Recombinant Saccharomyces cerevisiae Autophagy-related protein 32 (ATG32)

What is Atg32 and what is its fundamental role in yeast cells?

Atg32 is an outer mitochondrial membrane protein in Saccharomyces cerevisiae that functions as the primary receptor for mitophagy. It confers selectivity for mitochondrial sequestration as cargo and is necessary for recruitment of mitochondria by the autophagy machinery . The protein acts as a molecular tag that identifies mitochondria for degradation under specific conditions such as nutrient starvation or during quality control of damaged mitochondria . Cells lacking Atg32 (atg32Δ mutants) are specifically deficient in mitophagy while maintaining normal function in other selective autophagy pathways and non-specific macroautophagy .

How is Atg32 structurally organized?

Atg32 contains multiple functional domains that are critical for its role in mitophagy:

• An N-terminal domain that extends into the cytosol

• A transmembrane domain that anchors the protein to the outer mitochondrial membrane

• A C-terminal domain that extends into the intermembrane space

• A pseudo-receiver (PsR) domain (residues 1-199) that is essential for mitophagy initiation

The solution structure of the PsR domain was determined by NMR spectroscopy, revealing a previously undescribed structural element that regulates Atg32 activation . Additionally, the region between amino acids 200-341 appears dispensable for mitophagy function, as Atg32Δ200-341 still localizes to mitochondria and is expressed at comparable levels to full-length Atg32 .

What are the key protein interactions mediated by Atg32 during mitophagy?

Atg32 serves as a crucial adaptor that connects mitochondria to the core autophagy machinery through two primary interactions:

• Atg32-Atg11 interaction: Upon mitophagy induction, Atg32 is phosphorylated at Ser114 and Ser119 by casein kinase 2 (CK2), which enables binding to Atg11 . This interaction recruits mitochondria to the phagophore assembly site (PAS) .

• Atg32-Atg8 interaction: Atg32 also interacts with Atg8, which is anchored on the isolation membrane. This interaction facilitates the formation of the autophagosome surrounding the mitochondria .

These protein-protein interactions have been verified through yeast two-hybrid assays and co-immunoprecipitation experiments, confirming their physiological relevance .

How is Atg32 expression regulated at the transcriptional level?

Atg32 expression is subject to complex transcriptional regulation:

Analysis of ATG32 promoter activity using a pPROM-ATG32-β-galactosidase reporter construct has shown that ATG32 transcription increases as cells progress from exponential to stationary phase . Interestingly, while transcription increases, the actual protein levels decrease, suggesting post-transcriptional regulation plays a significant role in controlling Atg32 abundance .

What post-translational modifications regulate Atg32 function?

Atg32 activity is regulated by multiple post-translational modifications:

• Phosphorylation:

  • Under normal conditions, Ppg1 with the Far complex dephosphorylates Atg32 to prevent unwanted mitophagy

  • Upon mitophagy induction, CK2 phosphorylates Atg32 at Ser114 and Ser119, facilitating binding to Atg11

• Ubiquitination:

  • Atg32 is ubiquitinated at Lysine 282, as confirmed by LC-MS/MS analysis

  • This ubiquitination targets Atg32 for proteasomal degradation

  • Inhibition of proteasome activity with MG-132 prevents Atg32 degradation, stabilizing the protein during stationary phase, nitrogen starvation, and rapamycin treatment

These modifications create a sophisticated regulatory network that finely tunes Atg32 levels and activity in response to cellular conditions.

What is the significance of the pseudo-receiver (PsR) domain in Atg32?

The pseudo-receiver (PsR) domain (residues 1-199) of Atg32 plays a critical role in mitophagy:

• It is essential for the proteolysis of the C-terminal domain of Atg32 and subsequent recruitment of Atg11

• The solution structure determined by NMR spectroscopy revealed a previously undescribed structural element

• Deletion experiments have shown that the PsR domain is required for nitrogen starvation-induced mitophagy

Researchers have determined that even when the large portion of the cytosolic domain (residues 200-341) is removed, Atg32 still localizes to mitochondria and is expressed at levels comparable to the full-length protein . This suggests that the PsR domain contains the key functional elements required for mitophagy induction.

