Recombinant Oryza sativa subsp. japonica Formin-like protein 15 (FH15)

What is OsFH15 and what is its structural composition?

OsFH15 is a class I formin protein identified in Oryza sativa (rice) that contains 788 amino acids with an estimated molecular mass of 84.5 kDa. Structurally, OsFH15 contains:

• A signal peptide (SP) spanning amino acids 1-18

• A Pro-rich domain spanning amino acids 44-86

• A transmembrane domain (TM) spanning amino acids 148-168

• A typical FH1 domain spanning amino acids 257-297

• A characteristic FH2 domain spanning amino acids 340-768

The gene structure includes four exons and three introns, encoding a 2367 bp mRNA. This structural composition is consistent with other class I formins in plants, characterized by the presence of both FH1 and FH2 domains which are essential for its functions in cytoskeletal regulation .

What is the tissue-specific expression pattern of OsFH15 in rice?

OsFH15 exhibits specific expression patterns in rice, with primary localization in:

• Shoot apical meristem (SAM)

• Spikelets

• Spikelet hulls

• Seeds

This expression pattern corresponds with the protein's functional role in grain development, particularly in regulating spikelet hull size, which directly constrains final grain dimensions. The localized expression in these reproductive and developmental tissues suggests OsFH15's specialized function in controlling cellular expansion during grain formation .

What phenotypic changes are observed in OsFH15 mutants versus overexpression lines?

The genetic modification of OsFH15 expression produces contrasting phenotypes:

• OsFH15-Cas9 and OsFH15-RNAi mutants: Exhibit decreased grain size with reduced cell dimensions (length, width, and area) in the inner epidermal cells of the lemma compared to wild-type plants .

• OsFH15-overexpressed plants: Demonstrate increased grain size with larger cells, accompanied by more abundant microtubule (MT) and actin filament (AF) arrays .

These phenotypic observations provide clear evidence that OsFH15 functions as a positive regulator of cell expansion and consequent grain size in rice, with direct implications for crop improvement strategies .

How does OsFH15 interact with the cytoskeleton at the molecular level?

OsFH15 exhibits multiple interactions with cytoskeletal components:

• Actin nucleation: OsFH15's FH1FH2 domain significantly decreases the initial lag phase of actin polymerization in a concentration-dependent manner, indicating active nucleation activity. This nucleation function requires the FH1 domain, as the FH2 domain alone shows very weak nucleation activity .

• Barbed-end capping: OsFH15 caps the barbed ends of actin filaments (AFs), reducing both the elongation and depolymerization rates in a concentration-dependent manner. This capping activity contributes to stabilizing AFs .

• Filament bundling and crosslinking: OsFH15 can bind and bundle both AFs and microtubules (MTs). More significantly, it can crosslink AFs with MTs, showing a preferential binding of MTs to AFs .

• Profilin interaction: In the presence of rice profilin (OsPRF1), OsFH15 promotes increased rates of filament elongation from approximately 0.4 subunits/second to 1.4 subunits/second, demonstrating functional interaction with this actin-binding protein .

These molecular interactions explain how OsFH15 regulates cytoskeletal organization, which directly influences cell expansion and consequently grain size .

What experimental approaches have been used to create and validate OsFH15 mutants?

Researchers have employed multiple approaches to generate and validate OsFH15 mutants:

• CRISPR-Cas9 gene editing:

  • A 23 bp nucleotide sequence targeting coding regions of OsFH15 was selected

  • The sequence was ligated into a binary vector and transformed into wild-type rice Hwayoung via Agrobacterium-mediated transformation

  • 30 positive transgenic plants were identified in the T1 generation through hygromycin resistance

  • In the T2 generation, target sites with OsFH15 deletions were analyzed using PCR and Sanger sequencing

  • Two independent mutant lines (Cas9 #13 and Cas9 #17) were identified with 1 bp and 2 bp insertions, respectively, leading to reading frame shifts and premature stop codons

• RNA interference (RNAi):

  • RNAi constructs targeting OsFH15 were generated and introduced into rice

  • The resulting transgenic plants showed decreased expression of OsFH15, as verified by examining relative expression levels in 14-day-old seedlings with UBIQUITIN5 (UBQ5) as the reference gene

• Overexpression studies:

  • OsFH15 overexpression constructs were generated

  • Transgenic plants showed significantly increased OsFH15 expression levels

The non-hygromycin resistant mutants were isolated to exclude the influence of the Cas9 gene in subsequent analyses, ensuring that observed phenotypes were due solely to OsFH15 modification .

How does OsFH15 function in actin assembly and what is the role of different domains?

OsFH15's function in actin assembly involves multiple mechanisms and domain-specific activities:

• FH1FH2 domain function:

  • The FH1FH2 domain efficiently nucleates actin polymerization as demonstrated by pyrene-actin assays showing decreased initial lag phase of actin polymerization

  • This domain increases the number of new barbed ends in a concentration-dependent manner

  • Total internal reflection fluorescence microscopy (TIRFM) visualization confirms that FH1FH2 significantly increases actin filament formation

• FH2 domain function:

  • When isolated, the FH2 domain shows very weak nucleation activity

  • This suggests that the FH1 domain is necessary for OsFH15's effective actin assembly function

• Barbed-end dynamics:

  • OsFH15 caps the barbed ends of actin filaments, reducing both elongation and depolymerization rates

  • This capping activity helps stabilize actin filaments in vivo

• Profilin interaction:

  • In rice cells, much of the actin is sequestered by equimolar profilin

  • OsFH15 can promote actin assembly even in the presence of profilins (OsPRF1 or OsPRF2)

  • This characteristic enables OsFH15 to function as a positive actin-nucleator in the cellular environment

This multifaceted function in actin dynamics explains why actin filament levels decrease in OsFH15 mutant cells and increase in OsFH15 overexpression cells .

What methodological approaches can be used to study OsFH15's interaction with actin filaments and microtubules in vitro?

Several specialized techniques have been employed to study OsFH15's interactions with cytoskeletal components:

• Protein expression and purification:

  • Generation of truncated recombinant proteins containing the FH1FH2 domain and FH2 domain

  • Expression and purification from bacterial cells using appropriate fusion tags

• Pyrene-actin polymerization assays:

  • Incubation of pyrene-labeled actin monomers (10% pyrene labeled) with various concentrations of FH1FH2 and FH2

  • Monitoring actin polymerization by measuring pyrene fluorescence

  • This approach allows quantification of nucleation activity by measuring the lag phase reduction

• Total Internal Reflection Fluorescence Microscopy (TIRFM):

  • Direct visualization of actin polymerization using Oregon-green-labeled actin

  • Quantification of actin filament amount and growth rates

  • This technique provides visual confirmation of actin assembly promotion

• Seeded actin filament elongation assays:

  • Measurement of actin filament elongation rates in the presence of varying concentrations of OsFH15

  • This approach confirms barbed-end capping activity

• Dilution-mediated actin filament depolymerization assays:

  • Assessment of depolymerization rates to confirm barbed-end capping

  • This test exploits the fact that barbed-end depolymerization is more than 20-fold faster than pointed-end depolymerization

• Profilin-actin interaction studies:

  • Measuring actin assembly rates in the presence of both OsFH15 and profilins

  • TIRFM visualization of elongation rates under various conditions

These methodological approaches provide comprehensive insights into OsFH15's molecular mechanisms and its role in cytoskeletal regulation .

How can OsFH15 be utilized in rice crop improvement strategies?

OsFH15 presents several potential applications for rice crop improvement:

• Grain size enhancement: Since OsFH15 overexpression leads to increased grain size, targeted genetic modification of this gene could be employed to develop rice varieties with larger grains, potentially increasing yield per plant .

• Cytoskeletal engineering: The ability of OsFH15 to modulate both actin filaments and microtubules offers opportunities to engineer the cytoskeleton for improved cellular function and stress tolerance .

• Marker-assisted selection: Identification of natural variants of OsFH15 associated with improved grain characteristics could enable marker-assisted selection in breeding programs .

• Gene stacking approaches: OsFH15 could be combined with other grain size regulators in gene stacking approaches to achieve synergistic improvements in grain dimensions and yield .

When implementing these strategies, researchers should account for potential pleiotropic effects, as cytoskeletal modifications might affect multiple developmental processes beyond grain size .

What are the experimental challenges in studying OsFH15 function in vivo?

Researchers investigating OsFH15 face several methodological challenges:

• Protein localization complexity: As OsFH15 contains transmembrane domains, visualizing its precise subcellular localization requires specialized approaches that preserve membrane integrity while allowing high-resolution imaging .

• Redundancy with other formins: The rice genome contains multiple formin genes that may have partially overlapping functions, potentially masking phenotypes in single-gene knockout studies .

• Cytoskeletal visualization in plant tissues: Effectively visualizing actin filaments and microtubules in intact plant tissues, especially in developing grains, presents technical challenges requiring specialized probes and imaging techniques .

• Temporal dynamics: The dynamic nature of cytoskeletal interactions necessitates time-lapse imaging approaches that can capture rapid changes in protein associations and filament organization .

• Quantitative analysis: Accurately quantifying changes in cell size, cytoskeletal organization, and grain dimensions requires sophisticated image analysis tools and standardized measurement protocols .

Addressing these challenges requires a combination of advanced microscopy techniques, genetic tools, and biochemical approaches to comprehensively understand OsFH15 function .

What are optimal expression systems for producing recombinant OsFH15 protein?

When designing expression systems for recombinant OsFH15, researchers should consider:

• Domain-specific expression: Due to the complexity of the full-length protein (788 amino acids), expressing functional domains separately (particularly FH1FH2 or FH2) has proven successful. This approach has been used to study biochemical properties in vitro .

• Bacterial expression systems: E. coli systems have been successfully used for expressing the FH1FH2 and FH2 domains of OsFH15 with appropriate fusion tags for purification .

• Considerations for full-length protein: Expression of the complete protein may require eukaryotic expression systems due to the presence of transmembrane domains and potential post-translational modifications.

• Purification strategies: Fusion tags that do not interfere with protein function should be selected, with options for tag removal if necessary for functional studies .

• Protein stability: Buffer conditions must be optimized to maintain protein stability, particularly for the actin-binding domains which may have specific requirements for salt concentration and pH .

The choice of expression system should align with the intended experimental applications, whether for structural studies, activity assays, or interaction analyses .

How can researchers differentiate between the direct and indirect effects of OsFH15 on grain size?

Distinguishing direct from indirect effects of OsFH15 on grain size requires multifaceted experimental approaches:

• Cell-specific expression analysis:

  • Utilize cell-type-specific promoters to express OsFH15 in different tissues of the developing grain

  • Determine which cell types are most sensitive to OsFH15 levels in terms of size regulation

• Temporal control strategies:

  • Implement inducible expression systems to activate or suppress OsFH15 at specific developmental stages

  • This approach helps identify critical windows when OsFH15 function directly impacts grain development

• Cytoskeletal correlation analyses:

  • Quantitatively correlate changes in cytoskeletal organization with cell expansion and grain size

  • Use live-cell imaging to track dynamics of both actin filaments and microtubules in developing grains

• Protein-protein interaction studies:

  • Identify direct interaction partners of OsFH15 in developing grains

  • Use techniques like co-immunoprecipitation, yeast two-hybrid, or proximity labeling approaches

• Transcriptome and proteome analysis:

  • Compare gene expression and protein abundance profiles between wild-type, OsFH15 mutants, and overexpression lines

  • Identify pathways and processes altered by OsFH15 modification

These approaches collectively provide evidence for distinguishing direct cytoskeletal effects from potential secondary signaling or metabolic consequences of OsFH15 modification .

What are the current contradictions in our understanding of formin proteins in rice?

Several areas of contradiction or uncertainty exist in our understanding of rice formins:

• Functional redundancy: Despite numerous formin genes in rice, knockout of single genes like OsFH15 produces clear phenotypes, suggesting limited functional redundancy. This contrasts with expectations based on the large formin gene family .

• Domain functionality: While the FH2 domain alone in OsFH15 shows very weak nucleation activity, other plant formins retain significant activity in their isolated FH2 domains. These differences in domain functionality across formin proteins require further investigation .

• In vivo versus in vitro activity: Some activities observed in vitro, such as efficient actin nucleation, may not perfectly translate to in vivo functions due to competition with other actin-binding proteins and cellular conditions .

• Membrane association: Though OsFH15 contains a transmembrane domain suggesting membrane localization, its interaction with cytoskeletal components implies cytoplasmic activity, creating uncertainty about its precise subcellular functioning .

• Crop-specific differences: The functions of formin proteins may differ between rice and other cereal crops, limiting the ability to translate findings across species .

Addressing these contradictions requires comparative studies across different formin proteins and plant species, combined with detailed structure-function analyses .

What major knowledge gaps exist in OsFH15 research and what approaches might address them?

Several significant knowledge gaps remain in OsFH15 research:

• Regulatory mechanisms:

  • How OsFH15 expression and activity are regulated during development remains poorly understood

  • Potential approaches: Promoter analysis, identification of transcription factors, and investigation of post-translational modifications

• Signaling pathways:

  • The upstream signals that modulate OsFH15 function in response to environmental conditions are unknown

  • Potential approaches: Phosphoproteomic analysis to identify modification sites and interacting kinases/phosphatases

• Evolutionary conservation:

  • The degree to which OsFH15 function is conserved across different rice varieties and related grass species remains to be established

  • Potential approaches: Comparative genomics and complementation studies across species

• Interaction with hormonal pathways:

  • How OsFH15 activity interfaces with plant hormone signaling that also regulates grain size is not well characterized

  • Potential approaches: Analyze OsFH15 mutant responses to various plant hormones and examine expression in hormone mutant backgrounds

• Structure-function relationships:

  • The precise structural basis for OsFH15's ability to interact with both actin filaments and microtubules needs further elucidation

  • Potential approaches: Structural biology techniques including X-ray crystallography or cryo-electron microscopy of protein domains

Addressing these gaps will require integrative approaches combining molecular, cellular, genetic, and structural biology techniques .

What are the optimal methods for visualizing OsFH15-mediated cytoskeletal changes in rice cells?

Visualizing OsFH15-mediated cytoskeletal changes requires specialized approaches:

• Live-cell imaging techniques:

  • Stable transgenic rice lines expressing fluorescent markers for actin (e.g., Lifeact-GFP) and microtubules (e.g., GFP-tubulin)

  • Spinning disk confocal microscopy for rapid acquisition with minimal photodamage

  • Super-resolution techniques such as Structured Illumination Microscopy (SIM) for higher resolution imaging of fine cytoskeletal structures

• Fixed-cell methods:

  • Optimized fixation protocols that preserve cytoskeletal structures while allowing antibody penetration

  • Immunofluorescence with anti-actin and anti-tubulin antibodies

  • Phalloidin staining for F-actin visualization

• Quantitative analysis approaches:

  • FilamentTracker or similar software for measuring filament density, length, and orientation

  • Skeleton analysis for quantifying network complexity

  • Co-localization analysis for assessing OsFH15 association with cytoskeletal elements

• Temporal analysis:

  • Time-lapse imaging to capture dynamic cytoskeletal rearrangements

  • Photoactivation or photobleaching techniques to measure filament turnover rates

• Tissue-specific considerations:

  • Sample preparation methods optimized for different rice tissues, particularly developing grains

  • Clearing techniques for deeper tissue imaging

  • Microdissection approaches for accessing internal cell layers

These methodological approaches enable comprehensive visualization and quantification of OsFH15's effects on cytoskeletal organization in different cellular contexts .

What experimental design is optimal for analyzing OsFH15's impact on grain development?

An optimal experimental design for analyzing OsFH15's impact on grain development should include: