Recombinant Arabidopsis thaliana Membrane steroid-binding protein 1 (MSBP1)

What is the molecular structure and organization of MSBP1?

MSBP1 is encoded by the MSBP1 gene (also termed AtMP1, accession number AF153284) located on chromosome V of Arabidopsis thaliana. The gene consists of two exons (381 and 525 bp) and one intron (774 bp) . The protein belongs to the membrane-associated progesterone binding protein (MAPR) family and shows significant homology to porcine membrane progesterone binding protein (PGC1_PIG) with an E-value of 2 × 10^-18 .

For recombinant expression, MSBP1 can be successfully expressed in E. coli using isopropylthio-β-galactoside (IPTG) induction (1 mM) at 28°C, with optimal protein yield at approximately 2.5 hours post-induction . When analyzed by SDS-PAGE, purified recombinant MSBP1 displays the expected molecular mass of approximately 48 kD .

What steroid molecules does MSBP1 bind and with what affinities?

MSBP1 demonstrates binding capacity for multiple steroid molecules with different affinities in vitro:

• Progesterone: Kd = 31.2 ± 2.8 nM, Bmax = 4.13 ± 0.26 pmol/μg proteins

• 5-dihydrotestosterone (5-DHT)

• 24-epi-brassinolide (24-eBL), a plant hormonal steroid

• Stigmasterol, a membrane constituent steroid

The Kd value of MSBP1 to progesterone (31.2 nM) is higher than that of porcine MSBP to progesterone (11 nM) and Arabidopsis BRI1 to brassinolide (7.4 ± 0.9 nM) . When conducting binding assays, purified recombinant MSBP1 protein shows dose-dependent and specific binding to [³H]-progesterone, while control proteins with similar molecular mass (e.g., AtIPK2α·His) do not exhibit such binding capacity .

Competition binding assays using unlabeled steroids as competitors for [³H]-progesterone binding provide additional insights into binding specificity. Molecules like abscisic acid (ABA) can serve as negative controls in these assays .

How does MSBP1 negatively regulate cell elongation in plants?

MSBP1 inhibits cell elongation through several interconnected mechanisms:

• Transgenic plants overexpressing MSBP1 (O-MSBP1) display short hypocotyl phenotypes (approximately 75% the length of control plants), while antisense MSBP1 transgenic plants (A-MSBP1) exhibit elongated hypocotyls .

• The reduced cell elongation in MSBP1-overexpressing plants correlates with altered expression of cell wall-modifying genes, particularly expansins and extensins, indicating that enhanced MSBP1 affects a regulatory pathway for cell elongation .

• At the molecular level, MSBP1 specifically interacts with the extracellular domain of BAK1 (BRI1-Associated Kinase 1) in a brassinolide-independent manner . BAK1 is a co-receptor that enhances BRI1-mediated brassinosteroid signaling.

• MSBP1 accelerates BAK1 endocytosis, shifting the equilibrium of BAK1 toward endosomes . This reduces the availability of BAK1 at the plasma membrane for interaction with BRI1, thereby suppressing brassinosteroid signaling .

• Enhanced MSBP1 expression reduces the interaction between BRI1 and BAK1 in vivo , demonstrating that MSBP1 acts as a negative regulator at an early step of the brassinosteroid signaling pathway.

These mechanisms collectively explain how MSBP1 inhibits cell elongation by negatively regulating brassinosteroid signaling, which normally promotes cell expansion.

How is MSBP1 expression regulated by light conditions?

MSBP1 expression shows strong light-dependent regulation, making it an important component in photomorphogenesis:

• MSBP1 is suppressed in darkness and activated under light conditions . This expression pattern aligns with its role in light inhibition of hypocotyl elongation.

• Promoter-reporter gene fusion studies using an MSBP1 promoter-GUS construct reveal that MSBP1 is constitutively expressed in cotyledons but differentially expressed in hypocotyls under different light conditions .

• In hypocotyls, MSBP1 is dramatically activated by blue and far-red light, and slightly stimulated by red light . This suggests regulation by both phytochromes and cryptochromes, the major plant photoreceptors.

• MSBP1 shows highest expression under far-red light conditions, which induce very short hypocotyls, and lower expression under red light conditions, which result in longer hypocotyls . This correlation supports MSBP1's role in inhibiting hypocotyl elongation in response to specific light wavelengths.

For accurate characterization of light-regulated expression, researchers should grow seedlings under controlled light conditions (darkness, blue, red, far-red) and assess gene expression through RT-PCR or promoter-reporter constructs. Quantitative analysis should include measurements at different time points to capture expression dynamics.

What is the tissue-specific expression pattern of MSBP1?

