Recombinant Selaginella moellendorffii CASP-like protein SELMODRAFT_75865 (SELMODRAFT_75865)

What is SELMODRAFT_75865 and what is its significance in plant biology?

SELMODRAFT_75865 is a CASP-like protein from Selaginella moellendorffii (spikemoss), a lycophyte that represents an early vascular plant lineage. The protein belongs to the Casparian strip membrane domain protein (CASP) family that plays crucial roles in the formation of Casparian strips in plants. The significance of studying this protein lies in understanding the evolutionary conservation of Casparian strip formation mechanisms across land plants. CASP proteins specifically localize at the Casparian strip formation site and guide local lignin deposition, which is essential for creating apoplastic barriers in plant roots . In Arabidopsis, these proteins act as scaffolds to recruit enzymes like RBOHF, PER64, and ESB1 that are involved in lignin polymerization, and their precise localization is regulated by receptor-like kinases SGN1 and SGN3 . The presence of CASP-like proteins in Selaginella, which diverged earlier in plant evolution, suggests a deep conservation of this important cellular barrier mechanism.

How does the structure of SELMODRAFT_75865 compare with other CASP proteins?

The SELMODRAFT_75865 protein consists of 191 amino acids with a sequence (MGAYDGAEAPRAAPASTAANSRPSRLLLLHSLLLRLVAVVVSILVIAVMVHAKQRVMIFKAEWDNSKAFVALVAISAICLGYSFLQFILSAFHLCSKSWKSPTKCWAWMNFIADQILTYAMLGAAAAAAELAYIAKNGSSRAQWQPICSTFNTFCTRAGASIILSFIAVLALANSSAISAYHLFRRPSSSV) that shows characteristic features of CASP family proteins . Like other CASP proteins, it likely contains multiple transmembrane domains that anchor it to the plasma membrane at specific domains. Phylogenetic analyses have shown that CASP proteins are conserved across land plants, with Roppolo et al. demonstrating the conserved subcellular localization of CASP family proteins across different species . The protein is also known by the synonyms CASP-like protein 2U3 and SmCASPL2U3, with a UniProt ID of D8QNV6 . Structural predictions would suggest that SELMODRAFT_75865, like other CASP proteins, contains four transmembrane domains with cytosolic N- and C-termini, although experimental verification of this structural arrangement in the Selaginella protein would be valuable for comparative studies across evolutionary distances.

What is known about the evolutionary conservation of CASP-like proteins across plant species?

Phylogenetic analyses have revealed that homologs of Casparian strip regulatory genes, including CASP proteins, exist in most land plants . The regulatory network required for Casparian strip formation appears to be conserved from early-diverging plants like Selaginella to flowering plants like Arabidopsis and tomato. Research has demonstrated that the subcellular localization of CASP family proteins is conserved across different plant species, suggesting functional conservation throughout plant evolution . The presence of SELMODRAFT_75865 in Selaginella moellendorffii indicates that the CASP protein family emerged early in vascular plant evolution. The conservation extends beyond just the CASP proteins themselves to the entire regulatory network, including transcription factors like MYB36, which acts as a master regulator activating the expression of CASPs, ESBs, and PERs . Spatial expression patterns of these regulatory genes also appear to be conserved, with components like SHR and CIF expressed in vascular tissues while others like MYB36, SGN3, and PER64 are expressed in the innermost ground tissue layer across different species .

What purification strategies yield the highest purity and activity for SELMODRAFT_75865?

For efficient purification of SELMODRAFT_75865, affinity chromatography using the His-tag is the primary method of choice. The protein has been successfully expressed with an N-terminal His tag, allowing for efficient purification using immobilized metal affinity chromatography (IMAC) . For highest purity, a multi-step purification strategy is recommended: initial capture using IMAC followed by further purification steps such as ion exchange chromatography and size exclusion chromatography. These additional steps can help remove contaminating proteins and aggregates. The commercial preparation of the protein achieves greater than 90% purity as determined by SDS-PAGE . To maintain protein activity, it is crucial to optimize buffer conditions during purification and storage. The protein has been successfully stored in Tris/PBS-based buffer with 6% Trehalose at pH 8.0 . For long-term storage, aliquoting the protein and adding 5-50% glycerol (final concentration) before storing at -20°C/-80°C is recommended to prevent activity loss during freeze-thaw cycles .

