Recombinant Schizosaccharomyces pombe Protein sey1 (sey1)

What is the domain organization of S. pombe Sey1 protein?

S. pombe Sey1 consists of an N-terminal GTPase domain and a long, stalk-like, helical domain connected by a linker region. The stalk domain is composed of four 3-helix bundles (3HBs), with the last helix of each bundle extending into the first helix of the next bundle . The protein contains 762 amino acids with a transmembrane domain near the C-terminus. This structural organization is critical for its function in mediating endoplasmic reticulum fusion through GTP-dependent conformational changes .

How does Sey1p contribute to endoplasmic reticulum membrane dynamics?

Sey1p functions analogously to mammalian atlastins in mediating homotypic ER fusion. When expressed in proteoliposomes, purified Sey1p mediates GTP-dependent membrane fusion in vitro . In S. cerevisiae, absence of Sey1p results in delayed ER fusion in vivo and can be partially complemented by human ATL1 . Interestingly, S. cerevisiae has an alternative ER fusion mechanism that requires the ER SNARE protein Ufe1p, functioning in parallel with Sey1p . This dual mechanism ensures robust ER network maintenance, although S. pombe-specific studies would be needed to confirm a similar arrangement in fission yeast.

How is Sey1 protein function regulated in vivo?

Sey1 function is primarily regulated through its GTPase activity. Crystal structures of Sey1p in different nucleotide-bound states (GDP/AlF4−, GDP, and GMP-PNP) reveal distinct conformational changes that drive membrane fusion . The GTP-bound form promotes dimerization, where the GTPase domains of two Sey1p molecules interact, bringing opposing ER membranes into close proximity. GTP hydrolysis then induces conformational changes that facilitate membrane fusion. Additional regulation may occur through interactions with other ER proteins and phospholipids, although these mechanisms require further investigation in S. pombe specifically.

What expression systems are recommended for recombinant S. pombe Sey1 production?

For recombinant S. pombe Sey1 production, two main expression systems have proven effective:

• E. coli expression: Full-length Sey1 protein (1-762aa) has been successfully expressed with an N-terminal His tag in E. coli . This system is suitable for producing large quantities of protein for biochemical and structural studies.

• S. pombe native expression: S. pombe itself can serve as an expression host using vectors such as pESP-1 and pESP-2 with the nmt1 promoter for regulated expression . Unlike E. coli, S. pombe provides appropriate post-translational modifications, which may be critical for proper protein folding and function.

The optimal expression system depends on the research application. For structural studies of the cytosolic domain, E. coli expression is often sufficient. For functional studies requiring properly folded full-length protein with native modifications, the S. pombe expression system may be preferable.

What are the optimal buffer conditions for maintaining Sey1 protein stability?

For the cytosolic domain, standard protein buffers like Tris/PBS-based buffer at pH 8.0 with 6% trehalose have been used successfully . The full-length protein including the transmembrane domain requires detergent for stability. For crystallization, specific buffer optimization may be necessary, as demonstrated in the successful crystallization of Sey1p in complex with different nucleotides .

What purification strategies yield high-purity recombinant Sey1 protein?

A multi-step purification approach is recommended for high-purity Sey1 protein:

• Initial capture: For His-tagged Sey1, immobilized metal affinity chromatography (IMAC) using Ni-NTA resin is effective for initial purification .

• Tag removal: If necessary, the His-tag can be removed using appropriate proteases (thrombin or SUMO protease for His-SUMO tags).

• Polishing steps:

  • Size exclusion chromatography (SEC) to separate monomeric from oligomeric forms and remove aggregates

  • Ion exchange chromatography for removing contaminants with different charge properties

  • Affinity chromatography with nucleotide analogs (e.g., GTP-agarose) for functional protein enrichment

• Quality control: Assess protein purity by SDS-PAGE (>90% purity is typically achievable) , verify proper folding through circular dichroism, and confirm GTPase activity with functional assays.

For membrane-bound full-length Sey1, additional considerations include detergent selection and concentration throughout the purification process to maintain protein stability while minimizing micelle size.

How can researchers measure Sey1p's GTPase activity in vitro?

Several complementary methods can be used to measure Sey1p's GTPase activity:

• Malachite green phosphate assay: This colorimetric assay quantifies released inorganic phosphate from GTP hydrolysis. It allows continuous monitoring of GTPase activity but may be sensitive to buffer components.

• HPLC analysis: Separates and quantifies GTP, GDP, and GMP to directly measure nucleotide conversion. This method is highly accurate but more labor-intensive.

• Fluorescence-based assays: Using fluorescent GTP analogs (e.g., mant-GTP) allows real-time monitoring of nucleotide binding and hydrolysis through changes in fluorescence intensity or polarization.

• Radioactive assays: [γ-³²P]GTP hydrolysis assays provide high sensitivity for detecting low levels of activity but require radioisotope handling facilities.

