid superfamily, which includes derlins and intramembrane serine proteases. Rhomboid-like proteins have been found to control protein homeostasis in the ER, but also in other eukaryotic organelles and in bacteria, pointing toward conserved principles of membrane protein quality control across organelles and evolution.

The transcriptome analysis on urea response mechanism in the process of ergosterol synthesis by Cordyceps cicadae

Nitrogen source is required for the growth of Cordyceps cicadae and involved in the regulation of metabolite synthesis. In order to further investigate the regulatory effects of nitrogen sources on the ergosterol synthesis by C. cicadae. We first confirmed that urea could significantly increase the ergosterol synthesis. The transcriptome analysis showed that compared with biomass cultured in the control fermentation medium (CFM), 1340 differentially expressed genes (DEGs) were obtained by Gene Ontology (GO) annotation, and 312 DEGs were obtained by Kyoto Encyclopedia of Genes and Genomes (KEGG) annotation from the biomass cultured in CFM + CO(NH₂)₂. Urea up-regulated D-3-phosphoglycerate dehydrogenase gene transcription level and down-regulated enolase and L-serine/L-threonine ammonialyase gene transcription level, increased serine synthesis, allosterically activate pyruvate kinase, to promote the synthesis of pyruvate and CH₃CO ~ SCOA, the primer of ergosterol; Urea increase the genes transcription related with ergosterol synthesis by up-regulating the steroid regulatory element binding protein gene transcription levels. The transcriptome results were provided by those of qRT-PCR. Collectively, our finding provided valuable insights into the regulatory effect of nitrogen source on the ergosterol synthesis by C. cicadae.

Proximity proteomics in a marine diatom reveals a putative cell surface-to-chloroplast iron trafficking pathway

Iron is a biochemically critical metal cofactor in enzymes involved in photosynthesis, cellular respiration, nitrate assimilation, nitrogen fixation, and reactive oxygen species defense. Marine microeukaryotes have evolved a phytotransferrin-based iron uptake system to cope with iron scarcity, a major factor limiting primary productivity in the global ocean. Diatom phytotransferrin is endocytosed; however, proteins downstream of this environmentally ubiquitous iron receptor are unknown. We applied engineered ascorbate peroxidase APEX2-based subcellular proteomics to catalog proximal proteins of phytotransferrin in the model marine diatom Phaeodactylum tricornutum. Proteins encoded by poorly characterized iron-sensitive genes were identified including three that are expressed from a chromosomal gene cluster. Two of them showed unambiguous colocalization with phytotransferrin adjacent to the chloroplast. Further phylogenetic, domain, and biochemical analyses suggest their involvement in intracellular iron processing. Proximity proteomics holds enormous potential to glean new insights into iron acquisition pathways and beyond in these evolutionarily, ecologically, and biotechnologically important microalgae.

High-Throughput Identification of Nuclear Envelope Protein Interactions in Schizosaccharomyces pombe Using an Arrayed Membrane Yeast-Two Hybrid Library

The nuclear envelope (NE) contains a specialized set of integral membrane proteins that maintain nuclear shape and integrity and influence chromatin organization and gene expression. Advances in proteomics techniques and studies in model organisms have identified hundreds of proteins that localize to the NE. However, the function of many of these proteins at the NE remains unclear, in part due to a lack of understanding of the interactions that these proteins participate in at the NE membrane. To assist in the characterization of NE transmembrane protein interactions we developed an arrayed library of integral and peripheral membrane proteins from the fission yeast Schizosaccharomyces pombe for high-throughput screening using the split-ubiquitin based membrane yeast two -hybrid system. We used this approach to characterize protein interactions for three conserved proteins that localize to the inner nuclear membrane: Cut11/Ndc1, Lem2 and Ima1/Samp1/Net5. Additionally, we determined how the interaction network for Cut11 is altered in canonical temperature-sensitive cut11-ts mutants. This library and screening approach is readily applicable to characterizing the interactomes of integral membrane proteins localizing to various subcellular compartments.

