Recombinant Gorilla gorilla gorilla Cytochrome b-c1 complex subunit Rieske, mitochondrial (UQCRFS1)

How does the flexible linker region of UQCRFS1 affect electron transfer?

The flexible linker region of UQCRFS1 is critical for proper function of the cytochrome bc1 complex:

• The catalytic domain of UQCRFS1, which carries the [2Fe-2S] cluster, is connected to a transmembrane anchor by this flexible linker region

• This flexibility allows the catalytic domain to move between two positions: proximal to cytochrome b and proximal to cytochrome c1

• Research has shown that altering the length of this flexible linker significantly impacts enzyme function:

  • Addition of one alanine residue reduces ubiquinol-cytochrome c reductase activity by 50%

  • Addition of two alanine residues reduces activity by 90%

  • Deletion of an alanine residue reduces activity by approximately 40%

  • These modifications also affect the apparent Km for ubiquinol and decrease inhibition efficacy by stigmatellin

These findings indicate that the precise length of the flexible linker is essential for optimal ubiquinol interaction with the bc1 complex, supporting electron transfer mechanisms where ubiquinol must simultaneously interact with both the iron-sulfur protein and cytochrome b .

How is UQCRFS1 processed and incorporated into Complex III?

UQCRFS1 undergoes a complex processing and assembly pathway:

• Synthesis: UQCRFS1 is synthesized as a precursor protein in the cytosol with a mitochondrial targeting sequence (MTS)

• Import: The precursor is imported into mitochondria via the TOM and TIM23 pathways

• Matrix processing:

  • In the matrix, UQCRFS1 binds to MZM1L/LYRM7 chaperone

  • This chaperone stabilizes UQCRFS1 and recruits the Fe-S transfer complex

  • The 2Fe-2S cluster is incorporated into UQCRFS1

• Membrane integration:

  • BCS1L translocates and incorporates UQCRFS1 into pre-assembled Complex III in the inner membrane

  • This is the penultimate step in Complex III assembly and renders it catalytically active

• Proteolytic processing:

  • Unlike other imported proteins, UQCRFS1 is processed only after incorporation into Complex III

  • The MTS is cleaved, generating the mature protein

  • In mammals, the cleaved N-terminal fragment (called subunit 9 in bovine) remains bound to Complex III between core subunits UQCRC1 and UQCRC2

This unique processing mechanism represents "the first instance in which a cleaved targeting presequence has been shown to be retained in the cell, possibly exhibiting a second function in addition to its function in protein trafficking" .

What is the role of TTC19 in UQCRFS1 processing and Complex III function?

TTC19 plays a critical post-assembly quality control role related to UQCRFS1:

• During UQCRFS1 assembly, the precursor is cleaved, and its N-terminal part remains bound to Complex III between the two core subunits (UQCRC1 and UQCRC2)

• Research with TTC19-deficient human and mouse models revealed:

  • In the absence of TTC19, there is significant accumulation of UQCRFS1-derived N-terminal fragments

  • This accumulation is detrimental to Complex III function

  • TTC19 is involved in the removal of these N-terminal UQCRFS1 peptides (particularly those of 8 kDa and 12 kDa)

  • Without proper clearance of these fragments, Complex III shows aberrant electrophoretic mobility, defective enzymatic activity, and increased reactive oxygen species (ROS) production

The current understanding suggests that during UQCRFS1 incorporation and in situ processing, several peptides containing its MTS are produced and remain bound to Complex III. For proper structural integrity and function, these peptides must be removed in a process facilitated by TTC19 .

What are the optimal conditions for handling recombinant UQCRFS1 in laboratory settings?

