Recombinant Kluyveromyces lactis Mitochondrial import inner membrane translocase subunit TIM50 (TIM50)

What are the functional domains of TIM50 and how do they coordinate protein translocation?

TIM50 contains two primary functional domains that have distinct but complementary roles in mitochondrial protein import:

• Core Domain: This domain serves as the main recruitment point to the TIM23 complex and contains the primary presequence-binding site. Research using a split TIM50 approach demonstrates that the core domain effectively crosslinks to incoming precursor proteins in organello, confirming its role in substrate recognition .

• Presequence Binding Domain (PBD): This domain works in conjunction with the core domain to facilitate protein translocation.

The coordination between these domains is critical for TIM50 function. Experimental evidence shows that when both domains are expressed separately but simultaneously in a cell (50split cells), TIM50 remains functional despite the disjoined domains. Crosslinking studies have identified that the TIM50 core domain contains the main binding site that interacts with incoming precursor proteins .

The interdomain flexibility is also crucial - studies with linker mutants joining these domains demonstrate that the native linkage provides optimal function, while both rigid and overly flexible artificial linkers compromise TIM50's ability to support cell growth .

What methods are used to purify and assess the quality of recombinant TIM50?

For purification and quality assessment of recombinant K. lactis TIM50, researchers typically employ the following methodology:

Purification Protocol:

• Expression in E. coli with an N-terminal His-tag

• Cell lysis under native conditions

• Nickel-NTA affinity chromatography for initial purification

• Further purification by size exclusion chromatography if needed

Quality Assessment Methods:

• Purity Analysis: SDS-PAGE with verification of >90% purity using densitometry analysis

• Western Blot Confirmation: Using anti-TIM50 or anti-His antibodies to confirm identity

• Mass Spectrometry: Peptide mass fingerprinting to verify protein sequence integrity

• Functional Assessment: Crosslinking assays with mitochondrial presequence peptides to verify binding activity

When analyzing fresh preparations, researchers should be aware that TIM50 can appear as multiple bands around 30-32 kDa on Western blots due to different glycosylation states in yeast expression systems . For long-term storage, avoid repeated freeze-thaw cycles, as this significantly reduces activity; instead, store working aliquots at 4°C for up to one week .

How does TIM50 deficiency affect mitochondrial function and cellular physiology?

TIM50 deficiency produces several significant mitochondrial and cellular phenotypes that can be assessed using the following methodological approaches:

Mitochondrial Morphology and Dynamics Assessment:

• TIMM50-deficient neuronal cells show approximately twofold decrease in mobile mitochondria compared to control cells

• Mobile mitochondria travel less distance and at lower average speeds

• This can be quantified using live-cell imaging with fluorescently labeled mitochondria

Respiratory Chain Activity:

• Activities of complexes I, II, and IV are significantly decreased in TIM50-deficient models

• This can be measured using spectrophotometric assays of respiratory complex activities

ATP Production:

• Cellular ATP levels show approximately 25% reduction in TIMM50-deficient cells

• This can be quantified using luciferase-based ATP detection assays

Oxidative Stress Markers:

• Increased ROS generation as measured by dihydroethidium staining

• Decreased activities of antioxidant enzymes (SOD and catalase)

• These changes contribute to activation of stress pathways

Signaling Pathway Activation:

• Increased phosphorylation of ASK1, JNK and P38 MAPK pathways

• This can be assessed through Western blotting using phospho-specific antibodies

Importantly, TIM50 deficiency does not appear to directly affect mitochondrial dynamics-related proteins (Drp-1, Mfn1, Mfn2, or Nrf2), suggesting that the observed effects on mitochondrial mobility are indirect and likely result from ATP deficiency .

What role does TIM50 play in cardiac pathology and how can it be experimentally investigated?

TIM50 functions as a novel protective regulator against cardiac hypertrophy, with its expression significantly downregulated in both human dilated cardiomyopathy (DCM) hearts and experimentally-induced hypertrophic murine hearts. Researchers can investigate this role using the following experimental approaches:

Expression Analysis in Disease Models:

• Western blot analysis of TIM50 shows marked decrease in human DCM hearts compared to donor hearts

• Similar decreases observed in murine hearts after aortic banding (AB)

• Cultured neonatal rat cardiomyocytes (NRCMs) treated with angiotensin II (Ang II) also show decreased TIM50 expression

Loss-of-Function Studies:

• TIM50 knockout (TIM50 KO) mice show enhanced cardiac hypertrophy after AB

• Increased heart weight/body weight (HW/BW), lung weight/BW (LW/BW), and HW/tibial length (HW/TL) ratios

