Recombinant Mouse Protein transport protein Sec61 subunit gamma (Sec61g)

What is Sec61g and what is its primary function in cellular biology?

Sec61g is a subunit of the highly conserved heterotrimeric membrane protein complex known as Sec61 in eukaryotes (equivalent to SecYEG in bacteria). This complex serves as the primary component of the protein translocation machinery at the endoplasmic reticulum (ER) membrane. The fundamental function of Sec61g is to facilitate the translocation of secretory proteins into the ER lumen or the integration of membrane proteins into the ER membrane .

Structurally, Sec61g consists of seven helices (labeled A through G), with helices B, C, and G serving as transmembrane helices, while helices A, D, and E are located on the cytosolic side. Helix F is the longest and spans both cytosolic and transmembrane regions . This structural arrangement allows Sec61g to interact with both the ribosome and other components of the translocation machinery.

Methodologically, researchers studying Sec61g's structure typically employ techniques such as cryo-electron microscopy (cryo-EM) to visualize its three-dimensional configuration and interactions within the translocation complex. For instance, high-resolution cryo-EM has revealed how TRAP-γ (a component of the translocon-associated protein complex) interacts with both the 28S ribosomal RNA and ribosomal protein L38, as well as with the α and γ chains of Sec61 .

How does recombinant mouse Sec61g differ from human Sec61g in structure and function?

For researchers studying mouse models of human diseases, it's crucial to note that while the core functions are conserved, the translation of findings from mouse to human systems should be approached with caution. When designing experiments involving recombinant Sec61g, researchers should consider using both species' proteins in parallel studies to validate interspecies consistency.

What are the recommended methods for producing high-quality recombinant mouse Sec61g for structural studies?

To produce high-quality recombinant mouse Sec61g suitable for structural studies, researchers commonly employ bacterial or eukaryotic expression systems. When selecting an expression system, consider that membrane proteins like Sec61g often require eukaryotic systems such as insect cells or yeast to ensure proper folding and post-translational modifications.

A methodological approach includes:

• Gene optimization and vector design: Codon-optimize the mouse Sec61g gene for your expression system and incorporate appropriate purification tags (His-tag or FLAG-tag) that can be cleaved if necessary for structural studies.

• Expression system selection: For membrane proteins like Sec61g, mammalian cell lines (HEK293 or CHO cells) or insect cell systems (Sf9, High Five) are often preferred to ensure proper membrane integration and folding.

• Solubilization and purification: Use appropriate detergents (such as DDM, LMNG, or digitonin) for membrane extraction. These detergents should be gentle enough to maintain the native conformation of Sec61g but effective in solubilizing it from membranes.

• Validation: Employ circular dichroism (CD) spectroscopy or limited proteolysis to confirm proper folding before proceeding to structural studies.

• Complex reconstitution: For studies requiring the complete Sec61 complex, co-express Sec61g with Sec61α and Sec61β, or reconstitute the complex in vitro from purified components.

When conducting structural studies, cryo-EM has proven particularly useful for visualizing Sec61g in its native membrane environment and in complex with interaction partners, as demonstrated in recent studies achieving resolutions of ~2.86 Å for the ribosome-Sec61 complex .

What are the most effective experimental approaches for studying Sec61g-mediated protein translocation?

Studying Sec61g-mediated protein translocation requires specialized approaches that can capture this dynamic process. The most effective experimental strategies include:

• In vitro translocation assays: Reconstitute purified Sec61 complex into proteoliposomes and assess translocation of radiolabeled or fluorescently tagged substrate proteins. This approach allows quantitative measurement of translocation efficiency and can be used to compare wild-type and mutant Sec61g.

• Crosslinking studies: Use photo-activatable or chemical crosslinkers to capture transient interactions between Sec61g and translocating substrates or other components of the translocation machinery.

• Real-time fluorescence spectroscopy: Employ FRET (Förster Resonance Energy Transfer) techniques using labeled Sec61g and substrate proteins to monitor conformational changes during translocation.

• Comparative analysis across species: As demonstrated in research comparing translocation efficiency in bacteria, yeast, and mammalian cells, this approach can reveal evolutionarily conserved deficiencies of the Sec61/SecY complex, such as its inability to efficiently translocate proteins composed entirely of intrinsically disordered domains or β-strands .

• Structure-function studies using mutagenesis: Introduce specific mutations in Sec61g and assess their impact on translocation efficiency, which can be measured using reporter proteins that undergo glycosylation upon successful translocation.

Research has shown that the Sec61/SecY complex exhibits conserved limitations in translocating certain types of proteins. For example, proteins entirely composed of intrinsically disordered domains (IDDs) or β-strands are poorly translocated across different species, suggesting an evolutionary conservation of this limitation . This understanding is crucial when designing translocation experiments involving Sec61g.

How can researchers effectively knock down or overexpress Sec61g in mouse models to study its function?

For effective manipulation of Sec61g expression in mouse models, researchers can employ several strategic approaches:

For knockdown studies:

• RNA interference (RNAi): Design specific siRNAs or shRNAs targeting mouse Sec61g mRNA. For in vivo delivery, consider using adeno-associated virus (AAV) or lentiviral vectors. In cell culture models, this approach has been shown to suppress breast cancer cell proliferation, migration, and invasion, while promoting apoptosis .

• CRISPR/Cas9-mediated knockdown: Design guide RNAs targeting mouse Sec61g gene. This approach allows for more stable and often more complete knockdown compared to RNAi methods.

• Conditional knockout models: Generate floxed Sec61g mice that can be crossed with tissue-specific Cre-expressing lines to achieve tissue-specific deletion. This is particularly important since complete knockout may be embryonically lethal given the essential nature of protein translocation.

For overexpression studies:

• Transgenic approaches: Generate mice with Sec61g under the control of a strong, potentially tissue-specific promoter.

• Viral vector-mediated overexpression: Use AAV or lentiviral vectors carrying Sec61g cDNA to achieve overexpression in specific tissues.

• Inducible expression systems: Consider using Tet-On/Tet-Off systems to control the timing and level of Sec61g expression.

Validation methods:

• qRT-PCR and Western blotting to confirm altered expression levels

• Immunohistochemistry to assess the spatial distribution of altered expression

• Functional assays specific to the tissue being studied

For in vivo cancer studies, xenograft models have proven particularly informative. Research has demonstrated that knockdown of Sec61g inhibits breast tumor development in vivo, suggesting its oncogenic role . When designing such experiments, it's crucial to include appropriate controls and to monitor for potential compensatory mechanisms that might mask the true effects of Sec61g manipulation.

What is the current evidence linking Sec61g expression to cancer development and progression?

Substantial evidence indicates that Sec61g plays significant roles in cancer development and progression across multiple cancer types. Key findings include:

• Expression profile: Using Oncomine data, SEC61G has been found to be upregulated in almost all cancer types compared to normal tissues (p < 0.001, |log2 fold change| > 1.5) . This widespread upregulation suggests a common oncogenic mechanism involving Sec61g.

• Breast cancer: SEC61G is notably overexpressed in breast cancer tissues, and high expression levels predict poor patient outcomes. This has been demonstrated in both The Cancer Genome Atlas (TCGA) breast cancer cohort and independently validated in the Nanjing Medical University (NMU) breast cancer cohort .

• Functional impact in cancer cells: Knockdown studies have demonstrated that SEC61G suppression inhibits cancer cell proliferation, migration, and invasion while promoting apoptosis. In vivo studies using xenograft breast tumor models confirmed that SEC61G knockdown inhibits tumor development .

• Metabolic regulation: SEC61G positively regulates glycolysis in breast cancer cells, suggesting a role in metabolic reprogramming, a hallmark of cancer development .

• Glioblastoma: SEC61G assists EGFR-amplified glioblastoma in evading immune surveillance, providing a mechanistic explanation for its contribution to cancer progression .