The role of rhomboid superfamily members in protein homeostasis: Mechanistic insight and physiological implications

Cells are equipped with protein quality control pathways in order to maintain a healthy proteome; a process known as protein homeostasis. Dysfunction in protein homeostasis leads to the development of many diseases that are associated with proteinopathies. Recently, the rhomboid superfamily has attracted much attention concerning their involvement in protein homeostasis. While their functional role has become much clearer in the last few years, their systemic significance in mammals remains elusive. Here we delineate the current knowledge of rhomboids in protein quality control and how these functions are integrated at the organismal level.

Metabolism of Storage Lipids and the Role of Lipid Droplets in the Yeast Schizosaccharomyces pombe

Storage lipids, triacylglycerols (TAG), and steryl esters (SE), are predominant constituents of lipid droplets (LD) in fungi. In several yeast species, metabolism of TAG and SE is linked to various cellular processes, including cell division, sporulation, apoptosis, response to stress, and lipotoxicity. In addition, TAG are an important source for the generation of value-added lipids for industrial and biomedical applications. The fission yeast Schizosaccharomyces pombe is a widely used unicellular eukaryotic model organism. It is a powerful tractable system used to study various aspects of eukaryotic cellular and molecular biology. However, the knowledge of S. pombe neutral lipids metabolism is quite limited. In this review, we summarize and discuss the current knowledge of the homeostasis of storage lipids and of the role of LD in the fission yeast S. pombe with the aim to stimulate research of lipid metabolism and its connection with other essential cellular processes. We also discuss the advantages and disadvantages of fission yeast in lipid biotechnology and recent achievements in the use of S. pombe in the biotechnological production of valuable lipid compounds.

Sterol regulatory element-binding protein Sre1 regulates carotenogenesis in the red yeast Xanthophyllomyces dendrorhous

Xanthophyllomyces dendrorhous is a basidiomycete yeast that produces carotenoids, mainly astaxanthin. Astaxanthin is an organic pigment of commercial interest due to its antioxidant and coloring properties. X. dendrorhous has a functional SREBP pathway, and the Sre1 protein is the SREBP homolog in this yeast. However, how sterol regulatory element (Sre)1 promotes the biosynthesis of sterols and carotenoids in X. dendrorhous is unknown. In this work, comparative RNA-sequencing analysis between modified X. dendrorhous strains that have an active Sre1 protein and the WT was performed to identify Sre1-dependent genes. In addition, Sre1 direct target genes were identified through ChIP combined with lambda exonuclease digestion (ChIP-exo) assays. SRE motifs were detected in the promoter regions of several Sre1 direct target genes and were consistent with the SREs described in other yeast species. Sre1 directly regulates genes related to ergosterol biosynthesis as well as genes related to the mevalonate (MVA) pathway, which synthesizes the building blocks of isoprenoids, including carotenoids. Two carotenogenic genes, crtE and crtR, were also identified as Sre1 direct target genes. Thus, carotenogenesis in X. dendrorhous is regulated by Sre1 through the regulation of the MVA pathway and the regulation of the crtE and crtR genes. As the crtR gene encodes a cytochrome P450 reductase, Sre1 regulates pathways that include cytochrome P450 enzymes, such as the biosynthesis of carotenoids and sterols. These results demonstrate that Sre1 is a sterol master regulator that is conserved in X. dendrorhous.