• Increased cardiomyocyte cross-sectional area

• Impaired cardiac function measured by echocardiography

• Enhanced cardiac fibrosis as demonstrated by picrosirius red staining

Gain-of-Function Studies:

• Cardiac-specific TIM50 transgenic mice (TG mice) show protection against AB-induced hypertrophy

• Decreased HW/BW, LW/BW, and HW/TL ratios compared to control mice

• Improved cardiac function as assessed by echocardiography

• Reduced cardiac fibrosis

Molecular Mechanism Investigation:

• TIM50 regulates oxidative stress via antioxidant enzyme activities (SOD and catalase)

• Mediates apoptosis as measured by cleaved-caspase-3 expression

• Affects ASK1-JNK/P38 signaling pathways, which can be blocked by antioxidant (NAC) treatment

The experimental data is presented in the following table comparing cardiological parameters between wild-type (WT) and TIM50 KO mice:

Note: AB indicates aortic banding; HW, heart weight; BW, body weight; LW, lung weight; EF, ejection fraction; FS, fractional shortening

How can researchers design crosslinking experiments to study TIM50 interactions with precursor proteins?

To study TIM50 interactions with precursor proteins, researchers can employ several crosslinking strategies:

Site-Specific Photocrosslinking Approach:

• Generate TIM50 variants with site-specific incorporation of photo-reactive amino acids (e.g., benzoyl-phenylalanine, BPA) at position 415

• Express these variants in yeast cells using amber suppression technology

• Isolate mitochondria containing the modified TIM50

• Incubate with radiolabeled precursor proteins

• Activate crosslinking by UV irradiation

• Analyze crosslinked products by SDS-PAGE and autoradiography

Purification and Identification of Crosslinked Adducts:

• Scale up crosslinking reactions for preparative purposes

• Solubilize mitochondria using 1% Triton X-100

• Purify TIM50 and its crosslinking adducts by Ni-NTA affinity chromatography

• Separate specifically bound material by SDS-PAGE

• Excise bands of interest for peptide mass fingerprinting using mass spectrometry

Controls and Validation:

• Perform parallel experiments without UV irradiation

• Use TIM50 without photo-reactive amino acids as a negative control

• Include competition experiments with excess non-labeled precursor

• Validate interactions using complementary approaches such as co-immunoprecipitation

This approach has revealed that TIM50 forms dimers and that the core domain of TIM50 directly interacts with incoming precursor proteins. Importantly, when using this technique, researchers should be aware that crosslinking can capture both stable and transient interactions, necessitating careful interpretation of results .

What are the structural properties of TIM50 and how can they be computationally predicted or experimentally determined?

TIM50's structural properties can be investigated through both computational prediction and experimental techniques:

Computational Structure Prediction:

• AlphaFold and similar AI-based tools have been used to predict TIM50 structures

• SWISS-MODEL Repository provides models based on templates like 4qqf.3.A (monomer, QMEAN: 0.78) and 8q84.1.K (monomer, QMEAN: 0.61)

• These models cover different regions of the protein with varying degrees of confidence

Domain Structure Analysis:

• Core domain: Primarily responsible for integration into the TIM23 complex

• Presequence binding domain (PBD): Contains regions for recognizing mitochondrial targeting sequences

• Transmembrane domain: Anchors TIM50 to the inner mitochondrial membrane

• Interdomain linker: Critical for functional flexibility between domains

Experimental Structure Determination:

• X-ray crystallography has been used for partial structures

• Crosslinking coupled with mass spectrometry for determining proximities between domains

• Limited proteolysis to identify domain boundaries

• Circular dichroism spectroscopy to assess secondary structure content

How does the expression of recombinant TIM50 differ between systems, and what modifications optimize its functional expression?

The expression of recombinant TIM50 varies significantly between different expression systems, requiring specific optimization strategies:

E. coli Expression System:

• Full-length mature K. lactis TIM50 (aa 40-480) can be successfully expressed in E. coli

• Addition of N-terminal His-tag facilitates purification

• Optimal expression achieved using BL21(DE3) strains with IPTG induction

• Expression in E. coli yields non-glycosylated protein with >90% purity

• Yields approximately 5-10 mg protein per liter of culture

Yeast Expression Systems:

• Expression in K. lactis yields glycosylated forms of TIM50

• Western blot analysis reveals two TIM50-specific bands (approximately 30 kDa and 32 kDa) due to different glycosylation states

• For K. lactis expression, integration into the LAC4 promoter region via homologous recombination is effective

• Multicopy integration can be verified by genomic PCR using integration primers

• The pKLAC1 vector system allows for efficient expression

Optimization Strategies:

• Codon optimization based on the expression host improves yields

• Using only the mature form of the protein (removing the mitochondrial targeting sequence) enhances solubility

• Expression at lower temperatures (16-20°C) increases the proportion of properly folded protein

• Addition of 6% Trehalose in storage buffer enhances stability

• Avoiding repeated freeze-thaw cycles is critical for maintaining activity

When comparing expression systems, researchers should consider that while E. coli provides higher yields of non-glycosylated protein suitable for structural studies, yeast expression systems may produce protein with post-translational modifications more closely resembling the native state, which might be preferable for functional studies.