• Molecular mechanism: SEC61G is transcriptionally regulated by E2F1, a key cell cycle regulator. The E2F1/SEC61G axis has been shown to regulate glycolysis and chemosensitivity to Herceptin in breast cancer cells .

This compelling evidence suggests that SEC61G functions as an oncogene across multiple cancer types, making it a potential therapeutic target and prognostic marker.

How does Sec61g contribute to metabolic reprogramming in cancer cells?

Sec61g significantly contributes to metabolic reprogramming in cancer cells, particularly through its effects on glycolysis. The metabolic impact of Sec61g manifests through several mechanisms:

• Glycolysis regulation: Research has demonstrated that Sec61g positively regulates glycolysis in breast cancer cells. When SEC61G is knocked down, cancer cells show decreased glycolytic activity, suggesting that Sec61g upregulation in cancer may contribute to the Warburg effect, where cancer cells predominantly use glycolysis even in the presence of oxygen .

• E2F1/SEC61G axis: The transcription factor E2F1 directly regulates SEC61G expression by binding to its promoter. This E2F1/SEC61G axis has been shown to regulate glycolysis in breast cancer cells. Specifically, the metabolic effects of E2F1 knockdown can be partially rescued by SEC61G overexpression, confirming that SEC61G is a downstream mediator of E2F1's metabolic effects .

• Potential mechanisms: While the exact molecular mechanisms by which Sec61g influences glycolysis remain to be fully elucidated, several possibilities exist:

  • Sec61g may facilitate the translocation of specific glycolytic enzymes or regulators into the ER

  • It might influence the trafficking of glucose transporters to the cell membrane

  • It could affect signaling pathways that regulate metabolic enzymes

• Therapeutic implications: The role of Sec61g in cancer cell metabolism suggests that targeting Sec61g could disrupt cancer cell energy production, potentially enhancing the efficacy of conventional chemotherapies. For instance, research has shown that the E2F1/SEC61G axis regulates chemosensitivity to Herceptin in breast cancer cells .

Understanding these metabolic functions of Sec61g is crucial for researchers designing experiments to explore novel cancer therapeutic approaches.

What specific experimental models best demonstrate the oncogenic properties of Sec61g?

To effectively demonstrate the oncogenic properties of Sec61g, researchers have employed several complementary experimental models:

• Clinical sample analysis: Examination of SEC61G expression in patient tumor samples compared to normal tissues provides direct clinical relevance. The Cancer Genome Atlas (TCGA) data and the Nanjing Medical University (NMU) breast cancer cohort have been instrumental in establishing the correlation between SEC61G overexpression and poor clinical outcomes .

• In vitro cancer cell models:

  • Knockdown experiments: siRNA or shRNA-mediated knockdown of SEC61G in breast cancer cell lines has demonstrated decreased proliferation, migration, and invasion, while promoting apoptosis .

  • Overexpression experiments: Introducing exogenous SEC61G into cancer cells with low endogenous expression can demonstrate gain-of-function effects.

  • Rescue experiments: Overexpression of SEC61G can antagonize the effects of E2F1 knockdown in regulating breast cancer cell proliferation, invasion, and apoptosis, demonstrating its position in oncogenic signaling hierarchies .

• In vivo xenograft models: Implanting SEC61G-knockdown cancer cells into immunocompromised mice has shown reduced tumor growth compared to control cells, providing strong evidence for its oncogenic role in a physiologically relevant setting .

• Glioblastoma models: Studies in glioblastoma have demonstrated that SEC61G assists EGFR-amplified tumors in evading immune surveillance, offering insight into its role in cancer-immune system interactions .

• Glycolysis assays: Measuring changes in glycolytic parameters (glucose uptake, lactate production, extracellular acidification rate) in response to SEC61G manipulation directly demonstrates its metabolic impact .

These complementary approaches provide robust evidence for the oncogenic properties of Sec61g and offer researchers multiple experimental systems to investigate its functions and potential as a therapeutic target.

How does Sec61g interact with the TRAP complex and what are the functional implications?

