Recombinant Herpetosiphon aurantiacus ATP synthase subunit b (atpF)

Basic Research Questions

• What expression systems are most suitable for producing recombinant Herpetosiphon aurantiacus ATP synthase subunit b?

  E. coli is the preferred expression system for recombinant Herpetosiphon aurantiacus ATP synthase subunit b. Based on successful expression protocols for related ATP synthase components, the following methodological approach is recommended:

  • Clone the atpF gene (Haur_4068) into a pET expression vector with an N-terminal His₆-tag

  • Transform into E. coli Rosetta2(DE3) cells for efficient expression of proteins with rare codons

  • Induce expression at mid-log phase (OD₆₀₀ = 0.6) with 0.5-1.0 mM IPTG

  • Harvest cells after 4-6 hours expression at 30°C (or overnight at 16°C for improved solubility)

  • Purify using Ni-NTA affinity chromatography followed by size exclusion chromatography

  This approach has been successfully employed for similar proteins and can be adapted specifically for atpF from Herpetosiphon aurantiacus.

• What are the optimal storage conditions for maintaining stability of recombinant Herpetosiphon aurantiacus ATP synthase subunit b?

  Based on empirical data from characterized recombinant proteins with similar properties, the following storage protocol is recommended:

  • Short-term storage (up to one week): 4°C in Tris-based buffer with 50% glycerol

  • Long-term storage: -20°C or -80°C in aliquots to avoid repeated freeze-thaw cycles

  • Buffer composition: 50 mM Tris-HCl pH 8.0, 150 mM NaCl, 50% glycerol

  • Addition of reducing agents (1 mM DTT or 5 mM β-mercaptoethanol) may improve stability

  • Protein concentration should be maintained at 1-5 mg/mL for optimal stability

  It is critical to avoid repeated freeze-thaw cycles as this significantly decreases protein activity and integrity.

Advanced Research Questions

• How does Herpetosiphon aurantiacus ATP synthase subunit b differ structurally and functionally from homologs in other bacterial species?

  While direct structural data for Herpetosiphon aurantiacus ATP synthase subunit b is limited, comparative analysis with related species reveals significant variations:

  Species	Subunit b Features	Peripheral Stalk Structure	Functional Implications
  Herpetosiphon aurantiacus	Single copy of subunit b per ATP synthase monomer (predicted)	Standard peripheral stalk arrangement (predicted)	Conventional coupling mechanism between F₀ and F₁
  Chloroflexus aurantiacus	Four copies of subunit b per complex	Two peripheral stalks	Enhanced stability and possibly altered proton-to-ATP ratio
  E. coli	Two copies of identical subunit b	Single peripheral stalk	Standard coupling mechanism
  Yersinia pseudotuberculosis	Two copies of identical subunit b	Single peripheral stalk	Standard coupling mechanism
  Bacillus PS3	Two different interfaces of b-subunits with subunit a	Single peripheral stalk with unique interactions	Specific adaptations for thermophilic environment

  The related Chloroflexus aurantiacus, which belongs to the same Chloroflexota phylum, contains an unusual ATP synthase architecture with two peripheral stalks and two proton-conducting a-subunits. These structural modifications create unique proton translocation pathways that potentially double the number of protons translocated per ATP synthesis cycle .

  Research suggests that structural variations in ATP synthase from early photosynthetic bacteria like those in the Chloroflexota phylum represent evolutionary adaptations to specific environmental conditions and energy requirements .

• What methodological approaches can be employed to investigate the interaction between recombinant Herpetosiphon aurantiacus ATP synthase subunit b and other components of the ATP synthase complex?

  Several complementary approaches are recommended for investigating subunit interactions:

  1. Cross-linking coupled with mass spectrometry:

     • Use bifunctional cross-linkers like DSS or EDC

     • Digest cross-linked complexes with trypsin

     • Analyze cross-linked peptides by LC-MS/MS

     • Identify interaction interfaces through computational analysis of cross-linked residues

  2. Surface plasmon resonance (SPR):

     • Immobilize purified recombinant atpF on a sensor chip

     • Flow other ATP synthase components over the surface

     • Measure binding kinetics (kon and koff rates)

     • Determine binding affinities (KD values) for different components

  3. Cryo-electron microscopy:

     • Express and purify the complete ATP synthase complex

     • Prepare vitrified samples on EM grids

     • Collect high-resolution images

     • Perform image processing and 3D reconstruction

     • This approach has successfully revealed the structure of ATP synthases from related species

  4. Functional reconstitution assays:

     • Incorporate purified components into liposomes

     • Measure ATP synthesis activity using luciferin-luciferase assays

     • Assess the effect of atpF mutations on proton translocation and ATP synthesis

• How can site-directed mutagenesis of Herpetosiphon aurantiacus ATP synthase subunit b be used to probe its role in the assembly and function of the ATP synthase complex?