What methods can be used to monitor Atg32-mediated mitophagy in yeast?

Several experimental approaches can be employed to study mitophagy in yeast:

• Om45-GFP processing assay:

  • Om45 is a mitochondrial outer membrane protein fused to GFP

  • During mitophagy, mitochondria are delivered to the vacuole, and GFP is cleaved and remains relatively stable

  • The appearance of free GFP can be monitored by Western blotting as an indicator of mitophagy

• GFP-Atg8 processing assay:

  • Similar to Om45-GFP, but monitors general autophagy rather than specifically mitophagy

  • Used as a control to distinguish between general autophagy defects and mitophagy-specific defects

• Microscopy-based assays:

  • Fluorescence microscopy to track mitochondrial markers and their colocalization with vacuolar markers

  • Can be used to visualize the recruitment of Atg32 to specific mitochondrial sites during mitophagy induction

• Biochemical fractionation:

  • Isolation of mitochondria and measurement of mitochondrial protein degradation

  • Particularly useful for quantitative assessment of mitophagy rates

How can researchers create and validate Atg32 mutants for functional studies?

Creating and validating Atg32 mutants involves several crucial steps:

• Design of mutations:

  • Target specific domains (e.g., PsR domain)

  • Target sites of post-translational modifications (e.g., phosphorylation sites Ser114/Ser119, ubiquitination site Lys282)

  • Create truncation mutants to study domain functions (e.g., Atg32Δ200-341)

• Expression verification:

  • Western blotting to confirm expression levels comparable to wild-type Atg32

  • Cellular fractionation to verify proper mitochondrial localization

• Functional validation:

  • Mitophagy assays using Om45-GFP processing

  • Growth assays under conditions that require mitophagy

  • Protein-protein interaction studies (e.g., yeast two-hybrid or co-immunoprecipitation) to verify interactions with Atg11 and Atg8

For example, researchers have validated that Atg32Δ200-341 still localizes to mitochondria despite the loss of a large portion of the cytosolic domain, demonstrating that this region is not essential for targeting to the mitochondrial outer membrane .

What techniques are used to analyze Atg32 ubiquitination?

Several approaches can be used to study Atg32 ubiquitination:

• Proteasome inhibition assays:

  • Treatment with MG-132 (proteasome inhibitor) prevents Atg32 degradation

  • Western blotting reveals higher molecular weight bands corresponding to ubiquitinated forms of Atg32

• Protein purification and mass spectrometry:

  • Expression of tagged Atg32 (e.g., Atg32-V5-6xHIS) for affinity purification

  • Ni-NTA column purification followed by SDS-PAGE and Western blotting with anti-histidine and anti-ubiquitin antibodies

  • LC-MS/MS analysis of tryptic digests to identify ubiquitinated peptides

• Mutational analysis:

  • Mutation of ubiquitination sites (e.g., K282R) to assess functional consequences

  • Comparison of wild-type and mutant Atg32 stability and mitophagy efficiency

Using these techniques, researchers have identified Lysine 282 as a specific ubiquitination site in Atg32, as evidenced by the detection of the characteristic diglycine (GG) tag remaining after tryptic proteolysis of ubiquitinated proteins .

How do researchers distinguish between the roles of Atg32 in mitophagy versus other potential mitochondrial functions?

Distinguishing between different roles of Atg32 requires careful experimental design:

• Use of specific autophagy mutants:

  • Compare phenotypes in atg32Δ with mutants defective in core autophagy (e.g., atg1Δ, atg5Δ, atg8Δ)

  • Compare with mutants defective in selective autophagy adaptors (e.g., atg11Δ)

• Domain-specific mutations:

  • Create mutants affecting specific functions (e.g., phosphorylation sites vs. ubiquitination sites)

  • Assess different cellular responses and mitochondrial phenotypes

• Temporal analysis:

  • Monitor Atg32 levels and modifications at different time points

  • Correlate with mitochondrial function, morphology, and degradation rates

Researchers have observed that Atg32 levels decrease in the stationary phase even in autophagy-defective mutants (atg5Δ, atg8Δ, atg11Δ), suggesting that Atg32 might be involved in yet unknown pathways related to mitochondrial functions beyond its role in mitophagy .