Sec61g engages in specific interactions with the translocon-associated protein (TRAP) complex, with significant functional implications for protein translocation. Recent high-resolution structural studies have provided detailed insights into these interactions:

• Structural arrangement: Cryo-electron microscopy (cryo-EM) at 2.86-Å resolution has revealed that TRAP-γ binds alongside Sec61, forming a critical interface between the ribosome and the translocation machinery. The TRAP-γ subunit consists of seven helices (A-G), with helices B, C, and G functioning as transmembrane helices (TMHs) .

• Molecular interactions:

  • TRAP-γ interacts with 28S ribosomal RNA and ribosomal protein L38

  • TRAP-γ forms contacts with both Sec61α and Sec61γ chains

  • The transmembrane part of TRAP-γ and one C-terminal helix from each TRAP α, β, and δ chains form a seven TMH bundle

  • Helices B and F of TRAP-γ interface directly with the TRAP α, β, and δ helices

• Functional positioning: The N-terminal parts of the TRAP α, β, and δ chains cluster as a heterotrimer, which is positioned adjacent to the Sec61 exit channel in the ER lumen. This strategic positioning allows TRAP to interact with nascent polypeptide chains emerging from the translocation channel .

• Functional implications:

  • TRAP is believed to facilitate the translocation of substrates with weak (low hydrophobicity) or proline- and glycine-rich signal peptides

  • The complex helps ensure the opened conformation of Sec61

  • TRAP may interact with other accessory proteins such as oligosaccharyl-transferase (OST) during protein maturation

This detailed understanding of Sec61g-TRAP interactions provides important mechanistic insights for researchers investigating the protein translocation machinery and potential intervention points for therapeutic development.

What is known about the transcriptional regulation of Sec61g and how might this be exploited in research?

The transcriptional regulation of Sec61g involves specific transcription factors and regulatory mechanisms that researchers can exploit for both basic and translational research. Key aspects include:

• E2F1 as a primary regulator: Research has demonstrated that transcription factor E2F1 directly binds to the promoter of SEC61G and regulates its expression in breast cancer cells. This relationship has been functionally validated through knockdown experiments and chromatin immunoprecipitation (ChIP) assays .

• Regulatory implications in cancer:

  • The E2F1/SEC61G axis regulates critical cancer phenotypes including cell proliferation, invasion, and apoptosis

  • This axis also modulates glycolysis and chemosensitivity to Herceptin in breast cancer cells

  • SEC61G overexpression can antagonize the effects of E2F1 knockdown, confirming its position as a downstream effector

• Research applications:

  • Promoter analysis: Researchers can use the SEC61G promoter region containing E2F1 binding sites to study transcription factor dynamics in different cellular contexts

  • Reporter assays: Constructing luciferase reporters containing the SEC61G promoter allows quantitative assessment of transcriptional regulation under various experimental conditions

  • CRISPR-based approaches: Genome editing of E2F1 binding sites in the SEC61G promoter can help establish causality in regulatory relationships

• Therapeutic exploitation:

  • Small molecules targeting the E2F1-SEC61G interaction could potentially modulate SEC61G expression

  • Epigenetic modifiers affecting the chromatin state at the SEC61G promoter might provide alternative regulatory approaches

  • Synthetic biology approaches could leverage understanding of SEC61G regulation to create cancer-specific expression systems

• Research methodology:

  • ChIP-seq to identify additional transcription factors that may regulate SEC61G in different contexts

  • ATAC-seq to assess chromatin accessibility at the SEC61G locus

  • Single-cell transcriptomics to understand the heterogeneity of SEC61G regulation in mixed cell populations

Understanding the transcriptional regulation of Sec61g provides researchers with valuable tools to manipulate its expression in experimental settings and potentially develop targeted therapeutic approaches.

How do intrinsically disordered domains affect Sec61g-mediated protein translocation?

