Recombinant Protein SEY1 homolog 2 (GSPATT00022700001)

What is SEY1 and what is its primary cellular function?

SEY1 (Synthetic Enhancer of YOP1) is a dynamin-like GTPase that plays a critical role in homotypic endoplasmic reticulum (ER) fusion. It was initially identified in Saccharomyces cerevisiae as a protein that interacts genetically with Yop1p (hence the name Synthetic Enhancer of YOP1) . The protein consists of a dynamin-like GTPase domain with characteristic signature motifs, a helical bundle domain, two closely spaced transmembrane segments, and a C-terminal tail .

SEY1's primary function is to mediate the fusion of ER membranes, which is essential for maintaining the dynamic structure of the ER network. The mechanism involves GTP-dependent dimerization of SEY1 proteins located in opposing ER membranes, followed by conformational changes upon GTP hydrolysis that bring the membranes close enough to fuse . This process is fundamental to ER morphogenesis and homeostasis.

To experimentally assess SEY1's function, researchers have developed both in vivo fusion assays (using fluorescently labeled ER markers to track fusion events in mating yeast cells) and in vitro proteoliposome fusion assays . These methods have demonstrated that SEY1 mediates GTP-dependent membrane fusion similar to that of mammalian atlastins.

What experimental methods are most effective for studying SEY1 localization and dynamics?

To effectively study SEY1 localization and dynamics, researchers have employed several complementary approaches:

In vivo fluorescence-based assays:

• Fusion of fluorescent proteins (GFP, RFP) to SEY1 for live-cell imaging

• Development of mating assays with differentially labeled ER markers to track membrane fusion events in real-time

• Time-lapse microscopy to monitor dynamic changes in ER structure

Biochemical approaches:

• Subcellular fractionation followed by Western blotting

• Co-immunoprecipitation to identify interaction partners (e.g., Rtn1p and Yop1p in yeast)

• Protease protection assays to determine membrane topology

Advanced imaging techniques:

• Ultrastructure expansion microscopy to visualize ER and Golgi morphology changes upon SEY1 inhibition or deletion

• Super-resolution microscopy to observe nanoscale dynamics of SEY1-mediated membrane fusion

For quantitative assessment of SEY1-mediated fusion, researchers have developed an in vivo assay using mating yeast cells. In this approach, haploid cells expressing cytosolic GFP are mated with haploid cells expressing ER-localized RFP (ss-RFP-HDEL), and the equilibration of the ER marker between cells is monitored over time. In wild-type cells, equilibration occurs within approximately 4 minutes after cell fusion, while in sey1Δ cells, equilibration is delayed to approximately 25 minutes .

How does the GTPase activity of SEY1 mechanistically drive membrane fusion?

The GTPase activity of SEY1 is essential for driving membrane fusion through a series of coordinated conformational changes. Based on biochemical and structural studies, the mechanism appears to involve the following steps:

• GTP binding and dimerization: SEY1 proteins in opposing membranes bind GTP, which promotes dimerization of their GTPase domains. This initial interaction brings the membranes into close proximity .

• GTP hydrolysis and conformational change: Hydrolysis of GTP to GDP induces a conformational change in the SEY1 dimer, which exerts mechanical force to pull the membranes even closer together .

• Membrane destabilization and fusion: The mechanical force disrupts the lipid bilayers, leading to hemifusion and eventually complete fusion of the membranes.

• GDP release and recycling: After fusion, GDP is released, the SEY1 proteins dissociate, and the cycle can begin again.

Evidence for this mechanism comes from several experimental approaches:

• Liposome fusion assays: Proteoliposomes containing purified Sey1p fuse in a GTP-dependent manner in vitro. No fusion is observed in the absence of magnesium ions or when GTP is replaced with GDP or non-hydrolyzable GTPγS .

• Mutational analysis: The GTPase-deficient mutant Sey1p-K50A (with a mutation in the P-loop) shows drastically reduced GTPase activity and fails to mediate fusion in vitro or complement fusion defects in vivo .

• Oligomerization studies: Sedimentation velocity analysis of the Sey1p GTPase domain (Sey1-ΔTM) shows that it exists predominantly as a monomer in the absence of nucleotide or in the presence of GDP, but forms dimers in the presence of GDP and AlFx (a transition state mimic) .

A quantitative comparison of SEY1's GTPase activity shows:

Understanding this mechanism is essential for designing experiments to study SEY1 function and for developing potential inhibitors for therapeutic applications.

What is the relationship between SEY1 and SNARE-mediated ER fusion pathways?

