Recombinant Borrelia burgdorferi Uncharacterized protein BB_0173 (BB_0173)

What is BB_0173 and what are its key structural features?

BB_0173 is a conserved hypothetical protein in Borrelia burgdorferi, the causative agent of Lyme disease. Structurally, BB_0173 is characterized by several distinct domains and motifs. Most notably, it contains a von Willebrand Factor A (VWFA) domain, multiple transmembrane regions, and a Bacteroides aerotolerance-like domain. The protein also contains a Metal Ion Dependent Adhesion Site (MIDAS) motif within its VWFA domain . These structural elements suggest BB_0173 plays a role in metal ion binding and potentially in protein-protein interactions within the periplasmic space. The transmembrane topology analysis confirms that BB_0173 spans the membrane multiple times, creating a complex spatial arrangement that positions functional domains in specific cellular compartments .

Where is BB_0173 localized within the Borrelia cell?

Unlike its paralog BB_0172 (which is an outer membrane protein), BB_0173 is an inner membrane protein in Borrelia burgdorferi. This localization has been experimentally verified through multiple complementary approaches. Triton X-114 detergent phase separation assays demonstrate that BB_0173 is found in both the aqueous phase and protoplasmic cylinders but absent from the detergent phase, consistent with inner membrane association . Additionally, proteinase K protection assays show that BB_0173 remains unaffected by protease treatment of intact cells, even at concentrations of up to 200 μg/mL, further confirming its intracellular localization . The protein's orientation positions the VWFA domain facing the periplasmic space, where it can potentially interact with periplasmic or outer membrane components .

How conserved is BB_0173 among different bacterial species?

BB_0173 is highly conserved across pathogenic Borrelia species, suggesting an essential role in bacterial survival. Comparative genomic analysis reveals similar domain architecture not only in Borrelia species but also in other bacterial genera including Bacteroides fragilis, Rhizobium leguminosarum, Leptospira interrogans, and Leptospira biflexa . The conservation pattern encompasses not just BB_0173 but extends to neighboring genes (BB_0170 to BB_0176), which appear to form a functional unit with conserved domain organization across these diverse bacteria . This high degree of conservation across phylogenetically distant bacterial species strongly indicates that BB_0173 and its associated gene cluster perform critical functions that have been maintained throughout bacterial evolution.

How is BB_0173 expression regulated in different environmental conditions?

BB_0173 expression is significantly influenced by environmental oxygen levels, with minimal impact from temperature or pH changes. Gene expression analysis shows that BB_0173 is dramatically downregulated (21.3-fold decrease) under low-oxygen conditions compared to aerobic environments . This oxygen-dependent regulation appears to be more pronounced for BB_0173 than for some neighboring genes; for instance, BB_0170 expression decreased by only 3.09-fold under the same conditions . Interestingly, BB_0176, which encodes a MoxR-type ATPase often found near VWFA domain-containing proteins, showed the strongest downregulation (22.8-fold) under low-oxygen conditions . The expression pattern suggests that while temperature and pH (factors that change between tick vector and mammalian host) have limited regulatory effect, oxygen availability serves as a primary environmental cue controlling BB_0173 expression.

What is the relationship between BB_0173 expression and the tick-mammal transmission cycle?

While direct correlation with the tick-mammal cycle was not fully characterized in the search results, the oxygen-dependent regulation of BB_0173 provides important clues. B. burgdorferi experiences significantly different oxygen concentrations during its life cycle - relatively aerobic conditions in unfed ticks, microaerophilic environments in feeding ticks, and varying oxygen levels in mammalian tissues . The downregulation of BB_0173 under low-oxygen conditions suggests it may be more highly expressed in unfed ticks or specific mammalian environments with higher oxygen tension . This regulation pattern differs from BB_0172, which has been shown to be differentially regulated based on the environmental shift between tick vector and mammalian hosts . Understanding this differential expression may provide insights into stage-specific adaptive mechanisms employed by B. burgdorferi during transmission.

What techniques are effective for determining BB_0173 cellular localization?

The research employed multiple complementary approaches to definitively establish BB_0173's cellular localization:

Triton X-114 Detergent Phase Separation: This technique separates proteins based on their hydrophobicity, allowing differentiation between cytoplasmic, inner membrane, and outer membrane proteins. For BB_0173 analysis, bacterial cells were lysed and fractionated into aqueous phase (AQ), detergent phase (DT), and protoplasmic cylinders (PC). BB_0173 was detected in both AQ and PC fractions but absent from DT, consistent with inner membrane localization .

Proteinase K Protection Assay: This assay exploits the inability of proteases to cross the outer membrane, allowing selective degradation of surface-exposed proteins. Intact B. burgdorferi cells were treated with proteinase K at concentrations ranging from 0 to 200 μg/mL. Control outer membrane proteins (P66, OspC, VlsE) showed decreased detection with proteinase treatment, while BB_0173 remained unaffected, confirming its non-surface exposure .

Computational Prediction: Transmembrane topology prediction algorithms were used to analyze BB_0173 sequence prior to experimental validation, providing initial hypotheses about membrane spanning regions and domain orientation .

Together, these methods provide a robust approach for determining membrane protein localization in spirochetes and can be adapted for similar proteins in other bacterial systems.

How can researchers generate specific antibodies against BB_0173?

The search results detail an effective approach for generating BB_0173-specific antibodies:

• Antigen Selection: Two approaches were used simultaneously:

  • A recombinant truncated BB_0173 protein (rBB0173 T)

  • A 30-amino acid synthetic peptide (BB0173 pep, amino acids 105-134) derived from the large loop containing the VWFA domain

• Immunization Protocol:

  • Animal model: Chickens (housed at Texas A&M University poultry farm)

  • Immunization schedule: 50 μg of antigen mixed with TiterMaxTM Gold adjuvant administered intramuscularly at days 0, 14, and 28

  • Collection: Eggs collected daily from days 35-45 and frozen at -20°C

• Antibody Purification:

  • Antibodies were purified from egg yolks using established protocols

  • Both antigens yielded antibodies that recognized truncated antigen and full-length BB_0173 in borrelial whole cell lysates

  • The peptide-derived antibodies (BB0173 pep) provided superior specificity with lower background in detection assays

This chicken IgY antibody production approach offers advantages for spirochete research, including high antibody yields, avoidance of cross-reactivity with mammalian proteins, and ethical benefits compared to some mammalian antibody production methods.

What expression systems are suitable for recombinant BB_0173 production?

While the search results don't provide extensive details on expression systems specifically for BB_0173, they mention the production of a truncated recombinant form (rBB0173 T) used for antibody generation . Based on the protein's characteristics, researchers should consider:

• Expression Host Selection:

  • E. coli systems are commonly used for initial attempts but may require optimization for membrane proteins

  • For membrane proteins like BB_0173, specialized E. coli strains (C41, C43) designed for membrane protein expression may improve yields

  • Cell-free systems can be considered for toxic or difficult-to-express membrane proteins

• Construct Design Considerations:

  • Truncated constructs omitting transmembrane regions (as used for rBB0173 T) typically improve solubility

  • Addition of solubility-enhancing tags (MBP, SUMO) may improve expression

  • Codon optimization for the expression host

• Protein Extraction Methods:

  • Specialized detergent-based extraction for full-length protein with membrane domains

  • Milder extraction conditions for soluble domain constructs

When working with multi-domain membrane proteins like BB_0173, researchers often achieve better results by expressing individual domains separately rather than attempting full-length expression, particularly for structural or interaction studies.

What is the proposed function of BB_0173 in Borrelia burgdorferi?

Based on domain analysis, cellular localization, and expression patterns, BB_0173 is proposed to function as part of an aerotolerance complex in B. burgdorferi. The protein contains a Bacteroides aerotolerance-like domain and shows significant expression changes in response to oxygen levels . The aerotolerance function is supported by comparison to similar protein complexes in Bacteroides, which help bacteria survive in varying oxygen environments . BB_0173's inner membrane localization with periplasm-facing VWFA domain suggests it may act as a sensor or regulator responding to environmental oxygen conditions .

Unlike its paralog BB_0172 (which functions by binding mammalian integrin α3β1), BB_0173's intracellular localization indicates a distinct role, potentially in signaling or sensing . The presence of the MIDAS motif suggests metal ion binding capabilities, which may be involved in this proposed sensing function . This aerotolerance role would be critical for B. burgdorferi survival as it transitions between the relatively aerobic unfed tick environment and the microaerophilic conditions of the feeding tick and mammalian host.

How does BB_0173 compare to other VWFA domain-containing proteins in Borrelia?

BB_0173 is one of four identified VWFA domain-containing proteins in B. burgdorferi (BB_0172, BB_0173, BB_0175, and BB_0325), with significant differences from its better-characterized paralog BB_0172:

This comparison highlights the functional diversification of VWFA domain-containing proteins in B. burgdorferi, with BB_0172 evolving for host interaction while BB_0173 appears specialized for environmental sensing . The contrasting localizations with similarly positioned VWFA domains (facing external environments - host or periplasm) suggests these proteins may form part of a sensing system, with BB_0172 detecting host factors and BB_0173 monitoring internal/periplasmic conditions .