Based on vendor recommendations and research protocols, optimal handling of recombinant UQCRFS1 includes:

• Storage conditions:

  • Store at -20°C/-80°C upon receipt

  • Aliquot to avoid repeated freeze-thaw cycles

  • For working aliquots, store at 4°C for up to one week

  • For long-term storage, add 5-50% glycerol (final concentration) before aliquoting

• Reconstitution:

  • Briefly centrifuge vials prior to opening

  • Reconstitute lyophilized protein in deionized sterile water to 0.1-1.0 mg/mL

  • For buffers, Tris/PBS-based buffer with pH 8.0 is commonly used

• Experimental considerations:

  • His-tagged or GST-tagged versions are available for different purification strategies

  • For Western blotting applications, antibodies targeting the mature form recognize a band at approximately 23-29.7 kDa

  • When analyzing Complex III assembly, Blue Native Polyacrylamide Gel Electrophoresis (BN-PAGE) is recommended over SDS-PAGE

What methods are most effective for assessing UQCRFS1 incorporation into Complex III?

Several complementary methods can be used to assess UQCRFS1 incorporation into Complex III:

• Blue Native Polyacrylamide Gel Electrophoresis (BN-PAGE):

  • Allows separation of multi-protein complexes in their native conformation

  • Can detect assembled Complex III using antibodies against UQCRC2 or UQCRFS1

  • Particularly useful for identifying assembly defects or abnormal complex formation

• Import assays with radioactively labeled UQCRFS1:

  • In organello assays using isolated mitochondria

  • Time-course analysis to monitor UQCRFS1 processing and incorporation

  • Allows detection of N-terminal fragments generated during processing

• Functional assays:

  • Microscale respirometry (Seahorse) to measure oxidative phosphorylation function

  • Spectrophotometric measurement of ubiquinol-cytochrome c reductase activity

  • Kinetic analysis to determine Km values for ubiquinol

• Protein level analysis:

  • Western blotting with antibodies against UQCRFS1 and other Complex III subunits

  • Immunoprecipitation to study protein-protein interactions within the complex

• Immunofluorescence microscopy:

  • To confirm mitochondrial localization of UQCRFS1

  • Co-localization studies with other Complex III components

How do mutations in UQCRFS1 contribute to mitochondrial disorders?

Bi-allelic UQCRFS1 variants are associated with mitochondrial complex III deficiency, nuclear type 10 (MC3DN10). The pathomechanism includes:

• Clinical presentation:

  • Low Complex III activity in fibroblasts

  • Lactic acidosis

  • Fetal bradycardia

  • Hypertrophic cardiomyopathy

  • Alopecia totalis

• Cellular pathophysiology:

  • Reduced UQCRFS1 protein abundance

  • Impaired mitochondrial import of UQCRFS1

  • Defective Complex III assembly

  • Compromised cellular respiration

  • Energy production deficiency

• Molecular consequences:

  • Disruption of the 2Fe-2S cluster incorporation

  • Impaired electron transfer in the respiratory chain

  • Reduced ATP production

  • Increased reactive oxygen species (ROS) production

  • Metabolic acidosis

Complementation studies via lentiviral transduction and overexpression of wild-type UQCRFS1 have been shown to restore mitochondrial function and rescue the cellular phenotype in patient fibroblasts, confirming the causality of UQCRFS1 variants in Complex III deficiency .

What cellular adaptations occur in response to UQCRFS1 dysfunction?

Cellular adaptations to UQCRFS1 dysfunction include:

• Metabolic reprogramming:

  • Increased reliance on glycolysis for ATP production

  • Altered TCA cycle activity

  • Metabolic acidosis with lactic acid accumulation

• Mitochondrial responses:

  • Changes in mitochondrial morphology and distribution

  • Compensatory upregulation of other respiratory chain components

  • Activation of mitochondrial quality control mechanisms

  • Altered mitochondrial membrane potential

• Cellular stress responses:

  • Activation of retrograde signaling pathways

  • Upregulation of stress response genes

  • Altered calcium homeostasis

  • Increased production of reactive oxygen species

  • Potential activation of apoptotic pathways

• Tissue-specific effects:

  • Most pronounced in tissues with high energy requirements such as heart and skeletal muscle

  • Distinct patterns of adaptation in different cell types based on their reliance on oxidative phosphorylation

What are the key differences between Gorilla gorilla gorilla UQCRFS1 and human UQCRFS1?

Comparison of Gorilla gorilla gorilla and human UQCRFS1 reveals:

The high degree of conservation between gorilla and human UQCRFS1 reflects the critical function of this protein in mitochondrial respiration across primates. This conservation makes gorilla UQCRFS1 a valuable model for studying human mitochondrial disorders associated with UQCRFS1 dysfunction .