AMPKα Subunit Ssp2 and Glycogen Synthase Kinases Gsk3/Gsk31 are involved in regulation of sterol regulatory element-binding protein (SREBP) activity in fission yeast

Sterol regulatory element-binding protein (SREBP), a highly conserved family of membrane-bound transcription factors, is an essential regulator for cellular cholesterol and lipid homeostasis in mammalian cells. Sre1, the homolog of SREBP in the fission yeast Schizosaccharomyces pombe (S. pombe), regulates genes involved in the transcriptional responses to low sterol as well as low oxygen. Previous study reported that casein kinase 1 family member Hhp2 phosphorylated the Sre1 N-terminal transcriptional factor domain (Sre1N) and accelerated Sre1N degradation, and other kinases might exist for regulating the Sre1 function. To gain insight into the mechanisms underlying the Sre1 activity and to identify additional kinases involved in regulation of Sre1 function, we developed a luciferase reporter system to monitor the Sre1 activity through its binding site called SRE2 in living yeast cells. Here we showed that both ergosterol biosynthesis inhibitors and hypoxia-mimic CoCl₂ caused a dose-dependent increase in the Sre1 transcription activity, concurrently, these induced transcription activities were almost abolished in Δsre1 cells. Surprisingly, either AMPKα Subunit Ssp2 deletion or Glycogen Synthase Kinases Gsk3/Gsk31 double deletion significantly suppressed ergosterol biosynthesis inhibitors- or CoCl₂-induced Sre1 activity. Notably, the Δssp2Δgsk3Δgsk31 mutant showed further decreased Sre1 activity when compared with their single or double deletion. Consistently, the Δssp2Δgsk3Δgsk31 mutant showed more marked temperature sensitivity than any of their single or double deletion. Moreover, the fluorescence of GFP-Sre1N localized at the nucleus in wild-type cells, but significantly weaker nuclear fluorescence of GFP-Sre1N was observed in Δssp2, Δgsk3Δgsk31, Δssp2Δgsk3, Δssp2Δgsk31 or Δssp2Δgsk3Δgsk31 cells. On the other hand, the immunoblot showed a dramatic decrease in GST-Sre1N levels in the Δgsk3Δgsk31 or the Δssp2Δgsk3Δgsk31 cells but not in the Δssp2 cells. Altogether, our findings suggest that Gsk3/Gsk31 may regulate Sre1N degradation, while Ssp2 may regulate not only the degradation of Sre1N but also its translocation to the nucleus.

Metallopeptidase Stp1 activates the transcription factor Sre1 in the carotenogenic yeast Xanthophyllomyces dendrorhous

Xanthophyllomyces dendrorhous is a basidiomycete yeast known as a natural producer of astaxanthin, a carotenoid of commercial interest because of its antioxidant properties. Recent studies indicated that X. dendrorhous has a functional SREBP pathway involved in the regulation of isoprenoid compound biosynthesis, which includes ergosterol and carotenoids. SREBP is a major regulator of sterol metabolism and homeostasis in mammals; characterization in fungi also provides information about its role in the hypoxia adaptation response and virulence. SREBP protease processing is required to activate SREBP pathway functions in fungi. Here, we identified and described the STP1 gene, which encodes a metallopeptidase of the M50 family involved in the proteolytic activation of the transcription factor Sre1 of the SREBP pathway, in X. dendrorhous We assessed STP1 function in Δstp1 strains derived from the wild-type and a mutant of ergosterol biosynthesis that overproduces carotenoids and sterols. Bioinformatic analysis of the deduced protein predicted the presence of characteristic features identified in homologs from mammals and fungi. The Δstp1 mutation decreased yeast growth in the presence of azole drugs and reduced transcript levels of Sre1-dependent genes. This mutation also negatively affected the carotenoid- and sterol-overproducing phenotype. Western blot analysis demonstrated that Sre1 was activated in the yeast ergosterol biosynthesis mutant and that the Δstp1 mutation introduced in this strain prevented Sre1 proteolytic activation. Overall, our results demonstrate that STP1 encodes a metallopeptidase involved in proteolytic activation of Sre1 in X. dendrorhous, contributing to our understanding of fungal SREBP pathways.

Proteolytic Control of Lipid Metabolism

Synthesis and regulation of lipid levels and identities is critical for a wide variety of cellular functions, including structural and morphological properties of organelles, energy storage, signaling, and stability and function of membrane proteins. Proteolytic cleavage events regulate and/or influence some of these lipid metabolic processes and as a result help modulate their pleiotropic cellular functions. Proteins involved in lipid regulation are proteolytically cleaved for the purpose of their relocalization, processing, turnover, and quality control, among others. The scope of this review includes proteolytic events governing cellular lipid dynamics. After an initial discussion of the classic example of sterol regulatory element-binding proteins, our focus will shift to the mitochondrion, where a range of proteolytic events are critical for normal mitochondrial phospholipid metabolism and enforcing quality control therein. Recently, mitochondrial phospholipid metabolic pathways have been implicated as important for the proliferative capacity of cancers. Thus, the assorted proteases that regulate, monitor, or influence the activity of proteins that are important for phospholipid metabolism represent attractive targets to be manipulated for research purposes and clinical applications.

EGAD! There is an ERAD doppelganger in the Golgi

Disposal of membrane proteins in the late secretory pathway is thought to be exclusively facilitated by ESCRT-dependent lysosomal degradation. In this issue of The EMBO Journal, Schmidt et al define a previously uncharacterized endosome and Golgi-associated degradation (EGAD) pathway. This pathway, which has remarkable similarities to ERAD in the endoplasmic reticulum, operates in post-ER organelles via the proteasome and contributes to lipid homeostasis in eukaryotic cells.

Endosome and Golgi-associated degradation (EGAD) of membrane proteins regulates sphingolipid metabolism

Cellular homeostasis requires the ubiquitin-dependent degradation of membrane proteins. This was assumed to be mediated exclusively either by endoplasmic reticulum-associated degradation (ERAD) or by endosomal sorting complexes required for transport (ESCRT)-dependent lysosomal degradation. We identified in Saccharomyces cerevisiae an additional pathway that selectively extracts membrane proteins at Golgi and endosomes for degradation by cytosolic proteasomes. One endogenous substrate of this endosome and Golgi-associated degradation pathway (EGAD) is the ER-resident membrane protein Orm2, a negative regulator of sphingolipid biosynthesis. Orm2 degradation is initiated by phosphorylation, which triggers its ER export. Once on Golgi and endosomes, Orm2 is poly-ubiquitinated by the membrane-embedded "Defective in SREBP cleavage" (Dsc) ubiquitin ligase complex. Cdc48/VCP then extracts ubiquitinated Orm2 from membranes, which is tightly coupled to the proteasomal degradation of Orm2. Thereby, EGAD prevents the accumulation of Orm2 at the ER and in post-ER compartments and promotes the controlled de-repression of sphingolipid biosynthesis. Thus, the selective degradation of membrane proteins by EGAD contributes to proteostasis and lipid homeostasis in eukaryotic cells.

Membrane cholesterol as regulator of human rhomboid protease RHBDL4

In the last decade, intramembrane proteases have gained increasing attention because of their many links to various diseases. Nevertheless, our understanding as to how they function or how they are regulated is still limited, especially when it comes to human homologues. In this regard, here we sought to unravel mechanisms of regulation of the protease rhomboid-like protein-4 (RHBDL4), one of five active human serine intramembrane proteases. In view of our recent finding that human RHBDL4 efficiently cleaves the amyloid precursor protein (APP), a key protein in the pathology of Alzheimer's disease, we used established reagents to modulate the cellular cholesterol content and analyzed the effects of this modulation on RHBDL4-mediated processing of endogenous APP. We discovered that lowering membrane cholesterol levels increased the levels of RHBDL4-specific endogenous APP fragments, whereas high cholesterol levels had the opposite effect. Direct binding of cholesterol to APP did not mediate these modulating effects of cholesterol. Instead, using homology modeling, we identified two potential cholesterol-binding motifs in the transmembrane helices 3 and 6 of RHBDL4. Substitution of the essential tyrosine residues of the potential cholesterol-binding motifs to alanine increased the levels of endogenous APP C-terminal fragments, reflecting enhanced RHBDL4 activity. In summary, we provide evidence that the activity of RHBDL4 is regulated by cholesterol likely through a direct binding of cholesterol to the enzyme.