  Based on structural and functional studies of related ATP synthases, the following site-directed mutagenesis approach is recommended:

  1. Target regions for mutagenesis:

     • N-terminal transmembrane domain: mutations here likely affect membrane anchoring and interaction with subunit a

     • Middle coiled-coil region: mutations may disrupt peripheral stalk formation

     • C-terminal domain: mutations could affect interaction with the F₁ sector

  2. Key residues for mutational analysis:

     • Conserved charged residues (Arg, Lys, Glu, Asp) that may form salt bridges with other subunits

     • Hydrophobic residues in the transmembrane domain that interact with the membrane

     • Residues at predicted interfaces with other subunits based on homology modeling

  3. Functional analysis of mutants:

     • Express mutant proteins in E. coli

     • Assess assembly of the ATP synthase complex using blue native PAGE

     • Measure ATP synthesis and hydrolysis activities using enzyme assays

     • Analyze proton transport using pH-sensitive fluorescent dyes

  Studies on related bacterial ATP synthases have shown that mutations in the N-terminal region of subunit b can be particularly disruptive to assembly and function, as this region makes critical interactions with subunit a .

• What approaches can be used to study the oligomeric state of Herpetosiphon aurantiacus ATP synthase and the stoichiometry of subunit b in the complex?

  Determining the precise stoichiometry of subunit b in the Herpetosiphon aurantiacus ATP synthase complex is crucial, especially considering the unusual four-copy arrangement found in the related Chloroflexus aurantiacus. The following methodological approaches are recommended:

  1. Analytical ultracentrifugation:

     • Sediment velocity analysis to determine the size and shape of the complex

     • Equilibrium sedimentation to determine molecular weight and stoichiometry

  2. Native mass spectrometry:

     • Purify intact ATP synthase complexes in detergent micelles

     • Transfer to volatile buffers compatible with mass spectrometry

     • Determine accurate masses of intact complexes and subcomplexes

     • Calculate subunit stoichiometry from mass measurements

  3. Quantitative protein analysis:

     • Separate subunits by SDS-PAGE

     • Quantify band intensities using densitometry

     • Calculate molar ratios by dividing peak areas by corresponding molecular masses

  4. Cryo-EM structural analysis:

     • Purify intact ATP synthase complexes

     • Determine high-resolution structure by cryo-EM

     • Directly visualize and count subunit b copies in the structure

     • This approach has revealed the unusual architecture of ATP synthase from Chloroflexus aurantiacus

• How can the proton translocation mechanism of Herpetosiphon aurantiacus ATP synthase be experimentally characterized?

  Investigating the proton translocation mechanism requires specialized techniques:

  1. Liposome reconstitution and pH measurements:

     • Purify intact ATP synthase or reconstitute from purified components

     • Incorporate into liposomes with pH-sensitive fluorescent dyes

     • Monitor pH changes during ATP synthesis or hydrolysis

     • Quantify proton-to-ATP ratios under different conditions

  2. Single-molecule rotation analysis:

     • Immobilize ATP synthase complexes on surfaces

     • Attach fluorescent probes or beads to rotating components

     • Visualize and quantify rotation using fluorescence microscopy

     • Correlate rotational steps with ATP synthesis events

  3. Structure-based mutational analysis:

     • Target residues in the proton translocation pathway based on structural homology

     • Create point mutations and assess their effects on proton translocation

     • Key targets include conserved acidic residues in subunit a and the essential carboxylate in subunit c

  Recent studies on ATP synthases from photosynthetic bacteria reveal unique arrangements of proton inlets and outlets that facilitate efficient energy conversion. The Chloroflexus aurantiacus ATP synthase contains two proton inlets on the periplasmic side and two proton outlets on the cytoplasmic side, which may represent adaptations to the photosynthetic lifestyle .