MSBP1 exhibits a distinct tissue-specific expression pattern that provides insights into its diverse functions:

• RT-PCR analysis demonstrates that MSBP1 is expressed in most plant tissues, including cotyledons, stems, roots, leaves, and floral tissues .

• Promoter-reporter gene fusion studies reveal more detailed expression patterns:

  • High expression in aboveground parts of seedlings

  • Weak expression in root tissues

  • Clear expression in rosettes, bolts, flower stalks, and silique stalks

  • Expression in the pistil and stigma, but not in the anther

  • Little to no detectable expression in mature flowers and siliques

• The expression of MSBP1 in reproductive structures (specifically in the pistil and stigma but not in the anther) suggests it may have specialized functions in female reproductive development .

This heterogeneous expression pattern indicates that MSBP1 likely plays diverse roles throughout plant development, affecting processes beyond hypocotyl elongation. The differential expression in various organs suggests tissue-specific regulation mechanisms and potentially specialized functions in different plant parts.

How does MSBP1 interact with BAK1 to suppress brassinosteroid signaling?

MSBP1 suppresses brassinosteroid (BR) signaling through its specific interaction with BAK1 (BRI1-Associated Kinase 1):

• MSBP1 binds specifically to the extracellular domain of BAK1 in vivo in a brassinolide (BL)-independent manner . This interaction is a critical component of MSBP1's regulatory function.

• Both MSBP1 and BAK1 localize to the plasma membrane and endocytic vesicles as demonstrated by subcellular localization studies .

• MSBP1 accelerates BAK1 endocytosis, shifting the equilibrium of BAK1 toward endosomes . This alteration in BAK1 localization is central to the inhibitory mechanism.

• Enhanced MSBP1 expression reduces the interaction between BRI1 (the primary BR receptor) and BAK1 in vivo . Since BAK1 functions as a co-receptor enhancing BRI1-mediated BR signaling, this reduction in BRI1-BAK1 interaction leads to suppressed BR signaling.

• The suppressed cell expansion and BR responses caused by MSBP1 overexpression can be rescued by overexpressing BAK1 or its intracellular kinase domain . This genetic rescue experiment confirms that MSBP1 exerts its negative effects primarily through BAK1.

This mechanism represents a novel mode of BR signaling regulation that operates by controlling receptor complex formation and subcellular distribution rather than direct modification of signaling components. The interaction between MSBP1 and BAK1 demonstrates how membrane-localized proteins can modulate hormone signaling through protein trafficking.

What is the role of MSBP1 in autophagy and energy deprivation responses?

MSBP1 has been identified as a cargo in a specialized form of reticulophagy (selective autophagy of endoplasmic reticulum components) that is triggered by dark-induced starvation:

• The ER-localized MSBP1 interacts with ATG8-interacting proteins ATI1 and ATI2 . These interactions have been demonstrated through techniques such as yeast two-hybrid (Y2H) and bimolecular fluorescence complementation (BiFC).

• ATI1 and ATI2 function as cargo receptors for MSBP1 during autophagy . They facilitate the selective degradation of MSBP1 under energy deprivation conditions by bridging MSBP1 to the core autophagy machinery.

• This autophagy pathway is triggered specifically by dark-induced starvation (energy deprivation) rather than ER stress . This distinguishes it from previously reported forms of reticulophagy in plants.

• The pathway is regulated by the TOR (Target of Rapamycin) signaling pathway , a central regulator of cellular metabolism and autophagy in response to nutrient and energy status.

The selective degradation of MSBP1 during energy deprivation may represent an adaptive mechanism, potentially allowing plants to modulate BR signaling and growth responses under energy-limited conditions. This finding expands our understanding of plant responses during energy deprivation and highlights the role of selective autophagy in regulating specific proteins under stress conditions.

What are the optimal conditions for expressing and purifying recombinant MSBP1?

For successful expression and purification of recombinant MSBP1, researchers should follow these optimized protocols:

• Expression system:

  • Host: E. coli BL21(DE3) or similar expression strains

  • Vector: pET or similar expression vectors with appropriate affinity tags

  • Induction temperature: 28°C (critical for proper folding)

  • Induction agent: IPTG at 1 mM concentration

  • Induction duration: 2.5 hours (optimal for maximum yield while minimizing degradation)

• Purification strategy:

  • Affinity chromatography using tags such as His-tag

  • Buffer optimization to maintain protein stability and solubility

  • Expected molecular mass: approximately 48 kD

• Quality control measures:

  • SDS-PAGE to confirm protein size and purity

  • Western blotting with anti-MSBP1 or anti-tag antibodies

  • Functional validation through steroid binding assays

• Optimization considerations:

  • Testing different E. coli strains if expression is problematic

  • Varying IPTG concentrations (0.1-1.0 mM) if yield is low

  • Adjusting induction temperature (16-28°C) to balance yield and solubility

  • Adding stabilizing agents like glycerol to purification buffers

This protocol has been validated to produce functional recombinant MSBP1 capable of binding steroid ligands with affinities comparable to the native protein . The key factors for success are maintaining the appropriate induction temperature (28°C) and limiting induction time (2.5 hours).

