A lysosomal biogenesis map reveals the cargo spectrum of yeast vacuolar protein targeting pathways

AP-1 contributes to endosomal targeting of the ubiquitin ligase RNF13 via a secondary and novel non-canonical binding motif

Cellular trafficking between organelles is typically assured by short motifs that contact carrier proteins to transport them to their destination. The ubiquitin E3 ligase RING finger protein 13 (RNF13), a regulator of proliferation, apoptosis and protein trafficking, localizes to endolysosomal compartments through the binding of a dileucine motif to clathrin adaptor protein complex AP-3. Mutations within this motif reduce the ability of RNF13 to interact with AP-3. Here, our study shows the discovery of a glutamine-based motif that resembles a tyrosine-based motif within the C-terminal region of RNF13 that binds to the clathrin adaptor protein complex AP-1, notably without a functional interaction with AP-3. Using biochemical, molecular and cellular approaches in HeLa cells, our study demonstrates that a RNF13 dileucine variant uses an AP-1-dependent pathway to be exported from the Golgi towards the endosomal compartment. Overall, this study provides mechanistic insights into the alternate route used by this variant of the dileucine sorting motif of RNF13.

When less is more - a fast TurboID knock-in approach for high-sensitivity endogenous interactome mapping

In recent years, proximity labeling has established itself as an unbiased and powerful approach to map the interactome of specific proteins. Although physiological expression of labeling enzymes is beneficial for the mapping of interactors, generation of the desired cell lines remains time-consuming and challenging. Using our established pipeline for rapid generation of C- and N-terminal CRISPR-Cas9 knock-ins (KIs) based on antibiotic selection, we were able to compare the performance of commonly used labeling enzymes when endogenously expressed. Endogenous tagging of the µ subunit of the adaptor protein (AP)-1 complex with TurboID allowed identification of known interactors and cargo proteins that simple overexpression of a labeling enzyme fusion protein could not reveal. We used the KI strategy to compare the interactome of the different AP complexes and clathrin and were able to assemble lists of potential interactors and cargo proteins that are specific for each sorting pathway. Our approach greatly simplifies the execution of proximity labeling experiments for proteins in their native cellular environment and allows going from CRISPR transfection to mass spectrometry analysis and interactome data in just over a month.

Highly efficient expression of Rasamsonia emersonii lipase in Pichia pastoris: characterization and gastrointestinal simulated digestion in vitro

BACKGROUND

Acidic lipases with high catalytic activities under acidic conditions have important application values in the food, feed and pharmaceutical industries. However, the availability of acidic lipases is still the main obstacle to their industrial applications. Although a novel acidic lipase Rasamsonia emersonii (LIPR) was heterologously expressed in Escherichia coli, the expression level was unsatisfactory.

RESULTS

To achieve the high-efficiency expression and secretion of LIPR in Pichia pastoris GS115, the combinatorial optimization strategy was adopted including gene codon preference, signal peptide, molecular chaperone co-expression and disruption of vacuolar sorting receptor VPS10. The activity of the combinatorial optimization engineered strain in a shake flask reached 1480 U mL-1, which was 8.13 times greater than the P. pastoris GS115 parental strain. After high-density fermentation in a 5-L bioreactor, the highest enzyme activity reached as high as 11 820 U mL-1. LIPR showed the highest activity at 40 °C and pH 4.0 in the presence of Ca2+ ion. LIPR exhibited strong tolerance to methanol, indicating its potential application in biodiesel biosynthesis. Moreover, the gastrointestinal digestion simulation results demonstrated that LIPR was tolerant to pepsin and trypsin, but its activity was inhibited by sodium taurodeoxycholate.

CONCLUSION

This study provided an effective approach for the high expression of acidic lipase LIPR. LIPR was more appropriate for lipid digestion in the stomach than in intestine according to the gastrointestinal digestion simulation results. © 2024 Society of Chemical Industry.

A metabolically controlled contact site between vacuoles and lipid droplets in yeast

The lipid droplet (LD) organization proteins Ldo16 and Ldo45 affect multiple aspects of LD biology in yeast. They are linked to the LD biogenesis machinery seipin, and their loss causes defects in LD positioning, protein targeting, and breakdown. However, their molecular roles remained enigmatic. Here, we report that Ldo16/45 form a tether complex with Vac8 to create vacuole lipid droplet (vCLIP) contact sites, which can form in the absence of seipin. The phosphatidylinositol transfer protein (PITP) Pdr16 is a further vCLIP-resident recruited specifically by Ldo45. While only an LD subpopulation is engaged in vCLIPs at glucose-replete conditions, nutrient deprivation results in vCLIP expansion, and vCLIP defects impair lipophagy upon prolonged starvation. In summary, Ldo16/45 are multifunctional proteins that control the formation of a metabolically regulated contact site. Our studies suggest a link between LD biogenesis and breakdown and contribute to a deeper understanding of how lipid homeostasis is maintained during metabolic challenges.