The interaction between intrinsically disordered domains (IDDs) and Sec61g-mediated protein translocation reveals a fundamental limitation of the translocation machinery with significant implications for both basic research and therapeutic applications:

• Evolutionarily conserved deficiency: Research has demonstrated that neither the bacterial SecY nor the eukaryotic Sec61 translocon can efficiently transport proteins entirely composed of intrinsically disordered domains. This limitation is observed across bacteria, yeast, and mammalian cells, suggesting an evolutionary conservation of this translocation constraint .

• Experimental evidence:

  • In mammalian cells, proteins with intrinsically disordered domains showed impaired translocation even when equipped with efficient ER signal peptides

  • Similar impairment was observed in yeast systems using the Kre5p ER signal peptide

  • Bacterial systems also demonstrated poor translocation of IDD-containing proteins

• Structural requirements for effective translocation:

  • The addition of α-helical domains can restore translocation of IDD-containing proteins

  • In bacteria, α-helical domains must precede the IDD for effective translocation

  • In mammalian cells, C-terminally located α-helical domains are sufficient to promote translocation

• Methodological implications for recombinant protein design:

  • When designing recombinant proteins containing IDDs for secretion, researchers should incorporate strategically positioned α-helical domains

  • The position of these α-helical domains should be species-appropriate (N-terminal for bacterial expression, C-terminal sufficient for mammalian expression)

  • Signal peptide efficiency alone cannot overcome the translocation barrier for IDD-containing proteins

This understanding of how IDDs affect Sec61g-mediated translocation provides crucial guidance for researchers working with disordered proteins and offers insights into the evolutionary constraints on protein structure and trafficking.

What are the differences in Sec61g functionality between co-translational and post-translational protein translocation?

Sec61g participates in both co-translational and post-translational protein translocation, with distinct functional characteristics in each pathway:

Co-translational translocation:

• Ribosome interaction: In co-translational translocation, Sec61g forms part of the channel that directly interacts with the translating ribosome. Structural studies show that TRAP-γ (which interacts with Sec61g) binds to 28S ribosomal RNA and ribosomal protein L38, forming a bridge between the ribosome and the translocon .

• Signal recognition: The signal sequence emerges from the ribosome and is recognized by the signal recognition particle (SRP), which targets the ribosome-nascent chain complex to the Sec61 translocon.

• Channel dynamics: During co-translational translocation, the lateral gate of Sec61 undergoes dynamic repositioning, including the movement of TMH2 and TMH3 relative to TMH7 and TMH8, and displacement of the plug domain to facilitate protein passage .

Post-translational translocation:

• Chaperone dependence: In the absence of the targeting function of the ribosome, post-translational translocation relies heavily on molecular chaperones to maintain translocation competence of the fully synthesized protein.

• Energy requirements: Post-translational translocation often requires additional energy input, such as ATP hydrolysis by associated chaperones or other factors.

• Substrate specificity: Certain protein characteristics make them more suitable for post-translational rather than co-translational translocation, such as small size or rapid translation that outpaces SRP recognition.

Methodological considerations for researchers:

• Experimental design: When studying Sec61g function, researchers should specify whether they are examining co-translational or post-translational processes, as the associated factors and dynamics differ significantly.

• Reconstitution systems: In vitro reconstitution of Sec61-mediated translocation should include ribosomes and targeting factors for co-translational systems, while post-translational systems require appropriate chaperones.

• Inhibitor specificity: Certain inhibitors may differentially affect co-translational versus post-translational translocation. For example, the cyclotriazadisulfonamide derivative CK147 has been identified as a translocon inhibitor that binds the channel and interacts with the plug helix from the lumenal side .

Understanding these functional differences is crucial for accurate experimental design and interpretation when studying Sec61g-mediated protein translocation.

How might targeting Sec61g be exploited as a therapeutic strategy in cancer?

Targeting Sec61g represents a promising therapeutic strategy in cancer treatment, supported by multiple lines of evidence:

• Overexpression in cancer: SEC61G is overexpressed in almost all cancer types compared to normal tissues, with significant upregulation in breast cancer and glioblastoma . This widespread overexpression provides a potential cancer-specific therapeutic window.