How can MSBP1 steroid binding activity be accurately measured?

Accurate measurement of MSBP1 steroid binding activity can be achieved through several complementary approaches:

• Radioligand binding assays:

  • Use [³H]-progesterone as the primary radioligand

  • Incubate purified recombinant MSBP1 with varying concentrations of [³H]-progesterone

  • Separate bound and free ligand using filtration or precipitation methods

  • Measure bound radioactivity to determine binding parameters

  • Include control proteins (e.g., AtIPK2α·His) with similar molecular mass as negative controls

• Competition binding assays:

  • Use a fixed concentration of [³H]-progesterone with varying concentrations of unlabeled competitors

  • Test different steroids including progesterone, 5α-dihydrotestosterone, 24-eBL, and stigmasterol

  • Include non-steroid molecules (e.g., abscisic acid) as negative controls

  • Generate competition curves to determine relative binding affinities

• Binding parameter determination:

  • Calculate Kd (dissociation constant) to quantify binding affinity

  • Determine Bmax (saturation concentration) to measure binding capacity

  • For MSBP1, expected values are: Kd = 31.2 ± 2.8 nM, Bmax = 4.13 ± 0.26 pmol/μg proteins

• In vivo binding capacity assessment:

  • Isolate membrane fractions from wild-type and transgenic plants with altered MSBP1 expression

  • Measure progesterone binding activity of these membrane fractions

  • Correlate binding capacity with MSBP1 expression levels to validate physiological relevance

For reliable results, optimal assay conditions must include appropriate protein concentration, temperature control (typically 4°C for binding reactions), suitable buffer composition (pH 7.4-7.8), and effective methods for separating bound and free ligand. Multiple independent experiments with technical replicates are essential for statistical validation.

How do genetic modifications of MSBP1 affect plant phenotypes?

Genetic modifications of MSBP1 produce distinct phenotypes that provide insights into its physiological functions:

• MSBP1 overexpression lines (O-MSBP1):

  • Short hypocotyl phenotype (approximately 75% the length of control plants)

  • Increased steroid binding capacity in membrane fractions

  • Altered expression of cell wall-related genes (expansins and extensins)

  • Reduced sensitivity to exogenous progesterone and 24-epi-brassinolide (24-eBL)

  • Suppressed brassinosteroid signaling due to reduced BRI1-BAK1 interaction

• MSBP1 antisense/knockdown lines (A-MSBP1):

  • Elongated hypocotyl phenotypes compared to control plants

  • More pronounced phenotype when grown in light than in dark conditions

  • Reduced steroid binding capacity in membrane fractions

  • Enhanced sensitivity to exogenous progesterone and 24-eBL

• Genetic interaction data:

  • The phenotypes of MSBP1 overexpression can be rescued by overexpressing BAK1 or its intracellular kinase domain

  • This genetic rescue provides strong evidence that MSBP1 exerts its effects primarily through BAK1

These phenotypic analyses, combined with biochemical and molecular data, establish MSBP1 as a negative regulator of cell elongation that functions primarily through modulating brassinosteroid signaling via interaction with BAK1. The consistent correlation between MSBP1 expression levels, hypocotyl length, and steroid binding capacity provides robust evidence for its physiological role in regulating plant growth.

What is the mechanism of light-regulated MSBP1 activity in photomorphogenesis?

MSBP1 plays a crucial role in photomorphogenesis through a mechanism involving light-regulated expression and subsequent modulation of brassinosteroid signaling:

• Light-regulated expression:

  • MSBP1 is suppressed in darkness and activated by light, especially blue and far-red light

  • This regulation likely involves both phytochrome and cryptochrome photoreceptors

  • MSBP1 expression is highest under far-red light conditions that induce very short hypocotyls

• Integration with brassinosteroid signaling:

  • Light-activated MSBP1 expression leads to increased MSBP1 protein levels

  • MSBP1 interacts with BAK1 and accelerates its endocytosis

  • This reduces BRI1-BAK1 interaction, suppressing brassinosteroid signaling

  • Reduced BR signaling inhibits cell elongation in the hypocotyl

• Genetic evidence:

  • MSBP1 overexpression lines show short hypocotyls reminiscent of light-grown seedlings

  • MSBP1 antisense lines display longer hypocotyls, partially mimicking dark-grown seedlings

  • These phenotypes are observed in both light and dark conditions but are more pronounced in light

• Transcriptional regulation:

  • MSBP1 affects the expression of genes involved in cell elongation

  • This creates a regulatory cascade that amplifies the initial light signal

This mechanism represents an important link between light perception and growth regulation in plants. By modulating BR signaling in response to light conditions, MSBP1 helps coordinate developmental responses to the light environment, ensuring appropriate seedling establishment. The dual regulation by different light wavelengths suggests integration of multiple photoreceptor pathways through MSBP1 activity.