Identification of distinct active pools of yeast serine palmitoyltransferase in sub-compartments of the ER

Sphingolipids (SPs) are one of the three major lipid classes in eukaryotic cells and serve as structural components of the plasma membrane. The rate-limiting step in SP biosynthesis is catalyzed by the serine palmitoyltransferase (SPT). In budding yeast (Saccharomyces cerevisiae), SPT is negatively regulated by the two proteins, Orm1 and Orm2. Regulating SPT activity enables cells to adapt SP metabolism to changing environmental conditions. Therefore, the Orm proteins are phosphorylated by two signaling pathways originating from either the plasma membrane or the lysosome (or vacuole in yeast). Moreover, uptake of exogenous serine is necessary for the regulation of SP biosynthesis, which suggests the existence of differentially regulated SPT pools based on their intracellular localization. However, measuring lipid metabolic enzyme activity in different cellular sub-compartments has been challenging. Combining a nanobody recruitment approach with SP flux analysis, we show that the nuclear endoplasmic reticulum (ER)-localized SPT and the peripheral ER localized SPT pools are differentially active. Thus, our data add another layer to the complex network of SPT regulation. Moreover, combining lipid metabolic enzyme re-localization with flux analysis serves as versatile tool to measure lipid metabolism with subcellular resolution.

Multiomics of GCN4-Dependent Replicative Lifespan Extension Models Reveals Gcn4 as a Regulator of Protein Turnover in Yeast

We have shown that multiple tRNA synthetase inhibitors can increase lifespan in both the nematode C. elegans and the budding yeast S. cerevisiae by acting through the conserved transcription factor Gcn4 (yeast)/ATF-4 (worms). To further understand the biology downstream from this conserved transcription factor in the yeast model system, we looked at two different yeast models known to have upregulated Gcn4 and GCN4-dependent increased replicative lifespan. These two models were rpl31aΔ yeast and yeast treated with the tRNA synthetase inhibitor borrelidin. We used both proteomic and RNAseq analysis of a block experimental design that included both of these models to identify GCN4-dependent changes in these two long-lived strains of yeast. Proteomic analysis of these yeast indicate that the long-lived yeast have increased abundances of proteins involved in amino acid biosynthesis. The RNAseq of these same yeast uncovered further regulation of protein degradation, identifying the differential expression of genes associated with autophagy and the ubiquitin-proteasome system (UPS). The data presented here further underscore the important role that GCN4 plays in the maintenance of protein homeostasis, which itself is an important hallmark of aging. In particular, the changes in autophagy and UPS-related gene expression that we have observed could also have wide-ranging implications for the understanding and treatment of diseases of aging that are associated with protein aggregation.

Nuclear and cytoplasmic spatial protein quality control is coordinated by nuclear-vacuolar junctions and perinuclear ESCRT

Effective protein quality control (PQC), essential for cellular health, relies on spatial sequestration of misfolded proteins into defined inclusions. Here we reveal the coordination of nuclear and cytoplasmic spatial PQC. Cytoplasmic misfolded proteins concentrate in a cytoplasmic juxtanuclear quality control compartment, while nuclear misfolded proteins sequester into an intranuclear quality control compartment (INQ). Particle tracking reveals that INQ and the juxtanuclear quality control compartment converge to face each other across the nuclear envelope at a site proximal to the nuclear-vacuolar junction marked by perinuclear ESCRT-II/III protein Chm7. Strikingly, convergence at nuclear-vacuolar junction contacts facilitates VPS4-dependent vacuolar clearance of misfolded cytoplasmic and nuclear proteins, the latter entailing extrusion of nuclear INQ into the vacuole. Finding that nuclear-vacuolar contact sites are cellular hubs of spatial PQC to facilitate vacuolar clearance of nuclear and cytoplasmic inclusions highlights the role of cellular architecture in proteostasis maintenance.

Zinc homeostasis in Schizosaccharomyces pombe

Most metal ions such as iron, calcium, zinc, or copper are essential for all eukaryotes. Organisms must maintain homeostasis of these metal ions because excess or deficiency of metal ions could cause damage to organisms. The steady state of many metal ions such as iron and copper has been well studied in detail. However, how to regulate zinc homeostasis in Schizosaccharomyces pombe is still confusing. In this review, we provide an overview of the molecular mechanisms that how S. pombe is able to maintain the balance of zinc levels in the changes of environment. In response to high levels of zinc, the transcription factor Loz1 represses the expression of several genes involved in the acquisition of zinc. Meanwhile, the CDF family proteins transport excess zinc to the secretory pathway. When zinc levels are limited, Loz1 was inactivated and could not inhibit the expression of zinc acquisition genes, and zinc stored in the secretory pathway is released for use by the cells. Besides, other factors that regulate zinc homeostasis are also discussed.

