Recombinant Borrelia hermsii Membrane protein insertase YidC (yidC)

What is YidC in Borrelia species and what is its function?

YidC in Borrelia species functions as a dual-purpose membrane protein with both insertase and chaperone activities. As an evolutionary conserved member of the Oxa1 superfamily, YidC is essential for bacterial inner membrane biogenesis, significantly influencing membrane protein composition and lipid organization . YidC operates through two main mechanisms:

• In conjunction with the Sec translocon, aiding the proper folding of multi-pass membrane proteins

• Independently as an insertase and lipid scramblase, facilitating the insertion of smaller membrane proteins while contributing to bilayer organization

YidC substrates are typically limited in the length of polypeptide translocated, usually to less than 30 amino acids . This constraint distinguishes YidC-mediated insertion from Sec-dependent pathways, which can accommodate longer translocated segments.

The insertase activity is particularly crucial for integrating proteins such as phage coat proteins, ATP synthase subunit c, and small membrane proteins like SecG .

How does the structure of YidC in Borrelia compare to YidC in other bacteria?

While specific structural data for Borrelia YidC is limited, insights can be derived from related bacterial YidC structures due to the conserved nature of this protein family. YidC typically consists of:

• An N-terminal amphipathic helix (N-AH)

• A periplasmic domain (P1)

• Five or six transmembrane segments

• A cytoplasmic C-terminal region

The crystal structure of Thermotoga maritima YidC (TmYidC, PDB ID: 6Y86) provides valuable insights. In this structure, the N-terminal amphipathic helix lies on the periplasmic side of the membrane bilayer, forming an angle of approximately 15° with the membrane surface .

Comparative analysis with YidC from other bacteria reveals that while the core structure is conserved, species-specific variations exist, particularly in:

• The N-terminal amphipathic helix

• The periplasmic domain

• The C-terminal region

These variations contribute to species-specific interactions with the Sec translocon and other partners in membrane protein biogenesis pathways .

What experimental approaches are used to express and purify recombinant Borrelia YidC?

The expression and purification of recombinant Borrelia YidC typically follows protocols similar to those used for other bacterial membrane proteins:

Expression system:

• Escherichia coli is commonly used as an expression host

• Expression vectors containing an affinity tag (commonly His-tag) facilitate purification

• For the related Borrelia turicatae YidC, successful expression in E. coli with an N-terminal His-tag has been documented

Purification protocol:

• Cell lysis under conditions that preserve membrane protein integrity

• Membrane fraction isolation by differential centrifugation

• Solubilization using appropriate detergents

• Affinity chromatography (e.g., Ni-NTA for His-tagged proteins)

• Size exclusion chromatography for further purification

Storage considerations:

• Buffer containing stabilizing detergents

• Addition of glycerol (typically 6-50%) for prolonged storage at -20°C/-80°C

• Avoiding repeated freeze-thaw cycles

• Reconstitution into proteoliposomes for functional studies

What are the distinct ecological and clinical aspects of Borrelia hermsii relevant to YidC research?

Understanding the ecological and clinical context of Borrelia hermsii provides important background for YidC research:

B. hermsii is spread by Ornithodoros hermsi ticks and found in mountainous areas of western United States at moderate to high elevations. Infections are commonly associated with rustic, rodent-infested cabins . This ecological niche differs from B. turicatae, which is found in the south-central United States and associated with caves .

Clinically, B. hermsii causes soft tick relapsing fever (STRF) in humans, characterized by:

• High fever

• Headache, nausea, myalgias, and arthralgias

• Initial illness typically lasting approximately 3 days

• If untreated, febrile episodes can recur every 7–10 days for multiple cycles

In western U.S. states, where B. hermsii infections predominate:

• A summer peak is observed, with 71% of cases occurring during June–September

• Notable exposures include visits to cabins (74%) and camping (8%)

These ecological and clinical aspects may influence research priorities for B. hermsii YidC, particularly regarding its role in pathogenesis and potential as a therapeutic target.

How does the N-terminal amphipathic helix of YidC contribute to its insertase function in Borrelia species?

The N-terminal amphipathic helix (N-AH) of YidC plays several critical roles in its insertase function:

• Membrane anchoring: N-AH serves as an uncleaved signal sequence that anchors YidC in the membrane with the correct orientation. Molecular dynamics simulations of TmYidC show that N-AH lies on the periplasmic side of the membrane bilayer, forming an angle of approximately 15° with the membrane surface .

• Sec translocon interaction: N-AH mediates species-specific interactions with the Sec translocon. Functional studies with chimeric proteins of E. coli YidC and TmYidC provide evidence that N-AH is crucial for species-specific interaction with SecY .

• Substrate recognition and processing: The N-terminal region contributes to substrate recognition, particularly for Sec-independent insertase activity.

Experimental evidence demonstrates that chimeric YidC proteins with N-AH from E. coli YidC and the remainder from TmYidC can complement YidC depletion in E. coli, while full TmYidC cannot . This highlights the importance of the species-specific N-AH in proper function.

The isoelectric point and net charge of N-AH vary among different bacterial species. For instance, N-AH in E. coli YidC has an isoelectric point of 4.3 with one negatively charged residue, while TmYidC has an isoelectric point of 8.5 with one positively charged residue . These differences likely contribute to species-specific interactions with the Sec translocon and other partners.

What methods can be used to study the dual function of YidC as both an insertase and chaperone in Borrelia?

Investigating YidC's dual function in Borrelia requires approaches that differentiate between its insertase and chaperone activities:

• In vitro translation/insertion assays:

  • Prepare inverted membrane vesicles (INVs) from strains expressing wild-type or mutant YidC

  • Translate model substrates in the presence of these INVs

  • Assess membrane insertion and folding

  • This approach has shown that YibN-enriched INVs support enhanced insertion of substrates like Pf3 coat, M13 procoat H5, and F0c

• Protease protection assays:

  • Translate and insert model substrates

  • Treat with proteases (e.g., proteinase K)

  • Analyze membrane-protected fragments (MPFs)

  • This technique has demonstrated YidC's role in SecG insertion and orientation

• Site-directed mutagenesis:

  • Create mutations predicted to selectively affect insertase or chaperone function

  • Analyze mutants functionally in vivo and in vitro

  • Identify residues critical for each function

• Structural studies:

  • Crystallography or cryo-EM to capture different conformational states

  • Computational modeling and molecular dynamics simulations

  • Analyze conformational changes associated with different functions

  • Superimposition with known active structures (e.g., B. halodurans YidC)

• Substrate specificity analysis:

  • Systematically test various substrates dependent on insertase or chaperone functions

  • Compare insertion and folding efficiency across substrate types

How does the interaction between YidC and YibN affect membrane protein insertion in Borrelia?

The interaction between YidC and YibN significantly impacts membrane protein insertion in bacterial systems:

• Enhanced substrate insertion: YibN enhances the production and membrane insertion of YidC substrates. In vitro translation/insertion assays using inverted membrane vesicles (INVs) show that YibN-enriched INVs support a 1.5-1.8-fold stimulation of insertion for substrates like Pf3 coat, M13 procoat H5, and F0c .

• Physical interaction: YibN has been identified as a crucial component within the YidC protein environment using proximity-dependent biotin labeling (BioID). This association has been confirmed by affinity purification-mass spectrometry assays conducted on native membranes .

• Effect on membrane lipids: Overproduction of YibN stimulates membrane lipid production and promotes inner membrane proliferation, possibly by interfering with YidC lipid scramblase activity . These changes in membrane lipid composition and organization can indirectly affect protein insertion efficiency.

• Substrate-specific effects: YibN's enhancement varies among substrates. While significant stimulation is observed for wild-type SecG insertion, the effect is much less evident with the SecG I20E mutant . This suggests YibN's impact depends on specific substrate features or insertion mechanisms.

What are the challenges in expressing and purifying functional Borrelia YidC for structural studies?

Expressing and purifying functional Borrelia YidC for structural studies presents several significant challenges:

• Membrane protein expression difficulties:

  • Low expression levels compared to soluble proteins

  • Potential toxicity to host cells when overexpressed

  • Improper folding or aggregation in heterologous expression systems

• Solubilization and stability issues:

  • Identifying detergents that maintain YidC in a native, functional state

  • Preventing aggregation during solubilization and purification

  • Maintaining stability during concentration and crystallization

• Purification complexities:

  • Removing contaminants without destabilizing the protein

  • Preventing degradation during purification

  • Achieving high purity required for structural studies

  • Developing multi-step purification protocols that preserve function

• Functional verification requirements:

  • Ensuring purified YidC retains its dual insertase and chaperone functions

  • Developing activity assays applicable to purified protein

  • Correlating structural features with functional properties

• Crystallization obstacles:

  • Identifying conditions that promote crystal formation

  • Managing conformational heterogeneity

  • Optimizing crystal quality for high-resolution diffraction

  • Dealing with the detergent phase during crystallization

• Species-specific considerations:

  • Potential differences in stability compared to well-studied YidC homologs

  • Need for optimization of protocols specifically for Borrelia YidC

  • Consideration of the native membrane environment of Borrelia

What methods can be used to study the lipid scramblase activity of YidC in Borrelia?

Investigating YidC's lipid scramblase activity in Borrelia requires specialized approaches to monitor lipid movement across membrane bilayers:

• Fluorescent lipid analog assays:

  • Incorporate fluorescently labeled lipids (e.g., NBD-labeled phospholipids) into membranes

  • Monitor their translocation across the bilayer

  • Use quenching assays to distinguish between inner and outer leaflet localization

  • Compare membranes with and without YidC

• Reconstitution systems:

  • Purify and reconstitute Borrelia YidC into liposomes of defined composition

  • Control protein-to-lipid ratio and membrane composition

  • Measure scramblase activity in the purified system

  • Analyze the impact of different lipid compositions

• Mutational analysis:

  • Generate YidC mutants predicted to affect scramblase activity

  • Perform functional assays to assess impact on lipid translocation

  • Correlate with effects on membrane protein insertion

• Lipid composition analysis:

  • Conduct mass spectrometry-based lipidomics of membranes from wild-type and YidC-depleted Borrelia

  • Analyze lipid asymmetry using leaflet-specific labeling techniques

  • Assess changes in lipid organization and distribution

• Electron microscopy:

  • Visualize membrane morphology in cells with wild-type or altered YidC

  • Assess membrane proliferation and organization

  • Studies with YibN have shown that its overproduction stimulates membrane lipid production and promotes inner membrane proliferation, possibly by interfering with YidC lipid scramblase activity

How can researchers determine the substrate specificity of YidC in Borrelia hermsii?

Determining YidC substrate specificity in Borrelia hermsii requires multiple experimental approaches:

• Proteomics-based methods:

  • Perform comparative proteomics of membrane fractions from wild-type and YidC-depleted Borrelia

  • Identify proteins whose membrane integration is affected by YidC depletion

  • Conduct quantitative analysis using stable isotope labeling (SILAC) or label-free methods

• In vitro translation/insertion assays:

  • Prepare inverted membrane vesicles (INVs) from Borrelia with and without YidC

  • Test candidate substrates for YidC-dependent insertion

  • Analyze insertion efficiency and topology

  • Similar approaches have shown that YibN enhances the production and membrane insertion of YidC substrates such as M13 and Pf3 phage coat proteins, ATP synthase subunit c, and small membrane proteins like SecG

• Cross-linking studies:

  • Incorporate site-specific photoreactive amino acids into YidC

  • Capture YidC-substrate complexes during the insertion process

  • Identify cross-linked substrates by mass spectrometry

• Computational prediction:

  • Analyze the Borrelia proteome for proteins with features common to known YidC substrates

  • Apply machine learning approaches trained on known YidC substrates from other bacteria

  • Use molecular dynamics simulations of candidate substrate interactions with YidC

• In vivo reporter assays:

  • Create fusions of candidate substrates to reporter proteins

  • Analyze reporter activity as a proxy for proper membrane insertion

  • Compare between wild-type and YidC-depleted conditions

What are the implications of YidC research for understanding Borrelia pathogenesis?

Research on YidC in Borrelia species has several important implications for understanding pathogenesis:

• Essential cellular processes:

  • YidC is essential for bacterial viability, playing a crucial role in inner membrane biogenesis

  • Disruption of YidC function could impair multiple cellular processes dependent on proper membrane protein insertion

  • Understanding these essential processes provides insights into Borrelia biology and potential vulnerabilities

• Pathogen-specific adaptations:

  • Comparative analysis of YidC between Borrelia species and other bacteria could reveal pathogen-specific adaptations

  • Differences in substrate specificity, interaction partners, or regulatory mechanisms might contribute to the unique biology of these pathogens

  • B. hermsii and B. turicatae cause soft tick relapsing fever in humans, with distinct ecological niches and transmission cycles

• Virulence factor insertion:

  • Many bacterial virulence factors are membrane or secreted proteins

  • YidC may play a role in the insertion of virulence-associated proteins in Borrelia

  • Identifying YidC-dependent virulence factors could provide insights into pathogenesis mechanisms

• Host-pathogen interactions:

  • Membrane proteins are at the interface between pathogen and host

  • YidC-dependent proteins may mediate interactions with host cells or evasion of host immune responses

  • Understanding these interactions could reveal new therapeutic or vaccine targets

• Therapeutic target potential:

  • The essential nature and unique features of bacterial YidC make it a potential target for antimicrobial development

  • Compounds that selectively interfere with YidC function could impair bacterial viability

  • Structure-based drug design approaches could target species-specific features of Borrelia YidC

How can evolutionary analysis of YidC inform our understanding of membrane protein insertion in Borrelia?

Evolutionary analysis of YidC provides valuable insights into membrane protein insertion mechanisms in Borrelia:

• Ancient conservation:

  • YidC is evolutionarily conserved and was present alongside SecY in the cenancestor

  • This conservation underscores its fundamental importance in bacterial physiology

  • Comparative analysis across bacterial lineages illuminates the evolution of membrane protein biogenesis systems

• Functional adaptations:

  • Despite conservation of core functions, YidC homologs show adaptations to specific cellular contexts

  • The N-terminal amphipathic helix (N-AH) shows particular variability and functions as a recognition helix for YidC insertase function

  • These adaptations may reflect specialization for different substrate profiles or membrane environments

• Interaction network evolution:

  • The identification of YibN as a YidC interactor adds complexity to our understanding of membrane protein biogenesis

  • YibN enhances YidC-mediated insertion and influences membrane lipid organization

  • The evolutionary relationship between YidC and its interaction partners provides context for understanding Borrelia-specific adaptations

• Structure-function relationships:

  • Starting from a YidC-like ancestral protein, more membrane-penetrating conformations could have evolved through hydrophobic substitution mutations

  • The dual insertase and chaperone functions of YidC reflect ancient adaptations for membrane protein biogenesis

  • Understanding these evolutionary relationships can inform hypotheses about YidC function in Borrelia

• Species-specific interactions:

  • The N-terminal amphipathic helix mediates species-specific interactions with the Sec translocon

  • Chimeric proteins with N-AH from E. coli YidC and the remainder from TmYidC can complement YidC depletion in E. coli, while full TmYidC cannot

  • This species specificity likely extends to Borrelia YidC and may influence its functional partnerships