Recombinant Solanum demissum CASP-like protein SDM1_58t00016 (SDM1_58t00016)

What is known about the membrane topology and conserved domains of SDM1_58t00016?

SDM1_58t00016 exhibits the characteristic CASP protein topology with:

• Four transmembrane domains spanning the plasma membrane

• Two intracellular loops

• One extracellular loop

• N-terminal and C-terminal residues extending into the cytoplasm

The first (TM1) and third (TM3) transmembrane domains show particularly high conservation across the CASP/CASPL family, suggesting functional importance . Phylogenetic analyses have revealed that these conserved transmembrane domains are likely involved in CASP localization to specialized membrane domains . The protein associates with the plasma membrane, where it potentially contributes to the formation of specialized membrane microdomains that direct cell wall modifications, similar to other characterized CASP proteins .

What is the putative function of SDM1_58t00016 based on homology to other CASP proteins?

While the specific function of SDM1_58t00016 in Solanum demissum has not been directly characterized, insights can be derived from studies of homologous proteins:

• Casparian strip formation: In Arabidopsis, CASP proteins (CASP1-5) define and accumulate at Casparian strip membrane domains, creating protein exclusion zones and promoting membrane-cell wall adhesion . They form a scaffolding platform that recruits lignin polymerization machinery to precise locations, ensuring effective sealing of the apoplastic space .

• Membrane domain organization: CASP proteins organize specialized plasma membrane domains by excluding certain proteins and creating membrane-wall attachment sites. A full CASP knockout study revealed that these proteins enforce displacement of secretory foci through exclusion of vesicle tethering factors, facilitating rapid fusion of microdomains .

• Interaction with trafficking machinery: Proximity labeling experiments identified RabA-GTPases and exocyst subunits as potential CASP interactors, suggesting roles in regulating vesicle tethering and fusion at specialized membrane domains .

Based on these homologies, SDM1_58t00016 likely participates in organizing specialized membrane domains in Solanum demissum roots, potentially contributing to Casparian strip formation or other specialized cell wall modifications.

How do CASP-like proteins contribute to plant stress responses?

Research on various CASPL proteins indicates important roles in stress responses, which may provide clues about potential functions of SDM1_58t00016:

These diverse roles suggest CASPL proteins function in various stress response pathways. Given this functional diversity, SDM1_58t00016 might participate in stress tolerance mechanisms in Solanum demissum, potentially by regulating membrane domain organization or cell wall modifications in response to environmental challenges.

What expression systems are most suitable for recombinant SDM1_58t00016 production?

For successful expression of SDM1_58t00016, researchers should consider multiple systems based on their experimental requirements:

For membrane proteins like SDM1_58t00016, eukaryotic expression systems often yield better results due to their ability to properly fold and insert these proteins into membranes . When choosing an expression system, consider:

• The intended application (structural studies, functional assays, antibody production)

• Required protein yield and purity

• Need for post-translational modifications

• Available resources and expertise

SDM1_58t00016 has been successfully expressed in E. coli with an N-terminal His-tag , but for functional studies, insect or mammalian cell systems might provide more native-like protein conformations.

What purification strategies are effective for transmembrane proteins like SDM1_58t00016?

Purifying transmembrane proteins like SDM1_58t00016 presents unique challenges. The following methodological approach is recommended:

• Fusion tag selection:

  • His-tag: Most commonly used with SDM1_58t00016

  • Other options: FLAG, MBP, GST, trxA, Nus, Biotin, or GFP tags

  • Consider tag position (N- or C-terminal) based on protein topology

• Membrane extraction:

  • Screen detergents (e.g., DDM, LMNG, digitonin) for optimal solubilization

  • Use mild detergents to maintain native conformation

  • Consider nanodiscs or styrene maleic acid copolymers (SMALPs) for detergent-free extraction

• Chromatography steps:

  • Initial capture: Immobilized metal affinity chromatography (IMAC) for His-tagged protein

  • Intermediate purification: Ion exchange or hydroxyapatite chromatography

  • Polishing: Size exclusion chromatography to remove aggregates and ensure homogeneity

• Buffer optimization:

  • Include detergent at concentrations above critical micelle concentration

  • Add stabilizers like glycerol (5-50%) or trehalose (6%)

  • Maintain appropriate pH (typically pH 7-8)

  • Consider including reducing agents for proteins with cysteines

• Storage considerations:

  • Aliquot purified protein to avoid freeze-thaw cycles

  • Store at -20°C/-80°C for long-term storage

  • Add cryoprotectants like glycerol or trehalose

The search results indicate that recombinant SDM1_58t00016 has been successfully purified as a lyophilized powder in Tris/PBS-based buffer with 6% trehalose at pH 8.0 , demonstrating a viable purification approach for this protein.