What is the relationship between BB_0173 and the surrounding gene cluster (BB_0170 to BB_0176)?

The BB_0170 to BB_0176 gene cluster appears to form a functional unit with several distinctive features:

• Conserved Synteny: This gene arrangement is preserved across Borrelia species and shows similarities to gene clusters in Bacteroides, Rhizobium, and Leptospira, suggesting functional importance

• Complementary Domains: The cluster contains complementary functional domains including:

  • VWFA domains (BB_0173, BB_0172)

  • Tetratricopeptide repeat (TPR) domains (BB_0170, BB_0171)

  • SRC Homology 3 (SH3) domains

  • MoxR AAA+ ATPase (BB_0176)

  • BatA-like domains

• Coordinated Regulation: Multiple genes in this cluster show coordinated regulation in response to oxygen levels, with BB_0173 and BB_0176 showing the strongest downregulation under low-oxygen conditions (21.3-fold and 22.8-fold decreases, respectively)

• Functional Similarity to Bat Complex: The domain organization and arrangement show similarity to the Bacteroides aerotolerance (Bat) complex, with BB_0170 appearing to be a fusion protein with homology to both BatD and TPR domains found separately in Bacteroides

This evidence suggests that BB_0170 to BB_0176 likely function together as a multi-protein complex involved in sensing and responding to environmental oxygen conditions, with BB_0173 serving as a key component within this system .

How might BB_0173 contribute to Borrelia pathogenesis and survival in different host environments?

BB_0173's contribution to Borrelia pathogenesis likely centers on its role in helping the bacterium adapt to varying oxygen tensions encountered during its life cycle. As B. burgdorferi transitions between the tick vector and mammalian host, it experiences substantial changes in oxygen availability . The ability to sense and respond to these changes is critical for bacterial survival.

The aerotolerance function proposed for BB_0173 would be particularly important during:

• Tick Acquisition/Feeding Phase: When bacteria move from mammalian tissue to the tick midgut during blood feeding, experiencing a shift in oxygen tension

• Host Dissemination: As bacteria spread from the initial inoculation site to various tissues with different oxygen levels

• Persistent Infection: For maintaining infection in tissues with fluctuating oxygen levels due to inflammatory responses

The conservation of BB_0173 across pathogenic Borrelia suggests it may represent a fundamental adaptation required for the pathogenic lifestyle . While not directly involved in host interaction like BB_0172, BB_0173's role in environmental sensing and adaptation likely contributes to the bacterium's remarkable ability to establish persistent infections in diverse host tissues. This makes BB_0173 and its associated aerotolerance complex potential targets for novel therapeutic approaches aimed at disrupting bacterial adaptation during infection .

What approaches can be used to study potential protein-protein interactions involving BB_0173?

Given BB_0173's predicted role in a multi-protein complex, several complementary approaches could be employed to identify interaction partners:

• Bacterial Two-Hybrid System:

  • Particularly useful for membrane proteins when modified with appropriate membrane-targeting components

  • Can test specific hypothesized interactions (e.g., with other proteins in the BB_0170-BB_0176 cluster)

• Co-Immunoprecipitation with BB_0173-Specific Antibodies:

  • Using the chicken anti-BB_0173 antibodies described in the research

  • Requires careful optimization of membrane solubilization conditions to maintain protein-protein interactions

• Proximity-Based Labeling:

  • BioID or APEX2 fusions to BB_0173 could identify proximal proteins in the native cellular environment

  • Particularly valuable for transient or weak interactions in membrane complexes

• Cross-Linking Mass Spectrometry:

  • Chemical cross-linking followed by mass spectrometry analysis

  • Can capture direct protein-protein interactions in their native environment

• Genetic Approaches:

  • Suppressor mutation analysis following BB_0173 mutation

  • Synthetic lethality screening to identify functional relationships

When studying BB_0173 interactions, special attention should be paid to other proteins in the BB_0170-BB_0176 cluster, particularly BB_0176 (MoxR ATPase) which shows similar regulation patterns and is known to frequently associate with VWFA domain-containing proteins in various bacterial systems .

What are the challenges and considerations in targeting BB_0173 for therapeutic intervention?