How does UQCRFS1 processing differ between yeast, mammals, and other organisms?

UQCRFS1 processing shows significant differences across species:

• Yeast vs. Mammals:

  • Yeast Rip1 (UQCRFS1 homolog) has a much shorter MTS than mammalian UQCRFS1 (30 aa vs. 78 aa)

  • Processing mechanism differs:

    • Yeast: Two-step processing where MPP cleaves the first 22 amino acids, then MIP removes the next octapeptide

    • Mammals: Evidence suggests processing occurs after incorporation into Complex III

  • Retention of cleaved fragments:

    • Mammals: The cleaved N-terminal fragment remains bound to Complex III

    • Yeast: Cleaved presequence is typically degraded

• Plants:

  • MPP activity is integrated into Complex III in plants

  • Different processing pattern compared to mammals and yeast

• TTC19 distribution:

  • TTC19 orthologs exist in organisms with long UQCRFS1 MTS

  • TTC19 is absent in yeast (which has shorter MTS)

These differences suggest evolutionary adaptation of UQCRFS1 processing mechanisms that may relate to the complexity of respiratory chain assembly and regulation in different organisms .

How can UQCRFS1 be leveraged as a target for developing novel fungicides?

UQCRFS1 (Rieske iron-sulfur protein) offers unique advantages as a fungicide target:

• Strategic importance:

  • The cytochrome bc1 complex is already an important target for fungicides

  • Most commercial fungicides target the cytochrome b subunit, but resistance develops rapidly

  • UQCRFS1 offers an alternative target within the same complex

• Resistance management advantages:

  • Unlike cytochrome b (encoded by mitochondrial DNA), UQCRFS1 is nuclear-encoded

  • This difference may impact the mutation rate and resistance development

  • Targeting UQCRFS1 could overcome resistance problems associated with cytochrome b inhibitors

• Inhibitor classifications based on UQCRFS1 interaction:

  • Type I: Inhibitors that mobilize the rotation of the ISP

  • Type II: Inhibitors that restrict ISP rotation

  • Type III: Inhibitors that fix ISP rotation

  • The strength of ISP-inhibitor interactions correlates with inhibitor activity and resistance development

• Design considerations:

  • Target the flexibility and motion of the ISP and its essential role in electron transfer

  • Focus on interactions that fix the ISP in positions that prevent electron transfer

  • Design compounds that interact with both the ISP and other subunits for increased specificity

What methodologies are most effective for studying conformational changes in UQCRFS1 during electron transfer?

Advanced techniques for studying UQCRFS1 conformational dynamics include:

• Structural approaches:

  • Cryo-electron microscopy to capture different conformational states

  • X-ray crystallography with various inhibitors that trap specific conformations

  • NMR spectroscopy to study flexible regions and dynamic changes

• Spectroscopic methods:

  • EPR (Electron Paramagnetic Resonance) to monitor the redox state of the 2Fe-2S cluster

  • FTIR (Fourier-Transform Infrared Spectroscopy) to detect conformational changes

  • Resonance Raman spectroscopy to study the iron-sulfur center

• Molecular dynamics:

  • Computational simulations of UQCRFS1 movement during the catalytic cycle

  • In silico modeling of flexible linker region dynamics

  • Quantum mechanics/molecular mechanics (QM/MM) approaches to study electron transfer

• Site-directed mutagenesis approaches:

  • Strategic modification of the flexible linker region (as shown in research where adding or removing alanine residues affected function)

  • Introduction of cysteine residues for disulfide cross-linking to restrict movement

  • Incorporation of fluorescent labels at key positions for FRET analysis

• Time-resolved techniques:

  • Stopped-flow spectroscopy to capture transient states

  • Pulse radiolysis to initiate electron transfer

  • Time-resolved crystallography to visualize structural changes during catalysis

What role does the coordination between UQCRFS1 processing and Complex III assembly play in mitochondrial respiratory chain supercomplexes?

The intricate relationship between UQCRFS1 processing and respiratory chain supercomplex formation includes: