Recombinant Human Synaptogyrin-2 (SYNGR2)

What is Synaptogyrin-2 and what are its primary cellular functions?

Synaptogyrin-2 (SYNGR2), also known as Cellugyrin or UNQ352/PRO615, is a member of the synaptogyrin family of proteins involved in membrane trafficking and vesicle formation . This 224 amino acid protein plays several crucial roles in cellular function:

• Regulated exocytosis in various cell types

• Modulation of synaptophysin/SYP localization into synaptic-like microvesicles

• Formation and maturation of synaptic-like microvesicles

• GLUT4 storage and transport to the plasma membrane

• Assembly of cytoplasmic inclusion bodies required for viral replication

Molecularly, SYNGR2 may undergo tyrosine phosphorylation by Src kinase, which likely regulates its functional activities . This post-translational modification appears to influence its interactions with other proteins and its subcellular localization during cellular responses.

What is the structure and topology of SYNGR2 within cellular membranes?

SYNGR2 is a transmembrane protein with a specific topology in the membrane. Based on the evidence from the research literature, SYNGR2 features:

• Four transmembrane domains (TM1-4)

• N-terminal and C-terminal cytoplasmic regions

• A middle cytoplasmic loop (MOL) facing the cytoplasmic surface

• Two intraluminal loops (IL1/IL1A and IL2)

This topographical arrangement is critical for understanding SYNGR2's interactions with other proteins and its functional roles. The protein's structure allows different regions to interact with distinct binding partners - the cytoplasm-facing domains can interact with cellular machinery while the intraluminal domains may interact with vesicle contents or, in the case of infection, viral components .

How is SYNGR2 expression regulated during normal and pathological conditions?

SYNGR2 shows both constitutive expression and inducible upregulation depending on cellular conditions:

• In normal cells, SYNGR2 is constitutively expressed and primarily found in the soluble fraction of cell lysates

• During SFTS bunyavirus infection, SYNGR2 mRNA is dramatically upregulated (up to 275-fold increase at 36 hours post-infection)

• Viral infections like COVID-19 are associated with SYNGR2 upregulation (>1.035-fold and 1.35-fold compared to non-viral respiratory disorders)

What's particularly interesting from a research perspective is that during viral infection, a significant proportion of SYNGR2 protein translocates from the soluble to the insoluble fraction of cell lysates, indicating structural reorganization within the cell .

What are the optimal methods for detecting and quantifying SYNGR2 in research samples?

For comprehensive SYNGR2 analysis, researchers should consider multiple detection methods:

RNA Detection:

• Real-time RT-PCR with SYNGR2-specific primers has proven effective for detecting upregulation during viral infection

• When designing primers, target conserved regions of the SYNGR2 transcript

Protein Detection:

• Western blotting can detect both soluble and insoluble SYNGR2

• Critical methodological consideration: When studying infected cells, analyze both soluble and insoluble fractions separately, as SYNGR2 significantly shifts to insoluble fractions during viral infection

• SDS-PAGE followed by Coomassie Blue staining can visualize recombinant SYNGR2 protein

Microscopy-Based Detection:

• Immunofluorescence with validated anti-SYNGR2 antibodies

• Fluorescent fusion proteins (e.g., DsRed-SYNGR2) for live imaging studies

• Co-localization studies with markers for cellular compartments

What experimental systems are available for studying recombinant SYNGR2?

Researchers have several options for working with recombinant SYNGR2:

Commercial Recombinant Protein:

• Full-length human SYNGR2 protein (1-224 amino acids) expressed in wheat germ is available for applications including SDS-PAGE, ELISA, and Western blotting

• The amino acid sequence: MESGAYGAAKAGGSFDLRRFLTQPQVVARAVCLFVFSCIYGEGYSNHESKQMYCVFNRNEDACRYGSAIGVLAFLASAFFLVVDAYFPQISNATDRKYLVIGDLLFSALWTFLWFVGFCFLTNQWAVTNPKDVLVGADSVRAAITFSFFSIFSWGVLASLAYQRYKAGVDDFIQNYVDPTPDPNTAYASYPGASVDNYQQPPFTQNAETTEGYQPPPVY