How does MSBP1 contribute to the broader brassinosteroid signaling network?

MSBP1 serves as a critical negative regulator within the brassinosteroid (BR) signaling network through several interconnected mechanisms:

• Receptor complex regulation:

  • MSBP1 interacts with BAK1, a co-receptor that enhances BRI1-mediated BR signaling

  • It accelerates BAK1 endocytosis, reducing BAK1 availability at the plasma membrane

  • This disrupts BRI1-BAK1 interaction, an essential step in BR signal transduction

  • MSBP1 thus acts at the level of receptor complex formation, representing an early regulatory point

• Direct steroid binding capacity:

  • MSBP1 can bind 24-epi-brassinolide with high affinity in vitro

  • This binding capacity may contribute to its regulatory function by potentially sequestering BR molecules

  • MSBP1 may represent one of the steroid binding proteins (SBPs) suggested to be part of the BRI1 receptor complex

• Integration with environmental signals:

  • Light-regulated MSBP1 expression provides a mechanism to modulate BR signaling in response to light conditions

  • This allows integration of environmental cues with hormone signaling

• Downstream gene regulation:

  • MSBP1 affects the expression of BR-regulated genes involved in cell elongation, such as expansins and extensins

  • This amplifies its initial effect on receptor complex formation

• Relationship with other BR signaling components:

  • MSBP1's effects can be counteracted by overexpressing BAK1 or its intracellular kinase domain

  • This positions MSBP1 in opposition to positive regulators of BR signaling

This multi-faceted role positions MSBP1 as an important modulator of BR responses, helping to fine-tune signaling based on developmental and environmental contexts. By regulating receptor complex dynamics rather than directly affecting downstream signaling components, MSBP1 represents a distinct regulatory mechanism within the BR signaling network.

What is the potential connection between MSBP1 and plant metabolic regulation?

While direct evidence linking MSBP1 to metabolic regulation is limited in the search results, several potential connections can be inferred:

These connections suggest MSBP1 may represent an important link between environmental sensing, energy status, and growth regulation. By modulating BR signaling in response to energy availability, MSBP1 could help coordinate growth with metabolic capacity, ensuring optimal resource utilization under varying conditions.

How can advanced imaging techniques enhance our understanding of MSBP1 function?

Advanced imaging techniques offer powerful approaches to illuminate MSBP1's dynamic functions within plant cells:

• Super-resolution microscopy:

  • Techniques like STORM, PALM, or STED microscopy can visualize MSBP1 localization with resolution below the diffraction limit

  • This would allow precise mapping of MSBP1 distribution at the plasma membrane and in endocytic vesicles

  • Co-localization with BAK1 and BRI1 at nanometer resolution would provide insights into receptor complex dynamics

• Live-cell imaging with fluorescent protein fusions:

  • Generating stable transgenic lines with fluorescently tagged MSBP1 (e.g., MSBP1-GFP)

  • Using spinning disk confocal microscopy to track MSBP1 trafficking in real-time

  • Photoactivatable or photoswitchable fluorescent proteins could track specific MSBP1 populations

• FRET/FLIM analysis for protein interactions:

  • Förster Resonance Energy Transfer (FRET) combined with Fluorescence Lifetime Imaging (FLIM)

  • This would allow visualization of MSBP1-BAK1 interactions in living cells

  • Quantitative measurement of interaction dynamics under different conditions (light/dark, hormone treatments)

• FRAP and photoactivation for dynamic studies:

  • Fluorescence Recovery After Photobleaching (FRAP) to measure MSBP1 mobility in membranes

  • Photoactivation to track newly synthesized MSBP1 in response to environmental changes

  • These approaches would reveal dynamic aspects of MSBP1 behavior not captured by static imaging

• Correlative light and electron microscopy (CLEM):

  • Combining fluorescence imaging with electron microscopy

  • This would provide ultrastructural context for MSBP1 localization

  • Particularly valuable for studying MSBP1's role in autophagy and endocytosis

Implementation of these advanced imaging approaches would provide unprecedented insights into MSBP1's subcellular dynamics, interaction patterns, and response to environmental stimuli. This would complement biochemical and genetic approaches, creating a more comprehensive understanding of MSBP1 function.

What is the potential for using MSBP1 to engineer plant growth characteristics?

MSBP1's central role in regulating plant growth through brassinosteroid signaling makes it a promising target for engineering plant growth characteristics: