Effect of Sec61 interaction with Mpd1 on endoplasmic reticulum-associated degradation

NSP6 inhibits the production of ACE2-containing exosomes to promote SARS-CoV-2 infectivity

The emergence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has triggered a global pandemic, which severely endangers public health. Our and others' works have shown that the angiotensin-converting enzyme 2 (ACE2)-containing exosomes (ACE2-exos) have superior antiviral efficacies, especially in response to emerging variants. However, the mechanisms of how the virus counteracts the host and regulates ACE2-exos remain unclear. Here, we identified that SARS-CoV-2 nonstructural protein 6 (NSP6) inhibits the production of ACE2-exos by affecting the protein level of ACE2 as well as tetraspanin-CD63 which is a key factor for exosome biogenesis. We further found that the protein stability of CD63 and ACE2 is maintained by the deubiquitination of proteasome 26S subunit, non-ATPase 12 (PSMD12). NSP6 interacts with PSMD12 and counteracts its function, consequently promoting the degradation of CD63 and ACE2. As a result, NSP6 diminishes the antiviral efficacy of ACE2-exos and facilitates the virus to infect healthy bystander cells. Overall, our study provides a valuable target for the discovery of promising drugs for the treatment of coronavirus disease 2019.

IMPORTANCE

The outbreak of coronavirus disease 2019 (COVID-19) severely endangers global public health. The efficacy of vaccines and antibodies declined with the rapid emergence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) mutants. Angiotensin-converting enzyme 2-containing exosomes (ACE2-exos) therapy exhibits a broad neutralizing activity, which could be used against various viral mutations. Our study here revealed that SARS-CoV-2 nonstructural protein 6 inhibited the production of ACE2-exos, thereby promoting viral infection to the adjacent bystander cells. The identification of a new target for blocking SARS-CoV-2 depends on fully understanding the virus-host interaction networks. Our study sheds light on the mechanism by which the virus resists the host exosome defenses, which would facilitate the study and design of ACE2-exos-based therapeutics for COVID-19.

N-glycosylation at N57/100/110 affects CD44s localization, function and stability in hepatocellular carcinoma

The glycosylation levels of proteins in cancer cells are closely related to cancer invasion and migration. CD44 is a transmembrane glycoprotein that is significantly overexpressed in a variety of tumor cells and has been proven to promote the migration and motility of cancer cells, but the effect of its N-glycosylation modification on CD44 protein function in tumors is less studied. Here, we investigated the effect of six N-glycan chains (N25/57/100/110/120/255) on CD44s localization, function and stability in hepatocarcinoma cells. When the six sites were mutated, we found that CD44s lost its membrane localization in Huh7 and MHCC-97H cells. On this basis, we identified three glycosylation sites on CD44s (N57, N100 and N110) that played key roles in intracellular localization. When N57, N100 and N110 were mutated together, CD44 localized to the cytoplasm, while another three-site mutant (N25/N120/N255) was still anchored to the membrane. In addition, the ability of CD44-N57Q/N100Q/N110Q to promote the metastasis and invasion of Huh7 and 97H cells was weakened compared with that of CD44-N25Q/N120Q/N255Q. Furthermore, CD44-N57Q/N100Q/N110Q accumulated abnormally in the ER, and a high level of the ER stress (ERS) marker BiP was detected at the same time compared with wild-type CD44. When the lysosome inhibitor CQ was added, the content of mutant protein that triggered ERS significantly increased, which indicated that the degradation mode of CD44-N57Q/N100Q/N110Q after ERS was mainly through the lysosomal pathway (ERLAD). The results revealed that the N-glycosylation sites N57, N100 and N110 mutated on CD44s affected its function and degraded it by lysosomes after triggering ERS. These findings provide data for new studies on ER-related degradation, further promote the study of the glycan chain function of CD44 and furnish new ideas for the treatment of liver cancer metastasis.

Prospects of Using Protein Engineering for Selective Drug Delivery into a Specific Compartment of Target Cells

A large number of proteins are successfully used to treat various diseases. These include natural polypeptide hormones, their synthetic analogues, antibodies, antibody mimetics, enzymes, and other drugs based on them. Many of them are demanded in clinical settings and commercially successful, mainly for cancer treatment. The targets for most of the aforementioned drugs are located at the cell surface. Meanwhile, the vast majority of therapeutic targets, which are usually regulatory macromolecules, are located inside the cell. Traditional low molecular weight drugs freely penetrate all cells, causing side effects in non-target cells. In addition, it is often difficult to elaborate a small molecule that can specifically affect protein interactions. Modern technologies make it possible to obtain proteins capable of interacting with almost any target. However, proteins, like other macromolecules, cannot, as a rule, freely penetrate into the desired cellular compartment. Recent studies allow us to design multifunctional proteins that solve these problems. This review considers the scope of application of such artificial constructs for the targeted delivery of both protein-based and traditional low molecular weight drugs, the obstacles met on the way of their transport to the specified intracellular compartment of the target cells after their systemic bloodstream administration, and the means to overcome those difficulties.

Sec61 channel subunit Sbh1/Sec61β promotes ER translocation of proteins with suboptimal targeting sequences and is fine-tuned by phosphorylation

The highly conserved endoplasmic reticulum (ER) protein translocation channel contains one nonessential subunit, Sec61β/Sbh1, whose function is poorly understood so far. Its intrinsically unstructured cytosolic domain makes transient contact with ER-targeting sequences in the cytosolic channel vestibule and contains multiple phosphorylation sites suggesting a potential for regulating ER protein import. In a microscopic screen, we show that 12% of a GFP-tagged secretory protein library depends on Sbh1 for translocation into the ER. Sbh1-dependent proteins had targeting sequences with less pronounced hydrophobicity and often no charge bias or an inverse charge bias which reduces their insertion efficiency into the Sec61 channel. We determined that mutating two N-terminal, proline-flanked phosphorylation sites in the Sbh1 cytosolic domain to alanine phenocopied the temperature-sensitivity of a yeast strain lacking SBH1 and its ortholog SBH2. The phosphorylation site mutations reduced translocation into the ER of a subset of Sbh1-dependent proteins, including enzymes whose concentration in the ER lumen is critical for ER proteostasis. In addition, we found that ER import of these proteins depended on the activity of the phospho-S/T-specific proline isomerase Ess1 (PIN1 in mammals). We conclude that Sbh1 promotes ER translocation of substrates with suboptimal targeting sequences and that its activity can be regulated by a conformational change induced by N-terminal phosphorylation.

Identification and Phenotypic Characterization of Hsp90 Phosphorylation Sites That Modulate Virulence Traits in the Major Human Fungal Pathogen Candida albicans

The highly conserved, ubiquitous molecular chaperone Hsp90 is a key regulator of cellular proteostasis and environmental stress responses. In human pathogenic fungi, which kill more than 1.6 million patients each year worldwide, Hsp90 governs cellular morphogenesis, drug resistance, and virulence. Yet, our understanding of the regulatory mechanisms governing fungal Hsp90 function remains sparse. Post-translational modifications are powerful components of nature’s toolbox to regulate protein abundance and function. Phosphorylation in particular is critical in many cellular signaling pathways and errant phosphorylation can have dire consequences for the cell. In the case of Hsp90, phosphorylation affects its stability and governs its interactions with co-chaperones and clients. Thereby modulating the cell’s ability to cope with environmental stress. Candida albicans, one of the leading human fungal pathogens, causes ~750,000 life-threatening invasive infections worldwide with unacceptably high mortality rates. Yet, it remains unknown if and how Hsp90 phosphorylation affects C. albicans virulence traits. Here, we show that phosphorylation of Hsp90 is critical for expression of virulence traits. We combined proteomics, molecular evolution analyses and structural modeling with molecular biology to characterize the role of Hsp90 phosphorylation in this non-model pathogen. We demonstrated that phosphorylation negatively affects key virulence traits, such as the thermal stress response, morphogenesis, and drug susceptibility. Our results provide the first record of a specific Hsp90 phosphorylation site acting as modulator of fungal virulence. Post-translational modifications of Hsp90 could prove valuable in future exploitations as antifungal drug targets.

A new insight into the intestine of Pacific white shrimp: Regulation of intestinal homeostasis and regeneration in Litopenaeus vannamei during temperature fluctuation

Litopenaeus vannamei (L. vannamei) is an essential aquaculture shrimp throughout the world, but its aquaculture industry is threatened by temperature fluctuation. In this study, our histological results indicated that the shrimp intestine has a self-repairing ability during temperature fluctuation; however the potential mechanisms were still unknown. Therefore, transcriptome profiles of the intestine were collected from shrimp at 28 °C (C28), 13 °C (T13) and 28 °C after their temperature rose back (R28) and were analyzed. A total of 2229 differentially expressed genes (DEGs) (986 up- and 1243 downregulated) were identified in the C28 group, and 1790 DEGs (933 up- and 857 downregulated) were identified in the R28 group when compared to their expression levels in the T13 group. According to the functional annotation using KEGG, we found that the immune system was the most enriched section of organismal systems and that the shrimp can mobilize the body's immune response to regulate organism homeostasis during temperature fluctuation, although cold stress decreased the immunity. Additionally, metabolic inhibition is a strategy to cope with cold stress, and the regulation of lipid metabolism was especially important for shrimp during temperature fluctuation. Remarkably, the Hippo signaling pathway might help the repair of intestinal structure. Our research provides the first histological analysis and transcriptome profiling for the L. vannamei intestine during the temperature fluctuation stage. These results enrich our understanding of the mechanism of intestinal self-repair and homeostasis and could provide guidance for shrimp farming.