What are the technical challenges in studying Atg32 protein stability and regulation?

Several technical challenges must be addressed when studying Atg32:

• Protein modification complexity:

  • Atg32 undergoes multiple modifications (phosphorylation, ubiquitination)

  • These modifications may be interdependent and occur under specific conditions

• Protein aggregation issues:

  • Purified Atg32 has been observed to migrate at higher molecular weights than expected (~170 kDa instead of 58.9 kDa)

  • This may represent oligomerization or complex formation with other proteins

• Conditional expression:

  • Atg32 levels change dramatically under different growth conditions

  • Careful timing of experiments is critical for reproducible results

• Dual degradation pathways:

  • Atg32 appears to be degraded by both the proteasome and the vacuole

  • Inhibition of both pathways may be necessary to fully stabilize the protein

How does the regulation of Atg32 compare between different stress conditions?

Atg32 regulation varies under different stress conditions:

Understanding these condition-specific regulatory mechanisms is crucial for designing experiments that accurately capture the physiological roles of Atg32 in mitochondrial quality control and cellular adaptation to stress.

What is the relationship between Atg32 ubiquitination and phosphorylation?

The interplay between different post-translational modifications of Atg32 represents an important frontier in research:

• Sequential modification hypothesis:

  • Phosphorylation at Ser114/Ser119 may precede or influence ubiquitination at Lys282

  • These modifications may work together to regulate Atg32 activity and stability

• Experimental approaches:

  • Creation of phospho-mimetic and phospho-deficient mutants to study effects on ubiquitination

  • Analysis of ubiquitination patterns in CK2 mutants with reduced phosphorylation capability

  • Time-course experiments to determine the sequence of modifications

• Functional significance:

  • Phosphorylation appears primarily involved in activation and protein-protein interactions

  • Ubiquitination appears primarily involved in protein turnover and potentially inhibitory regulation

Further investigation of how these modifications interact could reveal sophisticated regulatory mechanisms that fine-tune mitophagy in response to different cellular conditions.

How might researchers develop tools to temporally control Atg32 activity?

Development of tools for temporal control of Atg32 activity would advance the field:

• Inducible expression systems:

  • Galactose-inducible promoters for controlled expression

  • Tetracycline-responsive systems for finer temporal control

• Optogenetic approaches:

  • Light-inducible dimerization systems to control Atg32-Atg11 interactions

  • Photocleavable inhibitory domains to rapidly activate Atg32

• Chemical genetics:

  • Engineering Atg32 to respond to small molecule inducers

  • Development of rapid degradation systems (e.g., auxin-inducible degrons) to control Atg32 levels

• Monitoring tools:

  • Development of FRET-based sensors to monitor Atg32 conformational changes or interactions

  • Split fluorescent protein systems to visualize Atg32-Atg11 interactions in real-time

These approaches would enable researchers to dissect the kinetics and threshold effects in mitophagy initiation and progression.

What methods can be used to map the complete interactome of Atg32 during different stages of mitophagy?

Comprehensive mapping of the Atg32 interactome requires multiple complementary approaches:

• Proximity labeling methods:

  • BioID or APEX2 fusions to Atg32 to label proximal proteins

  • Time-course experiments to capture dynamic interaction changes

• Quantitative proteomics:

  • SILAC or TMT labeling to compare interactors under different conditions

  • Cross-linking mass spectrometry to capture transient interactions

• Genetic screens:

  • Synthetic genetic array analysis to identify functional interactions

  • CRISPR screens to identify genes affecting Atg32-mediated mitophagy

• Structural biology approaches:

  • Cryo-EM studies of Atg32-containing complexes

  • Hydrogen-deuterium exchange mass spectrometry to map interaction interfaces

These approaches would help identify additional components of the mitophagy machinery and potentially reveal new functions of Atg32 beyond its established role in mitophagy.