How can researchers troubleshoot common issues in SELMODRAFT_75865 expression and purification?

When encountering low expression levels of SELMODRAFT_75865, several strategies can be implemented. First, optimize codon usage for the expression host, as plant genes often contain codons that are rare in prokaryotic systems. Second, test different expression temperatures and IPTG concentrations; lower temperatures (16-20°C) often improve the solubility of recombinant proteins. Third, consider using specialized E. coli strains like Rosetta-GAMI that supply rare tRNAs and enhance disulfide bond formation . For purification issues, if the protein forms inclusion bodies, attempt protein renaturation protocols or switch to expression systems that promote proper folding. When protein degradation occurs during purification, include protease inhibitors in all buffers and reduce the processing time. For proteins with low binding affinity to affinity resins, optimize imidazole concentrations in binding and washing buffers. If aggregation occurs post-purification, adjust buffer components, especially salt concentration and pH, or add stabilizing agents like glycerol or trehalose. Finally, when multiple freeze-thaw cycles cannot be avoided, store the protein in small aliquots to minimize repetitive thawing of the same sample .

What experimental approaches can be used to study the localization and function of SELMODRAFT_75865 in plant cells?

To investigate the localization and function of SELMODRAFT_75865 in plant cells, researchers can employ transient expression systems using fluorescent protein fusions. GFP-tagged SELMODRAFT_75865 can be expressed in plant protoplasts or through Agrobacterium-mediated transformation to observe its subcellular localization using confocal microscopy. Based on studies of other CASP proteins, SELMODRAFT_75865 would be expected to localize to specific plasma membrane domains where Casparian strips form . For functional studies, CRISPR-Cas9 gene editing can be used to generate knockout or knockdown lines in Selaginella moellendorffii, following protocols similar to those developed for other plant species . The mutant phenotypes can then be characterized by examining Casparian strip formation using fluorescent dyes like propidium iodide or berberine, which stain the lignified Casparian strips. Complementation studies using the recombinant protein can confirm the specificity of the observed phenotypes. Protein-protein interaction studies using techniques like co-immunoprecipitation, yeast two-hybrid assays, or bimolecular fluorescence complementation can identify binding partners of SELMODRAFT_75865, providing insights into its functional network.

How can researchers assess the role of SELMODRAFT_75865 in Casparian strip formation?

To assess the role of SELMODRAFT_75865 in Casparian strip formation, researchers should first establish a reliable system for visualizing Casparian strips in Selaginella moellendorffii. Fluorescent staining techniques using propidium iodide, berberine, or Basic Fuchsin can be employed to visualize the lignified structures. Researchers can then generate knockout or knockdown lines of SELMODRAFT_75865 using CRISPR-Cas9 gene editing, following protocols similar to those described for other plant genes . The integrity and development of Casparian strips in these mutant lines can be evaluated using confocal microscopy and compared to wild-type plants. Transport assays measuring the movement of fluorescent tracers or ions across the endodermis can provide functional data on barrier properties. Complementation experiments using the recombinant protein expressed under native or inducible promoters can confirm that any observed phenotypes are specifically due to the loss of SELMODRAFT_75865. Additionally, researchers can perform time-course analyses to determine if SELMODRAFT_75865 affects the timing of Casparian strip formation or only its structural integrity. Electron microscopy can provide detailed ultrastructural information about how the protein influences the deposition of lignin and other components of the Casparian strip.

What methods can be used to investigate protein-protein interactions involving SELMODRAFT_75865?

To investigate protein-protein interactions involving SELMODRAFT_75865, researchers can employ multiple complementary approaches. In vitro pull-down assays using the purified recombinant His-tagged SELMODRAFT_75865 as bait can identify binding partners from plant cell extracts, with subsequent mass spectrometry analysis to identify the captured proteins. Yeast two-hybrid screening can systematically test for binary interactions between SELMODRAFT_75865 and candidate proteins from the Casparian strip regulatory network, such as homologs of RBOHF, PER64, ESB1, and receptor kinases . For in vivo confirmation, co-immunoprecipitation experiments can be performed using antibodies against the His-tag or against SELMODRAFT_75865 itself. Bimolecular fluorescence complementation (BiFC) assays, where SELMODRAFT_75865 and potential interacting proteins are fused to complementary fragments of a fluorescent protein, can visualize interactions in living plant cells and provide spatial information about where these interactions occur. Förster resonance energy transfer (FRET) or split-luciferase assays offer additional means to detect protein interactions in vivo with high sensitivity. Finally, advanced techniques like proximity-dependent biotin identification (BioID) or cross-linking mass spectrometry can map the entire protein interaction network surrounding SELMODRAFT_75865 in its native cellular environment.

How does SELMODRAFT_75865 compare functionally with CASP proteins in Arabidopsis thaliana?

SELMODRAFT_75865 from Selaginella moellendorffii likely shares functional similarities with CASP proteins in Arabidopsis thaliana, though with evolutionary adaptations specific to lycophytes. In Arabidopsis, CASP proteins act as scaffolds at the Casparian strip domain, recruiting lignin polymerization machinery including RBOHF, PER64, and ESB1 . Comparative functional studies would require expression of fluorescently-tagged SELMODRAFT_75865 in Arabidopsis casp mutants to assess complementation capacity. Based on the conservation of the regulatory network across species, SELMODRAFT_75865 likely participates in similar protein complexes as Arabidopsis CASPs, though potentially with species-specific interaction partners. The expression pattern of SELMODRAFT_75865 in Selaginella should be analyzed and compared with the endodermis-specific expression of CASPs in Arabidopsis to determine if spatial regulation is conserved. Structural predictions suggest that SELMODRAFT_75865, like Arabidopsis CASPs, likely contains four transmembrane domains that anchor it to the plasma membrane at the Casparian strip domain. Differences in amino acid sequence might reflect adaptations to the specific cellular environment of Selaginella or to different interaction partners in the lycophyte Casparian strip regulatory network.

What can studies of SELMODRAFT_75865 reveal about the evolution of Casparian strip formation in land plants?

Studies of SELMODRAFT_75865 can provide crucial insights into the evolutionary history of Casparian strip formation mechanisms across land plants. Selaginella moellendorffii, as a lycophyte, represents an early-diverging vascular plant lineage that split from the lineage leading to seed plants over 400 million years ago. The presence of CASP-like proteins in Selaginella indicates that the molecular machinery for Casparian strip formation was already established early in vascular plant evolution . Comparative analyses of SELMODRAFT_75865 with CASP proteins from other plant lineages can reveal which features have been conserved throughout evolution and which have diverged. Functional studies examining whether SELMODRAFT_75865 can complement Arabidopsis casp mutants would indicate the degree of functional conservation despite hundreds of millions of years of independent evolution. Analysis of the promoter regions of SELMODRAFT_75865 compared to those of CASP genes in other species can identify conserved regulatory elements, providing insights into the evolution of tissue-specific expression patterns . The entire regulatory network for Casparian strip formation, including transcription factors like MYB36 and signaling components like SGN kinases, should be investigated in Selaginella to determine if the hierarchical regulation observed in Arabidopsis represents an ancestral state or a derived condition in angiosperms.

How does the amino acid sequence of SELMODRAFT_75865 inform its predicted structure and function?

The amino acid sequence of SELMODRAFT_75865 (191 amino acids) provides important clues about its structure and function. Sequence analysis reveals hydrophobic segments that likely form transmembrane domains, consistent with its predicted role as a membrane-localized protein involved in Casparian strip formation . The protein sequence (MGAYDGAEAPRAAPASTAANSRPSRLLLLHSLLLRLVAVVVSILVIAVMVHAKQRVMIFKAEWDNSKAFVALVAISAICLGYSFLQFILSAFHLCSKSWKSPTKCWAWMNFIADQILTYAMLGAAAAAAELAYIAKNGSSRAQWQPICSTFNTFCTRAGASIILSFIAVLALANSSAISAYHLFRRPSSSV) contains several hydrophobic stretches that would traverse the plasma membrane, interspersed with hydrophilic regions that would form loops either in the cytoplasm or extracellular space . Certain amino acid motifs might be involved in protein-protein interactions with other components of the Casparian strip machinery, such as peroxidases or NADPH oxidases. Comparative sequence analysis with well-characterized CASP proteins from Arabidopsis could identify conserved domains or motifs that are likely crucial for function. Secondary structure predictions would suggest a four-transmembrane domain protein with cytosolic N- and C-termini, similar to the structure of Arabidopsis CASP proteins. Potential post-translational modification sites in the sequence could indicate regulatory mechanisms affecting the protein's function or localization. Using the sequence for homology modeling based on known protein structures could provide additional insights into its three-dimensional conformation and potential binding interfaces.