While BB_0173 represents a potential therapeutic target given its conservation and likely essential role, several challenges must be considered:

• Inner Membrane Localization:

  • BB_0173's inner membrane localization with periplasm-facing functional domains creates accessibility challenges

  • Small molecule inhibitors would need to cross the outer membrane but not the inner membrane to reach the target domain

  • This contrasts with the relatively more accessible outer membrane protein BB_0172

• Functional Redundancy:

  • The presence of multiple VWFA domain-containing proteins in B. burgdorferi may provide functional redundancy

  • Effective intervention might require targeting multiple components of the aerotolerance system

• Structural Characterization Limitations:

  • Limited structural information about BB_0173 hampers structure-based drug design

  • Membrane proteins present significant challenges for traditional structural biology approaches

• Conservation Considerations:

  • High conservation across bacterial species could lead to broad-spectrum effects

  • While potentially advantageous for broad antimicrobial development, this might affect commensal bacteria

• Target Validation Needs:

  • Further validation of BB_0173's essentiality through conditional knockout systems is needed

  • Confirmation of the specific molecular function would guide intervention strategies

Despite these challenges, the conservation of BB_0173 and its apparent role in environmental adaptation make it a promising candidate for future therapeutic development, particularly for strategies aimed at preventing bacterial persistence during infection .

What are the key unanswered questions about BB_0173 function?

Despite significant progress in characterizing BB_0173, several critical questions remain:

• Molecular Mechanism: What is the precise molecular mechanism by which BB_0173 contributes to aerotolerance? Does it directly sense oxygen, or does it respond to secondary signals?

• Metal Binding: What is the functional significance of the MIDAS motif and metal ion binding? Which specific metal ions interact with BB_0173 under physiological conditions?

• Interaction Network: What are the direct interaction partners of BB_0173 in the periplasmic space or inner membrane? How does it communicate with other components of the proposed aerotolerance complex?

• Signal Transduction: How does BB_0173's sensing function translate into adaptive responses? What downstream pathways are activated or repressed?

• Essentiality: Is BB_0173 essential for B. burgdorferi survival in any particular environment? Would targeted disruption prevent bacterial persistence in hosts?

Addressing these questions will require comprehensive genetic, biochemical, and structural approaches, potentially revealing new insights into bacterial adaptation mechanisms and identifying novel therapeutic targets .

How might emerging technologies advance BB_0173 research?

Several emerging technologies could significantly accelerate BB_0173 research:

• Cryo-Electron Microscopy:

  • Recent advances in cryo-EM for membrane proteins could help determine BB_0173's structure in its native membrane environment

  • This would provide critical insights into domain organization and potential binding interfaces

• CRISPR-Based Strategies:

  • Optimized CRISPR interference or activation systems for Borrelia could allow precise temporal control of BB_0173 expression

  • This would help distinguish direct from indirect effects in complex phenotypes

• Single-Cell Analysis:

  • Single-cell RNA-seq and proteomics could reveal heterogeneity in BB_0173 expression across bacterial populations

  • Particularly relevant when studying adaptation to microenvironments within hosts

• Microfluidic Systems:

  • Controlled oxygen gradient devices could allow real-time observation of BB_0173-dependent responses

  • Would provide insights into the dynamics of adaptation to changing oxygen levels

• Synthetic Biology Approaches:

  • Reconstitution of the BB_0173 system in heterologous hosts could allow functional dissection

  • Minimal systems approach would help identify essential components of the aerotolerance complex

These technologies would complement traditional approaches and potentially overcome current limitations in understanding membrane protein function in spirochetes .

What implications does BB_0173 research have for understanding other bacterial adaptation mechanisms?

Research on BB_0173 has broader implications for understanding bacterial adaptation mechanisms:

• Conservation Across Diverse Bacteria: The high conservation of BB_0173 and its gene cluster across phylogenetically distant bacteria (including Bacteroides, Rhizobium, and Leptospira) suggests it represents a fundamental adaptation mechanism that evolved early and has been maintained across diverse bacterial lineages .

• Insight Into Bacterial Aerotolerance: Understanding BB_0173 function could provide a model for how various bacteria sense and adapt to different oxygen environments, a critical aspect of pathogen survival during host infection .

• Membrane Protein Sensing Systems: BB_0173 represents an example of inner membrane proteins with periplasm-facing functional domains serving as environmental sensors, a common but incompletely understood bacterial adaptation strategy .

• VWFA Domain Functional Diversity: The contrasting functions of BB_0172 and BB_0173, despite both containing VWFA domains, illustrate how bacteria can repurpose similar domains for different functions (external adhesion versus internal sensing) .

• Pathogen Adaptation Strategies: The study of BB_0173 provides insights into how vector-borne pathogens adapt to the dramatically different environments encountered during transmission cycles .

This research contributes to a growing understanding of bacterial environmental sensing and adaptation mechanisms, with potential applications for addressing other challenging bacterial infections that involve similar adaptation strategies .