• Functional significance in cancer biology:

  • Knockdown of SEC61G suppresses breast cancer cell proliferation, migration, invasion, and promotes apoptosis

  • SEC61G plays a critical role in glycolysis regulation, a key metabolic pathway in cancer cells

  • In glioblastoma, SEC61G assists EGFR-amplified tumors in evading immune surveillance

• Potential therapeutic approaches:

  • Small molecule inhibitors: Compounds like the cyclotriazadisulfonamide derivative CK147 bind to the Sec61 channel and interact with the plug helix from the lumenal side. Structural data reveals that CK147 resistance mutations surround the inhibitor binding site, providing a foundation for rational drug design .

  • RNA interference: siRNA or shRNA targeting SEC61G has shown efficacy in preclinical models, suggesting that therapeutic RNA interference could be a viable approach .

  • Combination therapies: The E2F1/SEC61G axis regulates chemosensitivity to Herceptin in breast cancer cells, suggesting that SEC61G inhibition could enhance the efficacy of existing therapies .

  • Immunotherapy combinations: Given SEC61G's role in immune evasion in glioblastoma, combining SEC61G inhibition with immunotherapies might overcome resistance mechanisms .

• Experimental design considerations:

  • Target validation: Researchers should confirm SEC61G dependency in their cancer model of interest using genetic approaches before proceeding to inhibitor development

  • Specificity assessment: Given the essential nature of protein translocation, selective targeting of cancer-specific functions of SEC61G is crucial

  • Biomarker development: Identifying patients most likely to respond to SEC61G-targeted therapy will require robust biomarkers, potentially including SEC61G expression levels or specific pathway activation signatures

The compelling evidence for SEC61G's role in cancer, combined with emerging structural insights into the Sec61 complex, provides a strong rationale for developing SEC61G-targeted cancer therapies.

What are the most useful experimental models for studying the effects of Sec61g inhibition?

Researchers investigating Sec61g inhibition should consider multiple experimental models to comprehensively evaluate efficacy, specificity, and potential toxicity:

• In vitro cellular models:

  • Cancer cell line panels: Use diverse cell lines with varying levels of SEC61G expression to identify potential biomarkers of sensitivity

  • Isogenic cell pairs: Engineer cell lines with SEC61G knockdown/knockout versus control to validate target dependency

  • 3D organoid cultures: These better recapitulate tissue architecture and can provide insights into microenvironmental effects of Sec61g inhibition

  • Primary patient-derived cells: These maintain the heterogeneity of the original tumor and may better predict clinical responses

• Biochemical and structural models:

  • Reconstituted proteoliposomes: Purified Sec61 complex incorporated into liposomes allows direct assessment of translocation inhibition by candidate compounds

  • Structural studies: Cryo-EM structures of inhibitor-bound Sec61 complex provide critical insights for rational drug design, as demonstrated with the CK147 inhibitor

• In vivo models:

  • Xenograft models: Human cancer cells implanted in immunocompromised mice allow assessment of tumor growth inhibition and pharmacokinetics

  • Genetically engineered mouse models (GEMMs): Tissue-specific Sec61g manipulation in mice can reveal potential systemic toxicities

  • Patient-derived xenograft (PDX) models: These maintain tumor heterogeneity and stromal components more faithfully than cell line xenografts

• Specialized functional assays:

  • Glycolysis assays: Given SEC61G's role in glycolysis regulation, measuring parameters like glucose uptake, lactate production, and extracellular acidification rate are particularly relevant

  • Protein translocation assays: Direct measurement of translocation efficiency using reporter substrates

  • Immune interaction models: For evaluating SEC61G's role in immune evasion, particularly relevant in glioblastoma

• Combination therapy models:

  • Herceptin combination studies: Since the E2F1/SEC61G axis regulates Herceptin sensitivity, models testing combination effects are valuable

  • Immunotherapy combinations: Particularly for evaluating SEC61G inhibition in enhancing immune recognition of tumors

When designing experiments utilizing these models, researchers should implement appropriate controls and consider potential compensatory mechanisms that might emerge during Sec61g inhibition.