When measuring Sey1p GTPase activity, it's crucial to consider:

• The requirement for Mg²⁺ as a cofactor (typically 5 mM MgCl₂)

• Temperature dependence (activity increases at physiological temperatures)

• Protein concentration effects (dimerization-dependent activity enhancement)

• The presence of liposomes (membrane association can modulate activity)

What in vitro assays can evaluate Sey1p-mediated membrane fusion?

Sey1p-mediated membrane fusion can be evaluated using reconstituted proteoliposome systems:

• Lipid mixing assays: Incorporate fluorescent lipid pairs (e.g., NBD-PE and Rh-PE) into separate populations of Sey1p-containing liposomes. Upon fusion, fluorescence resonance energy transfer (FRET) between these labels changes, allowing quantification of membrane fusion rates.

• Content mixing assays: Encapsulate complementary fluorescent molecules in separate liposome populations. Fusion results in interaction of these molecules, producing measurable fluorescence signals.

• Dynamic light scattering (DLS): Monitors changes in liposome size distribution during fusion events.

• Cryo-electron microscopy: Directly visualizes fusion intermediates and end products to understand the morphological changes during the fusion process.

Based on studies with S. cerevisiae Sey1p, the proteoliposome fusion assay should include GTP (typically 1 mM) and show clear GTP-dependence . Control experiments should include GTPase-deficient mutants and GTP analogs like GTPγS or GMP-PNP to distinguish between nucleotide binding and hydrolysis requirements.

How can researchers investigate the structural dynamics of Sey1p during its functional cycle?

Multiple biophysical techniques can capture Sey1p's structural dynamics:

• Hydrogen-deuterium exchange mass spectrometry (HDX-MS): Maps regions of the protein that undergo conformational changes upon nucleotide binding or dimerization.

• Single-molecule FRET: By introducing fluorescent labels at strategic positions, researchers can monitor distance changes between protein domains during the GTPase cycle in real-time.

• Time-resolved X-ray crystallography: Captures structural snapshots at different stages of the GTPase cycle. Crystal structures of Sey1p with different nucleotides (GDP/AlF₄⁻, GDP, and GMP-PNP) have provided significant insights .

• Molecular dynamics simulations: Computational approaches can model transitions between known structural states and predict conformational intermediates.

• Site-directed spin labeling combined with electron paramagnetic resonance (EPR): Measures distances between specific regions during conformational changes.

The crystal structures of Sey1p have revealed that the protein adopts different conformations depending on the bound nucleotide . For instance, in the GDP-bound state, the stalk domain associates with the GTPase domain of the same molecule, while in the GDP/AlF₄⁻ state (mimicking the transition state), the GTPase domains dimerize and the stalks cross each other, likely facilitating membrane fusion.

What site-directed mutagenesis approaches are most informative for Sey1 functional studies?

Strategic site-directed mutagenesis can dissect Sey1's functional domains:

• GTPase domain mutations:

  • K50A/K51A: Disrupts the P-loop (G1 motif), affecting GTP binding

  • T78A: Impairs the G2 motif involved in Mg²⁺ coordination

  • D159A: Disrupts the G3 motif essential for GTP hydrolysis

  • N231A: Modifies the G4 motif involved in guanine nucleotide specificity

• Dimerization interface mutations:

  • Residues at the GTPase domain interface that mediate dimerization

  • Linker region modifications that affect the crossing of stalks

• Stalk domain mutations:

  • Targeted changes in the four 3-helix bundles to affect their arrangement

  • Hinge region modifications to restrict conformational flexibility

• Membrane interaction region:

  • Transmembrane domain alterations to affect membrane anchoring

  • Charged residue substitutions near the membrane interface

When designing mutagenesis studies, it's informative to create an allelic series:

• GTPase-deficient mutants (e.g., D159N) that can bind but not hydrolyze GTP

• Binding-deficient mutants (e.g., K50A/K51A) that cannot engage GTP

• Dimerization-deficient mutants that bind and hydrolyze GTP but cannot mediate fusion

Each mutant should be characterized for: (1) protein expression and stability, (2) GTPase activity, (3) dimerization capability, and (4) membrane fusion activity to establish structure-function relationships.

How do the crystal structures of Sey1p inform our understanding of its mechanism?

Crystal structures of Sey1p's N-terminal cytosolic domain in different nucleotide-bound states have provided crucial insights into its mechanism :

These structures reveal that:

• In the GDP/AlF₄⁻ structure (mimicking the transition state), Sey1p forms a dimer where the GTPase domains face each other and the stalks cross, creating a configuration that could bring opposing membranes together .

• In the GDP-bound structure, the GTPase domain associates with its own stalk domain, representing a post-hydrolysis state where the protein has returned to a monomeric configuration .

• The GMP-PNP structure (mimicking the GTP-bound state) closely resembles the GDP/AlF₄⁻ structure, suggesting that GTP binding promotes dimerization .

These conformational changes suggest a mechanism where GTP binding induces dimerization, bringing opposing membranes together, and GTP hydrolysis causes conformational changes that drive membrane fusion.

How does Sey1 compare structurally and functionally with its homologs in other organisms?

Sey1 shares structural and functional similarities with homologs in other organisms, but with notable differences:

Key differences from the search results:

• Nucleotide-dependent dimerization differs between Sey1p and human atlastins (ATLs). Despite structural similarity, "these proteins likely undergo nucleotide-dependent dimerization differently" .

• Sey1p structures reveal that "the stalk domain (mainly 3HB-1) is associated with the GTPase domain of the same molecule" in the GDP-bound state , a feature not observed in ATL structures.

• In S. cerevisiae, Sey1p functions in parallel with an ER SNARE-mediated fusion pathway. When Sey1p is absent, "residual ER-ER fusion... required the ER SNARE Ufe1p" . Whether a similar redundant mechanism exists in S. pombe requires investigation.

• The stalk domain of Sey1p is "comparable to that of bacterial dynamin-like protein (BDLP) and is longer than Dynamin-1 or human guanylate-binding protein 1 (GBP1)" , suggesting potential evolutionary relationships.

Understanding these similarities and differences provides insights into the evolution of membrane fusion mechanisms and may guide the development of specific modulators of Sey1 function.

How can researchers create and characterize Sey1 knockout or conditional mutants in S. pombe?

Creating and characterizing Sey1 mutants in S. pombe requires several approaches:

Based on S. cerevisiae studies, researchers should look for changes in ER morphology (increased sheets, decreased tubules) and delayed ER fusion in Sey1-deficient S. pombe cells . The effects might be partial if S. pombe also possesses an alternative fusion mechanism similar to the SNARE-dependent pathway in S. cerevisiae.

What are the current limitations in our understanding of S. pombe Sey1 function?

Several significant gaps remain in our understanding of S. pombe Sey1:

• While structural studies provide insight into the mechanism of Sey1p , these have primarily been conducted with partial protein constructs. The behavior of the full-length protein including its transmembrane domain in an actual membrane environment remains incompletely characterized.

• The specific regulation of Sey1 activity in response to cellular conditions (e.g., metabolic state, cell cycle, stress) is not well understood.

• Though S. cerevisiae has an alternative SNARE-mediated ER fusion pathway , whether S. pombe possesses a similar redundant mechanism remains to be established.

• The complete interactome of Sey1 in S. pombe and how these interactions modulate its function are not fully mapped.

• The potential role of Sey1 in specific cellular processes such as meiosis, spore formation, and stress response has not been thoroughly investigated.

What novel experimental approaches could advance Sey1 research?

Several innovative approaches could drive future S. pombe Sey1 research:

• Cryo-electron tomography of native ER membranes to visualize Sey1-mediated fusion events in a near-native environment.

• Single-molecule studies to track individual Sey1 proteins during the fusion cycle, revealing kinetic intermediates and rate-limiting steps.

• Optogenetic approaches to rapidly control Sey1 activity or localization, allowing temporal dissection of its function.

• Systematic mutation libraries combined with high-throughput phenotyping to generate comprehensive structure-function maps.

• Comparative studies across multiple yeast species to elucidate the evolution of Sey1 function and its interaction with other ER-shaping mechanisms.

• Integration of proteomics, lipidomics, and structural biology to understand how Sey1 function is influenced by the lipid environment and interacting proteins.

• Development of small molecule modulators of Sey1 function as research tools for acute perturbation of ER fusion.

These approaches would provide multidimensional insights into Sey1 function and could potentially reveal unexpected roles beyond its established involvement in ER membrane fusion.

How might insights from S. pombe Sey1 research translate to other biological systems?

Research on S. pombe Sey1 has broader implications:

• Evolutionary insights: Comparing Sey1 function across species provides a window into the evolution of membrane fusion mechanisms. The structural similarities and differences between S. pombe Sey1, S. cerevisiae Sey1p, and human atlastins highlight both conserved and divergent aspects of these GTPases .

• Disease relevance: Human atlastins (Sey1 homologs) are implicated in hereditary spastic paraplegia. Understanding the fundamental mechanisms of Sey1 function could provide insights into disease pathogenesis and potential therapeutic approaches.

• Biotechnological applications: Knowledge of membrane fusion mechanisms could inform the development of membrane-modulating technologies, such as improved liposome delivery systems or cell fusion techniques.

• Synthetic biology: Engineered Sey1 variants could potentially be used to create novel cellular compartments or control membrane dynamics in synthetic biological systems.

• Comparative cell biology: Insights from S. pombe can complement studies in other model organisms, building a more comprehensive understanding of ER dynamics across eukaryotes.