Embedded in the Membrane: How Lipids Confer Activity and Specificity to Intramembrane Proteases

Proteases, sharp yet unforgivable tools of every cell, require tight regulation to ensure specific non-aberrant cleavages. The relatively recent discovered class of intramembrane proteases has gained increasing interest due to their involvement in important signaling pathways linking them to diseases including Alzheimer's disease and cancer. Despite tremendous efforts, their regulatory mechanisms have only started to unravel. There is evidence that the membrane composition itself can regulate intramembrane protease activity and specificity. In this review, we highlight the work on γ-secretase and rhomboid proteases and summarize several studies as to how different lipids impact on enzymatic activity.

SREBP-regulated lipid metabolism: convergent physiology - divergent pathophysiology

Cellular lipid metabolism and homeostasis are controlled by sterol regulatory-element binding proteins (SREBPs). In addition to performing canonical functions in the transcriptional regulation of genes involved in the biosynthesis and uptake of lipids, genome-wide system analyses have revealed that these versatile transcription factors act as important nodes of convergence and divergence within biological signalling networks. Thus, they are involved in myriad physiological and pathophysiological processes, highlighting the importance of lipid metabolism in biology. Changes in cell metabolism and growth are reciprocally linked through SREBPs. Anabolic and growth signalling pathways branch off and connect to multiple steps of SREBP activation and form complex regulatory networks. In addition, SREBPs are implicated in numerous pathogenic processes such as endoplasmic reticulum stress, inflammation, autophagy and apoptosis, and in this way, they contribute to obesity, dyslipidaemia, diabetes mellitus, nonalcoholic fatty liver disease, nonalcoholic steatohepatitis, chronic kidney disease, neurodegenerative diseases and cancers. This Review aims to provide a comprehensive understanding of the role of SREBPs in physiology and pathophysiology at the cell, organ and organism levels.

Oxygen-responsive transcriptional regulation of lipid homeostasis in fungi: Implications for anti-fungal drug development

Low oxygen adaptation is essential for aerobic fungi that must survive in varied oxygen environments. Pathogenic fungi in particular must adapt to the low oxygen host tissue environment in order to cause infection. Maintenance of lipid homeostasis is especially important for cell growth and proliferation, and is a highly oxygen-dependent process. In this review, we focus on recent advances in our understanding of the transcriptional regulation and coordination of the low oxygen response across fungal species, paying particular attention to pathogenic fungi. Comparison of lipid homeostasis pathways in these organisms suggests common mechanisms of transcriptional regulation and points toward untapped potential to target low oxygen adaptation in antifungal development.

Dsc E3 ligase localization to the Golgi requires the ATPase Cdc48 and cofactor Ufd1 for activation of sterol regulatory element-binding protein in fission yeast

Sterol regulatory element-binding proteins (SREBPs) in the fission yeast Schizosaccharomyces pombe regulate lipid homeostasis and the hypoxic response under conditions of low sterol or oxygen availability. SREBPs are cleaved in the Golgi through the combined action of the Dsc E3 ligase complex, the rhomboid protease Rbd2, and the essential ATPases associated with diverse cellular activities (AAA⁺) ATPase Cdc48. The soluble SREBP N-terminal transcription factor domain is then released into the cytosol to enter the nucleus and regulate gene expression. Previously, we reported that Cdc48 binding to Rbd2 is required for Rbd2-mediated SREBP cleavage. Here, using affinity chromatography and mass spectrometry experiments, we identified Cdc48-binding proteins in S. pombe, generating a list of many previously unknown potential Cdc48-binding partners. We show that the established Cdc48 cofactor Ufd1 is required for SREBP cleavage but does not interact with the Cdc48-Rbd2 complex. Cdc48-Ufd1 is instead required at a step prior to Rbd2 function, during Golgi localization of the Dsc E3 ligase complex. Together, these findings demonstrate that two distinct Cdc48 complexes, Cdc48-Ufd1 and Cdc48-Rbd2, are required for SREBP activation and low-oxygen adaptation in S. pombe.

