Recombinant Saccharomyces cerevisiae Mitochondrial import inner membrane translocase subunit TIM17 (TIM17)

What is the structural organization of Tim17 in S. cerevisiae?

Tim17 is a member of the Tim17 protein family that includes Tim23 and Tim22. The protein features four transmembrane domains forming a curved surface with a lateral cavity opening to the lipid bilayer. This structure is similar to that of Tim22, as revealed by modeling based on cryo-EM structures of Tim22 from yeast and human as templates. The model is in good agreement with AlphaFold predictions of Tim17 . Unlike previous assumptions, Tim17 does not form a channel for precursor protein translocation across the inner membrane but rather provides a lateral cavity for protein translocation at the bilayer interface .

How does Tim17 contribute to protein import into mitochondria?

Tim17, not Tim23 as previously thought, is the major subunit of the presequence translocase directly involved in translocation of presequence proteins across the inner membrane. Mitochondrial presequence proteins can be imported across the inner membrane at the Tim17 bilayer interface. The negatively charged patch on the intermembrane space side of the lateral transmembrane cavity of Tim17 acts as a translocation initiation site (TIS) to import presequences along the cavity across the inner membrane . This mechanism represents a significant shift in our understanding of mitochondrial protein import.

What experimental approaches are most effective for studying Tim17 localization?

For studying Tim17 localization, a combination of subcellular fractionation and confocal microscopy has proven effective. Researchers can express Tim17 or its mutant variants with C-terminal GFP tags using tetracycline-inducible expression vectors. After expression, cells can be harvested and fractionated into total, cytosolic, and mitochondrial fractions. These fractions can then be analyzed using antibodies against Tim17, GFP, and appropriate cytosolic and mitochondrial marker proteins (such as protein phosphatase 5 and VDAC, respectively) . Confocal microscopy with MitoTracker Red staining (which incorporates into mitochondria in a membrane potential-dependent manner) and DAPI (for nuclear and mitochondrial DNA visualization) provides complementary visual confirmation of localization .

What are the critical amino acid residues for Tim17 function in protein translocation?

The hydrophilic residues within the lateral cavity of Tim17 are crucial for mitochondrial matrix protein translocation across the inner membrane. Specifically, mutations N64L (in the second transmembrane domain) and S114L (in the fourth transmembrane domain), located on opposing sides of the cavity, result in strong import defects of presequence proteins destined for the mitochondrial matrix while maintaining normal processing of proteins sorted to the inner membrane . This indicates that these hydrophilic residues on the matrix side of the lateral cavity are essential for matrix protein translocation. Additionally, Tim17 contains conserved negative charges close to the intermembrane space side of the bilayer that are essential to initiate presequence translocation .

How can researchers effectively generate and analyze Tim17 mutants?

Researchers can generate Tim17 mutants using a plasmid shuffling approach. This method involves:

• Using a Tim17 shuffle strain (e.g., YPH499 tim17∆ + YEp352-TIM17)

• Transforming with a plasmid encoding the mutant version of Tim17 (e.g., pFL39-Tim17 with TRP1 selection)

• Selecting on 5-fluoroorotic acid (5-FOA) plates to remove the wild-type copy

The composition of 5-FOA plates should include:

• 9.33 mM 5-FOA

• 0.067% (wt/vol) yeast nitrogen base without amino acids

• 0.077% (wt/vol) complete supplement mixture amino acids without uracil

• 0.005% (wt/vol) uracil

• 2-3% (wt/vol) glucose

• 3% (wt/vol) agar

For conditional expression, a Tim17 galactose-regulatable strain can be generated by homologous recombination of a PCR cassette containing the GAL promoter and antibiotic resistance . Cysteine mutants can be studied in specific backgrounds, such as Tim17 SCF and Tim23 CF backgrounds, using appropriate vector constructions .

What methods are most effective for studying Tim17-precursor protein interactions?

Chemical crosslinking is a particularly effective method for studying Tim17-precursor protein interactions. The crosslinking efficiency to Tim17 correlates with Tim17 activity, as demonstrated by experiments with temperature-sensitive tim17 mutant mitochondria. After heat shock and subsequent chemical crosslinking, the strong reduction in precursor protein-Tim17 crosslink formation in tim17-4 and tim17-5 temperature-sensitive mitochondria indicates that this crosslinking directly correlates with Tim17 activity .

For tracking Tim17 and its interactions, researchers can create tagged versions such as Tim17 2xStrep (with a glycine-alanine-glycine linker followed by 2xStrep tag downstream of the Tim17 open reading frame) or HisSUMO*Tim23 . These constructs facilitate protein purification and interaction studies.

How does S. cerevisiae Tim17 differ from Tim17 in other organisms?