Molecular insights into endolysosomal microcompartment formation and maintenance

The endolysosomal system of eukaryotic cells has a key role in the homeostasis of the plasma membrane, in signaling and nutrient uptake, and is abused by viruses and pathogens for entry. Endocytosis of plasma membrane proteins results in vesicles, which fuse with the early endosome. If destined for lysosomal degradation, these proteins are packaged into intraluminal vesicles, converting an early endosome to a late endosome, which finally fuses with the lysosome. Each of these organelles has a unique membrane surface composition, which can form segmented membrane microcompartments by membrane contact sites or fission proteins. Furthermore, these organelles are in continuous exchange due to fission and fusion events. The underlying machinery, which maintains organelle identity along the pathway, is regulated by signaling processes. Here, we will focus on the Rab5 and Rab7 GTPases of early and late endosomes. As molecular switches, Rabs depend on activating guanine nucleotide exchange factors (GEFs). Over the last years, we characterized the Rab7 GEF, the Mon1-Ccz1 (MC1) complex, and key Rab7 effectors, the HOPS complex and retromer. Structural and functional analyses of these complexes lead to a molecular understanding of their function in the context of organelle biogenesis.

The VINE complex is an endosomal VPS9-domain GEF and SNX-BAR coat

Membrane trafficking pathways perform important roles in establishing and maintaining the endosomal network. Retrograde protein sorting from the endosome is promoted by conserved SNX-BAR-containing coat complexes including retromer which enrich cargo at tubular microdomains and generate transport carriers. In metazoans, retromer cooperates with VARP, a conserved VPS9-domain GEF, to direct an endosomal recycling pathway. The function of the yeast VARP homolog Vrl1 has been overlooked due to an inactivating mutation found in commonly studied strains. Here, we demonstrate that Vrl1 has features of a SNX-BAR coat protein and forms an obligate complex with Vin1, the paralog of the retromer SNX-BAR protein Vps5. Unique features in the Vin1 N-terminus allow Vrl1 to distinguish it from Vps5, thereby forming a complex that we have named VINE. The VINE complex occupies endosomal tubules and redistributes a conserved mannose 6-phosphate receptor-like protein from endosomes. We also find that membrane recruitment by Vin1 is essential for Vrl1 GEF activity, suggesting that VINE is a multifunctional coat complex that regulates trafficking and signaling events at the endosome.

All healthy cells have a highly organized interior: different compartments with specialized roles are in different places, and in order to do their jobs properly, proteins need to be in the right place. Endosomes are membrane-bound compartments that act as transport hubs where proteins are sorted into small vesicles and delivered to other parts of the cell. Two groups of proteins regulate this transport: the first group, known as VPS9 GEFs, switches on the enzymes that recruit the second group of proteins, called the sorting nexins. This second group is responsible for forming the transport vesicles via which proteins are distributed all over the cell. Defects in protein sorting can lead to various diseases, including neurodegenerative conditions such as Parkinson’s disease and juvenile amyotrophic lateral sclerosis.

Scientists often use budding yeast cells to study protein sorting, because these cells are similar to human cells, but easier to grow in large numbers and examine in the laboratory. Previous work showed that a yeast protein called Vrl1 is equivalent to a VPS9 GEF from humans called VARP. However, Vrl1 only exists in wild forms of budding yeast, and not in laboratory strains of the organism. Therefore, researchers had not studied Vrl1 in detail, and its roles remained unclear.

To learn more about Vrl1, Shortill et al. started by re-introducing the protein into laboratory strains of budding yeast and observing what happened to protein sorting in these cells. Like VARP, Vrl1 was found in the endosomes of budding yeast. However, biochemical experiments revealed that, while human VARP binds to a protein called retromer, Vrl1 does not bind to the equivalent protein in yeast. Instead, Vrl1 itself has features of both the VPS9 GEFs and the sorting nexins. Shortill et al. also found that Vrl1 interacted with a different protein in the sorting nexin family called Vin1. In the absence of Vrl1, Vin1 was found floating around the cell, but once Vrl1 was re-introduced into the budding yeast, Vin1 relocated to the endosomes. Vrl1 uses its VPS9 GEF part to move itself to the endosome membrane, and Vin1 controls this movement, highlighting the interdependence between the two proteins. Once they are at the endosome together, Vrl1 and Vin1 help redistribute proteins to other parts of the cell.

This study suggests that, like VARP, Vrl1 cooperates with sorting nexins to transport proteins. Since many previous experiments about protein sorting were carried out in yeast cells lacking Vrl1, it is possible that this process was overlooked despite its potential importance. These new findings could also help other researchers investigating how endosomes and protein sorting work, or do not work, in the context of neurodegenerative diseases.