What advanced imaging techniques are most suitable for studying SELMODRAFT_75865 localization and dynamics in plant tissues?

For studying SELMODRAFT_75865 localization and dynamics in plant tissues, super-resolution microscopy techniques offer significant advantages over conventional confocal microscopy. Techniques such as Structured Illumination Microscopy (SIM), Stimulated Emission Depletion (STED) microscopy, or Photoactivated Localization Microscopy (PALM) can overcome the diffraction limit and provide nanoscale resolution of protein localization at the Casparian strip domain. These approaches would require generating transgenic Selaginella lines expressing SELMODRAFT_75865 fused to appropriate fluorescent proteins. For studying protein dynamics, Fluorescence Recovery After Photobleaching (FRAP) can measure protein turnover rates and mobility within the membrane, while Fluorescence Correlation Spectroscopy (FCS) can provide information on diffusion coefficients and molecular interactions in living cells. To visualize SELMODRAFT_75865 in relation to other Casparian strip components, multiplexed imaging with different fluorophores can be employed. For correlative studies, combining fluorescence microscopy with electron microscopy (Correlative Light and Electron Microscopy, CLEM) would enable visualization of both protein localization and ultrastructural features of the Casparian strip. Finally, light sheet microscopy offers the advantage of reduced phototoxicity for long-term live imaging of SELMODRAFT_75865 dynamics during Casparian strip development in intact plant tissues.

How can researchers design experiments to elucidate the regulatory network controlling SELMODRAFT_75865 expression and function?

To elucidate the regulatory network controlling SELMODRAFT_75865 expression and function, researchers should employ a multi-faceted approach combining genomic, transcriptomic, and proteomic methodologies. Promoter analysis of SELMODRAFT_75865 using deletion constructs fused to reporter genes can identify cis-regulatory elements controlling its expression. Chromatin immunoprecipitation followed by sequencing (ChIP-seq) can identify transcription factors that directly bind to the SELMODRAFT_75865 promoter. Based on knowledge from Arabidopsis, homologs of MYB36, a master regulator of CASP genes, would be primary candidates for investigation . RNA-seq analysis comparing wild-type and mutant lines for putative upstream regulators can identify genes whose expression correlates with SELMODRAFT_75865. CRISPR-Cas9-mediated knockout of candidate regulatory genes, such as Selaginella homologs of SHR, SCR, and MYB36, followed by analysis of SELMODRAFT_75865 expression and Casparian strip formation, can establish causative relationships . For post-translational regulation, phosphoproteomic analysis can identify phosphorylation sites on SELMODRAFT_75865 that might be targeted by kinases like SGN1 and SGN3 homologs. Co-expression network analysis can reveal genes with expression patterns similar to SELMODRAFT_75865, potentially identifying new components of the regulatory network. Finally, heterologous expression systems can be used to reconstitute the minimal regulatory circuit sufficient for proper SELMODRAFT_75865 localization and function.

What experimental approaches can address the challenges of studying transmembrane proteins like SELMODRAFT_75865?

Studying transmembrane proteins like SELMODRAFT_75865 presents unique challenges that require specialized experimental approaches. For structural studies, expression systems that properly fold membrane proteins, such as cell-free systems supplemented with lipids or insect cells, may be preferable to E. coli . Detergent screening is crucial for solubilizing the protein while maintaining its native conformation and activity; non-ionic detergents like DDM, LMNG, or digitonin are often suitable starting points. For reconstitution studies, the protein can be incorporated into liposomes or nanodiscs to create a native-like membrane environment for functional assays. Cryo-electron microscopy has emerged as a powerful technique for determining structures of membrane proteins without the need for crystallization. For in-cell studies, split-GFP systems where one fragment is fused to SELMODRAFT_75865 and the other is targeted to specific cellular compartments can confirm topology predictions. Crosslinking mass spectrometry can identify interaction interfaces between SELMODRAFT_75865 and its binding partners within the membrane. For functional studies, electrophysiological techniques or transport assays using liposomes reconstituted with purified SELMODRAFT_75865 can assess potential channel or transport activities. Finally, molecular dynamics simulations based on homology models can provide insights into how the protein might interact with the lipid bilayer and undergo conformational changes during function.

How can heterologous expression systems be optimized for functional studies of SELMODRAFT_75865?

Optimizing heterologous expression systems for functional studies of SELMODRAFT_75865 requires careful consideration of multiple factors. For E. coli expression, codon optimization of the SELMODRAFT_75865 gene for E. coli usage can significantly improve expression levels . Using specialized strains like C41/C43(DE3) or Lemo21(DE3) that are adapted for membrane protein expression can reduce toxicity issues. The addition of fusion partners like MBP, GST, or SUMO can enhance solubility, while inclusion of a cleavable signal sequence can improve membrane targeting . Expression conditions should be systematically optimized, testing different temperatures (typically 16-30°C), inducer concentrations, and induction times. For eukaryotic expression systems, yeast (P. pastoris or S. cerevisiae) offers advantages for membrane protein expression, with inducible promoters like AOX1 allowing tight regulation . Insect cell systems using baculovirus vectors provide a eukaryotic environment with efficient post-translational modifications. For plant-based expression, transient systems using Agrobacterium infiltration of Nicotiana benthamiana leaves can rapidly produce the protein in a native-like environment. When expressing SELMODRAFT_75865 for functional studies, it is crucial to verify that the expressed protein is correctly folded and localized, which can be assessed through activity assays, circular dichroism spectroscopy, or microscopy techniques if fluorescent tags are included.

What are the most promising research directions for understanding SELMODRAFT_75865 function in plant development?

The study of SELMODRAFT_75865 opens several promising research directions that could significantly advance our understanding of plant development and evolution. Comparative functional genomics approaches examining CASP-like proteins across diverse plant lineages could elucidate how Casparian strip formation mechanisms evolved from early land plants to angiosperms. Development of Selaginella moellendorffii as a model lycophyte system, including optimization of transformation protocols and establishment of mutant collections, would facilitate functional studies of SELMODRAFT_75865 in its native context . Investigation of protein interaction networks centered around SELMODRAFT_75865 could reveal conserved and lineage-specific components of the Casparian strip machinery. Environmental stress studies examining how SELMODRAFT_75865 expression and function respond to drought, salinity, or nutrient deficiency could provide insights into the adaptive significance of Casparian strip regulation in early vascular plants. Systems biology approaches integrating transcriptomic, proteomic, and metabolomic data could place SELMODRAFT_75865 within broader regulatory networks controlling plant development. Finally, application of synthetic biology principles to engineer modified versions of SELMODRAFT_75865 with altered properties could lead to crops with enhanced stress resistance through modified barrier properties in their roots. These research directions would not only advance our fundamental understanding of plant biology but could also contribute to agricultural innovations addressing climate change challenges.

What methodological advances would facilitate research on CASP-like proteins across diverse plant species?

Methodological advances in several areas would significantly facilitate research on CASP-like proteins across diverse plant species. Development of universal antibodies recognizing conserved epitopes in CASP-like proteins would enable comparative studies of protein localization across plant lineages without requiring transgenic approaches. Refinement of CRISPR-Cas9 protocols optimized for non-model plants, including delivery methods suited to species with thick cell walls or complex tissues, would expand the range of species amenable to functional genomics . High-throughput phenotyping methodologies for assessing Casparian strip integrity and function in diverse plant species would accelerate comparative studies. Improved heterologous expression systems specifically designed for plant membrane proteins could enhance structural and biochemical characterization of CASP-like proteins . Development of computational tools for identifying CASP-like proteins and predicting their functions based on sequence alone would facilitate genome mining across the plant kingdom. Standardized protocols for isolating intact Casparian strips and associated protein complexes would advance proteomic analyses. Advancements in cryo-electron microscopy techniques for membrane protein complexes could reveal the structural basis of CASP protein function and interactions. Finally, establishment of plant cell culture systems that recapitulate Casparian strip formation in vitro would provide simplified experimental systems for dissecting the mechanistic functions of CASP-like proteins independent of whole-plant complexity.