While the search results focus primarily on S. cerevisiae Tim17 and Trypanosoma brucei Tim17 (TbTim17), notable differences exist between these homologs. Both possess four transmembrane domains, but their targeting mechanisms differ. TbTim17 contains at least two internal targeting signals (ITS): one within TM1 (amino acids 31-50) and another in TM4 + loop 3 (amino acids 120-136) . Both signals are required for proper targeting and integration into the membrane. Additionally, a positively charged residue (K122) is critical for mitochondrial localization of TbTim17 .

In contrast to S. cerevisiae Tim17, where the transmembrane domains form loop structures during import, TbTim17 appears to use a different mechanism. Studies with TbTim17 suggest that the signals in TM1 and TM4 work cooperatively for import and insertion, possibly via sequential interaction with translocase subunits rather than as loop structures .

What experimental strategies can be used to investigate targeting signals in Tim17?

To investigate targeting signals in Tim17, researchers can create a series of deletion mutants that systematically remove specific transmembrane domains. For example, with TbTim17, researchers created mutants removing TM1 (ΔN50), TM1-TM2 (ΔN100), TM1-TM3 (ΔN120), and TM4 (ΔC31), each attached to GFP at the C-terminal end for localization tracking .

Additionally, in silico structural modeling can be employed using programs like RaptorX (without templates) or Swiss modeling (based on existing structures like the cryo-EM structure of human Tim22) to predict the effects of these truncations on protein structure .

For a comprehensive analysis, these approaches should be combined with:

• Subcellular fractionation to assess protein distribution

• Confocal microscopy to visualize localization

• Functional assays to determine the impact on protein import

How can researchers optimize protein import assays for studying Tim17 function?

For studying Tim17 function through protein import assays, researchers should:

• Isolate mitochondria from wild-type and mutant strains under comparable conditions

• Ensure the isolated mitochondria maintain comparable membrane potential (Δψ) across the inner membrane, which can be verified using appropriate dyes or assays

• Use radiolabelled presequence proteins targeted to different mitochondrial compartments:

  • Matrix-targeted proteins (requiring complete translocation)

  • Inner membrane-sorted proteins (requiring lateral sorting)

  • Carrier proteins (dependent on the TIM22 complex)

This approach allows researchers to distinguish between general import defects and specific defects in particular import pathways. For instance, Tim17 N64L and S114L mutants specifically exhibited strong import defects of radiolabelled presequence proteins destined to the mitochondrial matrix, while maintaining normal processing of inner membrane-sorted proteins and Dic1 assembly (dependent on the TIM22 complex) .

What are the recommended approaches for studying Tim17 interactions with other TIM complex components?

To study Tim17 interactions with other TIM complex components, researchers can employ:

• Affinity purification: Using tagged versions of Tim17 (such as Tim17 2xStrep) to pull down interaction partners

• Chemical crosslinking: To capture transient interactions

• Co-immunoprecipitation: Using antibodies against Tim17 or other TIM complex components

• Blue native electrophoresis: To analyze intact TIM complexes

The creation of double-shuffle strains (e.g., YPH499 tim17∆ tim23∆ + YEp352-TIM17, YEp352-TIM23) allows for the simultaneous manipulation of both Tim17 and Tim23, facilitating the study of their interactions and functional relationships .

How should researchers interpret discrepancies in Tim17 mutant phenotypes?

When interpreting discrepancies in Tim17 mutant phenotypes, researchers should consider:

• Temperature effects: Some Tim17 mutants (e.g., Tim17 N64L) show growth defects only at elevated temperatures, suggesting conditional phenotypes

• Protein stability: Verify that mutant proteins are expressed at levels comparable to wild-type

• Complex assembly: Check whether TIM23 complexes form normally in mutant mitochondria

• Membrane potential: Ensure that the mitochondrial membrane potential is maintained, as it is essential for protein import

It's important to compare different types of precursor proteins (matrix-targeted, inner membrane-sorted, and carrier proteins) to distinguish between general and pathway-specific defects, as demonstrated with the Tim17 N64L and S114L mutants .

What are common pitfalls in studying Tim17 function and how can they be avoided?

Common pitfalls in studying Tim17 function include:

• Inadequate controls: Always include appropriate wild-type controls and verify that mutant proteins are expressed at comparable levels

• Overlooking membrane potential effects: Membrane potential disruption can cause general import defects independent of specific Tim17 functions

• Misinterpreting localization data: When using GFP-tagged proteins, confirm that the tag does not interfere with protein function or localization

• Insufficient characterization of mutants: Thoroughly characterize mutants using multiple approaches (growth assays, protein levels, complex formation, import assays)

To avoid these pitfalls, researchers should:

• Include appropriate controls in all experiments

• Verify membrane potential in isolated mitochondria

• Use complementary approaches (biochemical, microscopic, genetic) to validate findings

• Consider the effects of experimental conditions (temperature, induction time) on protein expression and function