Expression Vectors:

• Plasmid vectors for expressing tagged versions include:

  • pRK5-F-SYNGR2 (FLAG-tagged)

  • EGFP-NSs and DsRed-SYNGR2 for fluorescence fusion proteins

Cell Lines for Expression Studies:

• HepG2, HeLa, and HEK293 cells have all been successfully used to study SYNGR2 expression and function

• Jurkat T cells have been employed for SYNGR2 knockout studies using CRISPR/Cas9 technology

What approaches can be used to study SYNGR2 protein-protein interactions?

Several methodologies have proven effective for investigating SYNGR2 interactions:

Co-immunoprecipitation (Co-IP):

• Successfully used to demonstrate interaction between SYNGR2 and viral proteins like NSs

• Protocol: Prepare cell lysates from infected or transfected cells, immunoprecipitate with either anti-SYNGR2 or anti-target protein antibodies, followed by SDS-PAGE and Western blot analysis

Surface Plasmon Resonance (SPR):

• Effective for analyzing binding of specific SYNGR2 peptides to potential interacting partners

• Implementation: Biotinylated SYNGR2 peptides can be immobilized on Neutravidin-coated sensors, followed by flowing potential binding partners over the surface

• SPR has revealed differential binding of viral proteins to specific SYNGR2 domains (e.g., SARS-CoV-2 spike domain 1 preferentially binds to the N-terminal region)

Confocal Microscopy for Co-localization:

• Effective for visualizing SYNGR2 translocation in response to interacting partners

• In cells co-transfected with SYNGR2 and viral proteins, SYNGR2 changes its localization pattern to co-localize with inclusion bodies

What role does SYNGR2 play in viral replication and pathogenesis?

SYNGR2 has emerged as a significant factor in viral infection cycles:

SFTS Bunyavirus (SFTSV):

• SYNGR2 is dramatically upregulated (up to 275-fold) during SFTSV infection

• Interacts with the viral non-structural protein NSs

• Facilitates the formation of viral inclusion bodies (IBs) that serve as viral factories for RNA replication

• Silencing SYNGR2 with specific shRNA significantly reduces viral RNA replication and infectious virus titers

SARS-CoV-2:

• SYNGR2 is upregulated (>1.035-fold) in COVID-19 patients compared to those with non-viral respiratory disorders

• SARS-CoV-2 spike protein (both full-length and S1 domain) shows binding affinity to SYNGR2 peptides in SPR analysis

• The S1 domain preferentially binds to the N-terminal region of SYNGR2

Common Mechanism Hypothesis:
Research suggests SYNGR2 represents a common pathway exploited by both bacterial toxins and viruses to gain host cell entry . This pathway appears to involve:

• Interaction with cholesterol-rich microdomains

• Formation of synaptic-like microvesicles containing SYNGR2 (SLMV Cg+)

• Facilitation of pathogen internalization and trafficking

How can researchers manipulate SYNGR2 expression to study its role in viral infection?

CRISPR/Cas9 Knockout:

• Successfully employed to generate SYNGR2 knockout Jurkat cells (Jurkat Cg-) using commercially available reagents

• This approach allows direct comparison of wild-type (Cg+) and knockout (Cg-) cells in viral infection studies

shRNA Silencing:

• Effective for reducing SYNGR2 expression

• Correlates with reduced formation of large inclusion bodies and decreased viral replication

Experimental Readouts for Effectiveness:

• Viral RNA replication (measured by RT-PCR)

• Infectious virus titers

• Formation of inclusion bodies (size and number)

• Flow cytometry analysis of cells infected with fluorescent reporter viruses

What experimental systems are available for studying SYNGR2-virus interactions?

Viral Systems:

• SFTS bunyavirus (SFTSV): Well-established model for studying SYNGR2's role in viral replication

• Vesicular stomatitis virus (VSV) pseudotyped with SARS-CoV-2 spike protein (VSV-ΔGFP/SARS-CoV2-S)

Cellular Models:

• HepG2, HeLa, HEK293: Used for expression studies and infection models

• Jurkat cells: T cell model with available CRISPR knockout

• Calu-3 cells: Respiratory epithelial cell line

Biochemical Interaction Studies:

• Surface plasmon resonance with biotinylated SYNGR2 peptides representing different domains

• The following peptides have been used successfully:

  • N-terminus (N-Term)

  • C-terminus (C-Term 1 and C-Term 2)

  • Middle loop (MOL)

  • Intraluminal loops (IL1, IL1A, IL2)

How does SYNGR2 contribute to inclusion body formation during viral infection?

Research suggests SYNGR2 plays a critical role in restructuring lipid droplets into viral inclusion bodies:

Mechanism:

• SYNGR2 interacts with viral proteins like NSs and translocates into insoluble structures

• It facilitates the conversion of regular lipid droplets into enlarged inclusion bodies (IBs)

• These IBs serve as viral factories for RNA replication

Experimental Approach for Studying This Process:

• Co-transfection with fluorescently tagged SYNGR2 and viral proteins

• Time-course confocal microscopy to track IB formation

• Quantification of IB size and number

  • IBs larger than 0.2 μm in diameter increase significantly at 12 and 24 hours post-transfection

• Correlate IB formation with viral replication metrics

This research area represents a frontier in understanding how viruses repurpose cellular structures and could lead to novel antiviral strategies targeting this process.

What is the relationship between SYNGR2's membrane topology and its functional interactions?

The specific topology of SYNGR2 in membranes appears critical for its interactions with viral components:

Structure-Function Relationships:

• The four transmembrane domains anchor SYNGR2 in vesicle membranes

• Different regions show preferential binding to various viral proteins:

  • N-terminal region: Strong binding to SARS-CoV-2 S1 domain (530 RU in SPR)

  • Intraluminal loop 1 (IL1A): Preferential binding to full-length SARS-CoV-2 spike (220 RU)

  • Middle loop (MOL): Highest affinity for VSV-G protein (140 RU)

Proposed Experimental Approach:

• Generate domain-specific mutations or deletions in SYNGR2

• Assess the effect on:

  • Viral protein binding (using SPR or co-IP)

  • SYNGR2 trafficking and localization

  • Viral replication efficiency

Understanding these structure-function relationships could inform the design of peptide inhibitors that disrupt SYNGR2-pathogen interactions.

What methodologies can be used to target SYNGR2 for potential antiviral applications?

Given SYNGR2's apparent role as a "universal facilitator" for pathogen entry, several approaches could be explored:

Competitive Inhibition:

• Design peptides based on the binding interfaces between SYNGR2 and viral proteins

• SPR data on binding affinities to different SYNGR2 domains provides a starting point for rational design

Inhibition of SYNGR2 Upregulation:

• Target transcription factors or signaling pathways responsible for SYNGR2 upregulation during infection

• Monitor effectiveness using RT-PCR and Western blotting as described in research protocols

Disruption of SYNGR2 Trafficking:

• Develop compounds that prevent SYNGR2 translocation into inclusion bodies

• Evaluate using confocal microscopy to track fluorescently tagged SYNGR2

Research Progress Metrics:

• Reduction in viral RNA replication

• Decrease in infectious virus titers

• Disruption of inclusion body formation

• Maintenance of normal cellular functions

These approaches represent "non-anti-microbial" strategies that could complement traditional antiviral approaches by targeting host factors essential for viral replication .

SYNGR2 Domain-Specific Binding Affinities to Viral Proteins

Note: Values represent maximum response units (RU) measured by Surface Plasmon Resonance (SPR) . Higher values indicate stronger binding affinity.

SYNGR2 Expression Changes During Viral Infection

Note: Values represent fold increase in SYNGR2 mRNA measured by real-time RT-PCR compared to non-infected controls .