How can TIM50 be used as a model to study the effects of protein mutations on mitochondrial import pathways?

TIM50 provides an excellent model for studying the effects of mutations on mitochondrial import pathways through several experimental approaches:

Patient-Derived Cell Models:

• Patient fibroblasts carrying the TIM50 mutation c.446C>T; p.Thr149Met show mitochondrial import defects

• Primary fibroblast cells can be generated from 4mm punch biopsies using standard procedures

• Genomic DNA purification and sequencing confirm the presence of mutations

• These cells show specific defects in the mitochondrial import pathway that can be characterized

Knockdown and Rescue Experiments:

• Generate TIM50 knockdown cells using shRNA or CRISPR-Cas9

• Complement with wild-type or mutant versions of TIM50

• Assess import efficiency of reporter proteins with mitochondrial targeting sequences

• Measure effects on mitochondrial membrane potential and protein steady-state levels

• This approach demonstrates that even low levels of TIM50 suffice to maintain most of the mitochondrial matrix and inner membrane proteome

Proteomics Approaches:

• Use quantitative proteomics to assess the impact of TIM50 deficiency on:

  • Mitochondrial proteome composition

  • Assembly of respiratory chain complexes

  • Expression of non-mitochondrial proteins

• Mass spectrometry reveals that reduction in TIM50 levels leads to decreased levels of many OXPHOS and MRP complex subunits

• Interestingly, the steady-state levels of certain potassium channels (KCNA2 and KCNJ10) are also affected, linking TIM50 mutations to neurological phenotypes

Mitochondrial Function Assessments:

• Measure ATP production in TIM50-deficient vs. control cells

• Track mitochondrial mobility in neuronal cells using live imaging

• Assess mitochondrial membrane potential using potentiometric dyes

• These measurements have shown approximately 25% reduction in cellular ATP levels and a twofold decrease in mitochondrial mobility in TIM50-deficient neuronal cells

This multi-faceted approach allows researchers to establish direct links between specific TIM50 mutations and both biochemical and physiological phenotypes, providing insights into disease mechanisms.

What techniques can be used to study the regulation of TIM50 by post-translational modifications?

Studying post-translational modifications (PTMs) of TIM50 requires specialized techniques for detection, quantification, and functional characterization:

Detection and Mapping of PTMs:

• Mass Spectrometry Approaches:

  • Tryptic digestion followed by LC-MS/MS analysis

  • Phospho-enrichment techniques (TiO2, IMAC) for phosphorylation sites

  • Glycopeptide enrichment (hydrazide chemistry, lectin affinity) for glycosylation mapping

  • Targeted multiple reaction monitoring (MRM) for quantifying specific modifications

• Site-Specific Antibodies:

  • Generate antibodies against known modification sites

  • Use for Western blotting and immunoprecipitation experiments

  • Apply for immunohistochemistry to assess tissue distribution of modified TIM50

Functional Characterization:

• Site-Directed Mutagenesis:

  • Generate phosphomimetic (S/T→D/E) or phospho-dead (S/T→A) mutations

  • Create K→R mutations to prevent ubiquitination

  • Express these mutants and assess their:

    • Integration into the TIM23 complex

    • Ability to bind precursor proteins

    • Effects on mitochondrial protein import

• Pharmacological Modulation:

  • Use kinase inhibitors to block phosphorylation events

  • Apply deubiquitinating enzyme inhibitors to stabilize ubiquitinated forms

  • Treat with glycosylation inhibitors to assess effects on TIM50 function

Regulation by Heme:
Research on the yeast K. lactis has demonstrated that heme can exert feedback control on mitochondrial import of certain proteins. While not directly proven for TIM50, similar regulatory mechanisms might exist:

• Look for heme regulatory motifs (HRMs) in TIM50 sequence

• Perform mutagenesis of potential HRMs

• Assess protein import efficiency under varying heme concentrations

• This approach has been successful in studying the regulation of 5-aminolaevulinate acid synthase import in K. lactis

The study of PTMs provides critical insights into how TIM50 function is dynamically regulated in response to cellular conditions and metabolic states.