How can researchers investigate the membrane localization and domain formation properties of SDM1_58t00016?

Investigating membrane localization and domain formation properties of SDM1_58t00016 requires specialized approaches for membrane proteins:

• Fluorescence microscopy techniques:

  • Generate fluorescent protein fusions (e.g., SDM1_58t00016-GFP)

  • Express in heterologous systems (plant protoplasts, tobacco leaf epidermal cells)

  • Perform co-localization with known membrane markers

  • Use FRAP (Fluorescence Recovery After Photobleaching) to assess protein mobility

• Protein exclusion zone analysis:

  • Co-express with general membrane markers like mCitrine-SYP122

  • Analyze complementary localization patterns

  • Compare with known CASP proteins that form exclusion zones

• Membrane-cell wall attachment assays:

  • Plasmolysis experiments using hyperosmotic solutions (e.g., 0.8M mannitol)

  • Observe membrane detachment patterns

  • Compare with wild-type cells showing "band plasmolysis" characteristic of Casparian strips

• Electron microscopy:

  • Immunogold labeling with antibodies against SDM1_58t00016 or its tags

  • Analyze ultrastructural localization

  • Examine membrane domain organization at high resolution

• Biochemical fractionation:

  • Isolate membrane microdomains using detergent-resistant membrane protocols

  • Analyze protein composition by mass spectrometry

  • Compare SDM1_58t00016 distribution with known domain markers

These approaches can reveal whether SDM1_58t00016 forms specialized membrane domains similar to other CASP proteins, which typically exhibit protein exclusion, membrane-wall attachment, and recruitment of specific interaction partners .

What approaches can identify potential interaction partners of SDM1_58t00016?

Identifying interaction partners is crucial for understanding SDM1_58t00016 function. Several complementary approaches are recommended:

• Proximity labeling methods:

  • BioID: Fuse SDM1_58t00016 to a biotin ligase (BirA) to biotinylate nearby proteins

  • APEX2: Fuse to engineered ascorbate peroxidase for proximity labeling

  • These methods are particularly valuable for membrane proteins as they capture transient and weak interactions in their native environment

• Co-immunoprecipitation strategies:

  • Generate antibodies against SDM1_58t00016 or use tag-specific antibodies

  • Stabilize interactions with reversible crosslinkers if necessary

  • Analyze by mass spectrometry

  • Validate with reciprocal co-IPs

• Split-protein complementation assays:

  • Yeast split-ubiquitin system (optimized for membrane proteins)

  • Bimolecular fluorescence complementation (BiFC) in planta

  • Split luciferase assays for dynamic interaction studies

• Heterologous expression and functional complementation:

  • Express in Arabidopsis casp mutants to test for interaction with known CASP partners

  • Assess co-localization with CASP-interacting proteins (e.g., RabA-GTPases, exocyst components)

  • Test complementation of interaction-dependent phenotypes

• In vitro binding assays:

  • Pulldown assays using recombinant SDM1_58t00016

  • Surface plasmon resonance or microscale thermophoresis for binding kinetics

  • Liposome-based assays for membrane context-dependent interactions

Research on Arabidopsis CASP proteins found interactions with:

• RabA-GTPases (vesicle trafficking regulators)

• Exocyst components (vesicle tethering complex)

• Lignin polymerization machinery (RBOHF, ESB1, PER64, UCC1)

These represent potential interaction partners to investigate for SDM1_58t00016, as functional conservation may extend to interaction networks.

How can researchers investigate the role of SDM1_58t00016 in plant-pathogen interactions?