Proximity-dependent labeling methods for proteomic profiling in living cells

Characterizing the proteome composition of organelles and subcellular regions of living cells can facilitate the understanding of cellular organization as well as protein interactome networks. Proximity labeling-based methods coupled with mass spectrometry (MS) offer a high-throughput approach for systematic analysis of spatially restricted proteomes. Proximity labeling utilizes enzymes that generate reactive radicals to covalently tag neighboring proteins with biotin. The biotinylated endogenous proteins can then be isolated for further analysis by MS. To analyze protein-protein interactions or identify components that localize to discrete subcellular compartments, spatial expression is achieved by fusing the enzyme to specific proteins or signal peptides that target to particular subcellular regions. Although these technologies have only been introduced recently, they have already provided deep insights into a wide range of biological processes. Here, we describe and compare current methods of proximity labeling as well as their applications. As each method has its own unique features, the goal of this review is to describe how different proximity labeling methods can be used to answer different biological questions. WIREs Dev Biol 2017, 6:e272. doi: 10.1002/wdev.272 For further resources related to this article, please visit the WIREs website.

A guide to the rhomboid protein superfamily in development and disease

Rhomboid proteins are considered to be the most widespread membrane proteins across all forms of life. This superfamily comprises both active intramembrane serine proteases that catalyze the release of factors from the membrane, and a eukaryotic subset of non-catalytic members in which rhomboid architecture supports deviating functions. Although rhomboid was discovered in genetic studies of insect development, rhomboid research has broadened dramatically over the past 15 years; rhomboid enzymes are now the best biophysically understood of all intramembrane proteases, and are considered promising therapeutic targets for diseases ranging from parasitic infections to Parkinsonian neurodegeneration. Perhaps the most rapid progress has come with the catalytically inert rhomboid proteins, some of which regulate protein trafficking and/or function, and their prominence is underscored by clinical mutations. Such a diverse collection of advances mark an excellent point to review the state of this vibrant area of research, not because central questions have been answered, but instead because a firm grip in key areas has been established, and the field is now poised for breakthroughs.

A Golgi rhomboid protease Rbd2 recruits Cdc48 to cleave yeast SREBP

Hypoxic growth of fungi requires sterol regulatory element-binding protein (SREBP) transcription factors, and human opportunistic fungal pathogens require SREBP activation for virulence. Proteolytic release of fission yeast SREBPs from the membrane in response to low oxygen requires the Golgi membrane-anchored Dsc E3 ligase complex. Using genetic interaction arrays, we identified Rbd2 as a rhomboid family protease required for SREBP proteolytic processing. Rbd2 is an active, Golgi-localized protease that cleaves the transmembrane segment of the TatA rhomboid model substrate. Epistasis analysis revealed that the Dsc E3 ligase acts on SREBP prior to cleavage by Rbd2. Using APEX2 proximity biotinylation, we demonstrated that Rbd2 binds the AAA-ATPase Cdc48 through a C-terminal SHP box. Interestingly, SREBP cleavage required Rbd2 binding of Cdc48, consistent with Cdc48 acting to recruit ubiquitinylated substrates. In support of this claim, overexpressing a Cdc48-binding mutant of Rbd2 bypassed the Cdc48 requirement for SREBP cleavage, demonstrating that Cdc48 likely plays a role in SREBP recognition. In the absence of functional Rbd2, SREBP precursor is degraded by the proteasome, indicating that Rbd2 activity controls the balance between SREBP activation and degradation.