SEY1 and SNARE proteins represent two distinct but partially redundant pathways for ER membrane fusion in yeast. This redundancy likely ensures the robustness of ER network maintenance .

Evidence for parallel pathways:

• Genetic interactions: In S. cerevisiae, the absence of Sey1p results in delayed ER fusion (approximately 25 minutes for equilibration compared to 4 minutes in wild-type cells), but fusion still eventually occurs .

• SNARE requirement: The residual ER-ER fusion in cells lacking Sey1p requires the ER SNARE protein Ufe1p. Temperature-sensitive ufe1-1 mutants show moderately delayed ER fusion (10 minutes), but ufe1-1 sey1Δ double mutants show severely impaired fusion (median time >60 minutes) .

• Rescue experiments: Overexpression of Sey1p in ufe1-1 cells reduces the median time of ER-ER fusion from 10 to 6 minutes, suggesting that increased activity of one pathway can partially compensate for defects in the other .

Experimental approach to study both pathways:

To investigate the relative contributions of SEY1 and SNARE-mediated pathways, researchers have developed a quantitative in vivo fusion assay using fluorescently labeled ER markers in mating yeast cells. The time required for equilibration of an ER marker (ss-RFP-HDEL) between cells provides a measure of fusion efficiency .

These findings suggest a model where SEY1 and SNAREs constitute separate fusion machineries that can function independently but likely cooperate during normal ER dynamics. This dual system may provide regulatory flexibility and ensure ER integrity under various cellular conditions .

How is SEY1 being explored as a drug target in Plasmodium species?

Recent research has identified SEY1 as a novel and promising drug target in Plasmodium species, the causative agents of malaria. This discovery represents a significant advancement in antimalarial drug development strategies .

Evidence for SEY1 as a druggable target:

• Target identification: Proteomic affinity chromatography and chemical genetics approaches identified PfSEY1 (P. falciparum SEY1, PF3D7_1416100) as one of the highest-ranked potential targets for imidazolopiperazine (IZP) compounds like GNF179 .

• Essentiality: SEY1 is predicted to be an essential gene in P. falciparum, making it an attractive target for antimalarial drug development .

• Direct binding evidence:

  • GNF179 decreases the melting temperature of Plasmodium SEY1

  • GNF179 binds to recombinant Plasmodium SEY1 in surface plasmon resonance experiments

  • Molecular docking predicts binding to conserved GTPase motifs

• Functional inhibition: GNF179 inhibits the GTPase activity of PvSEY1 (P. vivax SEY1), providing a mechanistic explanation for its antimalarial effects .

• Resistance mechanisms: Overexpression of Plasmodium SEY1 confers resistance to GNF179, confirming SEY1 as a target of this compound .

Experimental approaches to validate SEY1 as a drug target:

This multifaceted evidence strongly suggests that SEY1 is a valid target for antimalarial drug development, potentially opening new avenues for combating drug-resistant malaria .

What methods are most effective for expression and purification of functional recombinant SEY1 proteins?

Expression and purification of functional recombinant SEY1 proteins present significant challenges due to their complex structure, including transmembrane domains and GTPase activity requirements. Based on successful approaches described in the literature, the following methodologies are recommended:

Expression systems:

• Bacterial expression (E. coli): Suitable for soluble domains (e.g., GTPase domain without transmembrane regions)

  • Use of specialized strains (BL21(DE3), Rosetta) to address codon usage bias

  • Expression at lower temperatures (16-18°C) to improve folding

  • Fusion tags (GST, MBP) to enhance solubility

• Yeast expression (S. cerevisiae, K. phaffii [P. pastoris]): Better for full-length protein

  • Demonstrated success with PvSEY1-myc expression in K. phaffii GS115

  • More appropriate folding environment for eukaryotic proteins

• Insect cell expression (Sf9, Hi5): Optimal for complex eukaryotic proteins

  • Baculovirus expression system provides eukaryotic post-translational modifications

  • Better membrane protein folding and insertion into membranes

Purification strategies:

• For full-length SEY1 (including transmembrane domains):

  • Solubilization with mild detergents (DDM has been successfully used)

  • Two-step affinity purification using appropriate tags

  • Size exclusion chromatography to ensure homogeneity

• For soluble domains (e.g., Sey1-ΔTM):

  • Standard affinity chromatography (GST, His-tag)

  • Ion exchange chromatography

  • Size exclusion chromatography

Quality control methods:

• Functional assays:

  • GTPase activity measurement using colorimetric phosphate detection

  • In vitro liposome fusion assays

  • GTP binding assays

• Structural integrity assessment:

  • Circular dichroism spectroscopy

  • Thermal shift assays

  • Limited proteolysis

Reconstitution for functional studies:
For in vitro fusion assays, purified SEY1 can be reconstituted into proteoliposomes. A successful protocol involves:

• Mixing purified protein with lipids in detergent

• Detergent removal by dialysis or adsorption to Bio-Beads

• Flotation experiments to confirm successful reconstitution

• Fusion assays using fluorescently labeled lipids

This approach has been used to demonstrate that Sey1p mediates fusion of vesicles in a concentration-dependent manner, with a strict requirement for GTP and magnesium ions .

How can researchers investigate the role of SEY1 in ER stress responses?

Investigating the role of SEY1 in ER stress responses requires a multi-faceted approach that combines genetic manipulation, biochemical assays, and advanced imaging techniques. While the provided search results don't directly address SEY1's role in ER stress, the following methodological framework would be appropriate based on known SEY1 functions:

Genetic approaches:

• Gene deletion/knockdown studies:

  • Generate SEY1 knockout or knockdown cell lines/organisms

  • Use conditional systems (e.g., tetracycline-inducible) for essential genes

  • CRISPR-Cas9 genome editing for precise mutations in GTPase domain

• Overexpression studies:

  • Express wild-type or mutant SEY1 under inducible promoters

  • Create fusion proteins with fluorescent tags for localization studies

  • Use heterologous expression systems (as demonstrated with PvSEY1 in K. phaffii)

ER stress induction and monitoring:

• Chemical ER stressors:

  • Tunicamycin (N-glycosylation inhibitor)

  • Thapsigargin (SERCA pump inhibitor)

  • DTT (reducing agent disrupting disulfide bonds)

• ER stress markers:

  • Unfolded protein response (UPR) activation: BiP/GRP78, CHOP, XBP1 splicing

  • Phosphorylation of eIF2α and PERK

  • ATF6 cleavage and nuclear localization

Experimental workflow:

• Subject wild-type and SEY1-modified cells to ER stressors

• Assess UPR activation via Western blotting, qRT-PCR of UPR targets

• Monitor ER morphology changes using fluorescence microscopy

• Evaluate cell viability and recovery after stress

Advanced methodologies:

• Real-time visualization of ER dynamics:

  • Live-cell imaging with ER-targeted fluorescent proteins

  • Photobleaching techniques (FRAP) to assess ER connectivity

  • The mating assay developed for SEY1 function could be adapted to study ER fusion under stress conditions

• Proteomic approaches:

  • Quantitative proteomics to identify changes in the SEY1 interactome during ER stress

  • Phosphoproteomics to detect stress-induced post-translational modifications of SEY1

• ER fragmentation analysis:

  • Ultrastructure expansion microscopy to visualize ER morphology changes

  • Quantification of ER tubule connectivity and network complexity

This methodological framework would enable researchers to systematically investigate how SEY1-mediated ER fusion contributes to cellular responses to ER stress, potentially revealing new therapeutic targets for diseases associated with ER dysfunction.

What are the most promising future research directions for SEY1 homologs?

Several promising research directions emerge from current understanding of SEY1 proteins:

• Structural biology approaches:

  • Determination of high-resolution structures of full-length SEY1 proteins from different species

  • Cryo-EM studies of SEY1 dimers during different stages of the GTPase cycle

  • Computational modeling of the conformational changes during membrane fusion

• Drug discovery targeting SEY1:

  • Structure-based design of specific inhibitors of Plasmodium SEY1

  • High-throughput screening for novel SEY1 modulators

  • Optimization of lead compounds like GNF179 derivatives with improved selectivity

• Comparative studies across species:

  • Further investigation of functional conservation and divergence between SEY1 homologs

  • Examination of organism-specific adaptations and regulatory mechanisms

  • Evolutionary analysis to understand why organisms have either SEY1 or atlastin homologs but not both

• Integration with cellular stress responses:

  • Investigation of SEY1's role in the unfolded protein response

  • Examination of how SEY1-mediated ER fusion is coordinated with other ER quality control mechanisms

  • Study of SEY1 contribution to organelle contact sites and inter-organelle communication

• Therapeutic applications:

  • Beyond antimalarials, exploration of SEY1 as a target in other pathogenic organisms

  • Investigation of SEY1 dysfunction in human diseases related to ER morphology

  • Development of strategies to modulate SEY1 activity for therapeutic benefit