SDM1_58t00016 comes from Solanum demissum, a wild potato species known for its resistance to the late blight pathogen Phytophthora infestans . This presents an intriguing opportunity to explore potential roles in disease resistance:

• Gene expression analysis during pathogen challenge:

  • Perform RT-qPCR or RNA-seq during P. infestans infection

  • Compare expression in resistant vs. susceptible lines

  • Analyze co-expression with known defense genes

• Genetic manipulation approaches:

  • Generate overexpression and knockout/knockdown lines

  • Assess disease susceptibility phenotypes

  • Examine Casparian strip integrity and function during infection

• Effector interaction studies:

  • Test for direct interactions with P. infestans effectors, particularly RXLR effectors

  • Investigate potential roles as a "guardee" or "decoy" protein in effector-triggered immunity

  • Examine co-localization with effectors during infection

• Barrier function analysis:

  • Assess apoplastic barrier integrity using tracer dyes

  • Examine pathogen penetration at cellular barriers

  • Investigate cell wall modifications mediated by SDM1_58t00016

• Comparative genomics approach:

  • Compare SDM1_58t00016 sequences between resistant and susceptible Solanum species

  • Identify potential selection signatures

  • Correlate sequence polymorphisms with resistance phenotypes

The search results indicate that Solanum demissum proteins can interact with P. infestans effectors, as demonstrated for the BSL1 protein, which associates with the AVR2 effector . This suggests SDM1_58t00016 might similarly play roles in pathogen perception or immune signaling.

What are the most promising directions for structural studies of SDM1_58t00016?

Structural characterization of membrane proteins like SDM1_58t00016 is challenging but would provide valuable insights into function. Several approaches show promise:

• Cryo-electron microscopy (cryo-EM):

  • Most promising approach for membrane proteins

  • Sample preparation options:

    • Detergent-solubilized protein

    • Reconstitution into nanodiscs

    • Reconstitution into lipid nanodiscs with native-like lipid composition

  • Potential for visualizing oligomeric states and interaction interfaces

• X-ray crystallography:

  • Challenging for membrane proteins but possible with:

    • Lipidic cubic phase crystallization

    • Antibody fragment-mediated crystallization

    • Fusion with crystallization chaperones like T4 lysozyme

• NMR spectroscopy approaches:

  • Solution NMR for individual domains or loops

  • Solid-state NMR for full-length protein in membrane mimetics

  • Can provide dynamic information not captured by static methods

• Integrative structural biology:

  • Combine lower-resolution data from multiple techniques

  • Incorporate computational predictions and evolutionary information

  • Validate models with targeted mutagenesis and functional assays

• Molecular dynamics simulations:

  • Model SDM1_58t00016 in lipid bilayers

  • Investigate conformational dynamics

  • Predict interaction interfaces and ligand binding sites

These structural studies would be particularly valuable for understanding:

• How transmembrane domains organize within the membrane

• Mechanisms of protein exclusion zone formation

• Molecular basis for interaction with RabA-GTPases and other partners

• Structural basis for membrane-cell wall attachment

Recombinant SDM1_58t00016 produced in E. coli with >90% purity would provide a starting point for such structural investigations, though expression systems might need optimization for structural biology applications.

What genomic approaches would advance understanding of SDM1_58t00016 regulation in different environmental contexts?

Understanding the regulation of SDM1_58t00016 under different environmental conditions requires sophisticated genomic approaches:

• Chromatin-based regulatory analysis:

  • Chromatin immunoprecipitation sequencing (ChIP-seq) to identify transcription factors binding the SDM1_58t00016 promoter

  • ATAC-seq to map open chromatin regions around the gene

  • DNA affinity purification sequencing (DAP-seq) to identify potential regulators

  • Focus on MYB transcription factors, as MYB36 regulates CASP genes in Arabidopsis

• Transcriptome profiling under diverse conditions:

  • RNA-seq across tissues, developmental stages, and stress conditions

  • Single-cell RNA-seq for cell-type specific expression patterns

  • Time-course analyses during development and stress responses

  • Correlation analysis with known stress-responsive genes

• Epigenetic regulation analysis:

  • Bisulfite sequencing to map DNA methylation

  • ChIP-seq for histone modifications

  • Investigation of stress-induced epigenetic changes

• Comparative genomics approaches:

  • Analysis of SDM1_58t00016 promoter architecture across Solanum species

  • Identification of conserved regulatory elements

  • Correlation of natural variation with environmental adaptations

• Functional validation:

  • Promoter-reporter constructs to test regulatory elements

  • CRISPR-based targeting of regulatory regions

  • Manipulation of candidate regulators and assessment of SDM1_58t00016 expression

These approaches could reveal how SDM1_58t00016 regulation is integrated into broader stress response networks, potentially uncovering applications for enhancing stress resilience in cultivated potato varieties. The multifaceted roles of CASPL genes in stress responses suggest SDM1_58t00016 regulation may be similarly complex and environmentally responsive.