Recombinant Saccharomyces cerevisiae Inheritance of peroxisomes protein 2 (INP2)

What is INP2 and what is its primary function in Saccharomyces cerevisiae?

INP2 (Inheritance of peroxisomes protein 2) is a peroxisomal membrane protein in Saccharomyces cerevisiae that functions as the peroxisome-specific receptor for the class V myosin motor, Myo2p. It plays a critical role in peroxisome inheritance by linking peroxisomes to the cellular translocation machinery that drives organelle movement to the developing bud during cell division. INP2 performs an antagonistic function to INP1, which is responsible for retaining peroxisomes in mother cells .

To study INP2 function, researchers commonly use fluorescent protein tagging (such as INP2-GFP fusion proteins) combined with live-cell imaging to visualize the dynamics of peroxisome movement during yeast cell division. Molecular techniques like genomic integration of fluorescent markers are essential for visualizing the temporal and spatial regulation of INP2 during the cell cycle .

How is peroxisome inheritance mediated by INP2 in budding yeast?

Peroxisome inheritance in budding yeast occurs through an active, directional transport process that requires INP2 to serve as an adaptor between peroxisomes and the Myo2p motor protein. The process follows these steps:

• INP2 is expressed in a cell cycle-dependent manner and accumulates on a subset of peroxisomes

• INP2 physically interacts with the cargo-binding domain of Myo2p

• This interaction enables peroxisomes to be transported along actin cables toward the developing bud

• The process is regulated through phosphorylation of INP2, which is coupled to cell cycle progression

Experimental evidence shows that in cells lacking INP2, peroxisomes remain in the mother cell and are not transported to the bud, demonstrating INP2's essential role in this process . Live-cell imaging using genomically encoded fluorescent chimeras such as INP2-GFP and peroxisomal markers like mRFP-SKL has been invaluable in elucidating these mechanisms .

What methodologies are most effective for studying INP2 localization and dynamics?

Several complementary experimental approaches have proven effective for studying INP2:

Researchers typically use integrative vectors to efficiently incorporate multiple fluorescent markers, enabling simultaneous imaging of peroxisomes, other cellular compartments, and the cytoskeleton . For example, studies have used INP2-GFP in combination with peroxisomal marker mRFP-SKL to track the correlation between INP2 levels and peroxisome inheritance in real-time .

How does the interaction between INP2 and Myo2p facilitate peroxisome transport?

The INP2-Myo2p interaction involves specific binding between INP2 on the peroxisomal membrane and the cargo-binding domain in the C-terminus of the Myo2p motor protein. This interaction has been characterized through several experimental approaches:

• Binding studies have identified the specific regions of Myo2p required for peroxisome binding

• Mutations in the cargo-binding domain of Myo2p (e.g., myo2-Y1483A) specifically impair peroxisome inheritance without affecting other functions

• The interaction is dynamic and regulated by the cell cycle and peroxisome positioning

Research has shown that point mutations in Myo2p that specifically disrupt peroxisome inheritance do not affect the transport of other organelles, allowing researchers to dissect the molecular requirements for peroxisome-specific transport . When the INP2-Myo2p interaction is disrupted, peroxisomes fail to be transported to the bud, and INP2 aberrantly accumulates on peroxisomes in the mother cell, indicating a feedback mechanism that monitors peroxisome inheritance .

What phenotypes are observed in inp2 deletion mutants?

Deletion of the INP2 gene (inp2Δ) results in several observable phenotypes that highlight its critical role in peroxisome inheritance:

• Complete abolishment of peroxisome inheritance to daughter cells

• Retention of all peroxisomes in mother cells during cell division

• No effect on the inheritance of other organelles (e.g., vacuoles)

• Moderate growth defects on media requiring functional peroxisomes

Studies comparing wild-type and inp2Δ strains have shown that while inp2Δ single mutants show no significant growth defects on glucose media, they exhibit increased doubling times on carbon sources that require peroxisomal metabolism, such as methanol . Interestingly, double mutants lacking both peroxisome fission (pex11Δ) and inheritance (inp2Δ) machinery show more severe growth defects, suggesting that peroxisome fission and inheritance together ensure peroxisome maintenance in wild-type cells .

How is INP2 function conserved across different yeast species?

INP2 function has been found to be conserved across multiple yeast species, despite relatively low sequence conservation. Research findings include:

• Weakly conserved INP2-related proteins have been identified in 18 different species of budding yeast

• The methylotrophic yeast Hansenula polymorpha contains an INP2 homolog that performs the same function as S. cerevisiae INP2

• H. polymorpha INP2 localizes to peroxisomes, interacts with Myo2p, and is essential for peroxisome inheritance

• This conservation indicates the evolutionary importance of the peroxisome inheritance mechanism

To identify INP2 homologs, researchers have employed in silico analyses to search for proteins with similar functional domains across yeast genomes. The functional conservation has been experimentally validated through localization studies, protein-protein interaction assays, and analysis of peroxisome inheritance in different yeast species . These findings suggest that despite sequence divergence, the fundamental mechanism of myosin-dependent peroxisome inheritance is preserved across evolutionarily diverse yeast species.

What mechanisms regulate INP2 expression and degradation during the cell cycle?

INP2 is regulated through multiple mechanisms that coordinate its levels with the cell cycle and peroxisome inheritance status:

• Transcriptional regulation: INP2 mRNA levels oscillate with the cell cycle

• Post-translational regulation: INP2 is a phosphoprotein whose phosphorylation state changes during cell cycle progression

• Feedback regulation: Peroxisome inheritance status affects INP2 protein levels on individual peroxisomes

• Degradation mechanisms: Proteolytic machinery likely degrades INP2 in response to successful peroxisome inheritance

Research has demonstrated that while total INP2 levels oscillate with the cell cycle, the accumulation of INP2 on specific peroxisomes is controlled by peroxisome inheritance status. This was elegantly shown using myo2 mutants specifically impaired in peroxisome binding, where INP2 aberrantly accumulates on peroxisomes in mother cells despite normal cell cycle progression . This suggests the existence of a regulatory feedback mechanism that monitors peroxisome positioning and regulates INP2 levels accordingly, ensuring proper organelle distribution between mother and daughter cells.

How does the interplay between peroxisome fission and inheritance affect cellular function?

The coordination between peroxisome fission (mediated by proteins like Pex11) and inheritance (mediated by INP2) is crucial for maintaining peroxisome function across generations:

• In wild-type cells, peroxisome fission creates new peroxisomes that can be distributed via the INP2-Myo2p machinery

• When both mechanisms are disrupted (e.g., in pex11 inp2 double mutants), cells show enhanced growth defects on media requiring peroxisomal metabolism

• De novo peroxisome biogenesis serves as a rescue mechanism when inheritance fails, but this process is relatively slow

• The interplay ensures that both mother and daughter cells maintain functional peroxisomes

Studies of pex11 inp2 double mutant strains have shown that the combined defects in peroxisome fission and inheritance result in increased doubling times on methanol media compared to either single mutant . Live cell imaging experiments revealed that in these double mutants, no peroxisomes were inherited to daughter cells, which must then rely on de novo peroxisome formation - a slower and less efficient process. These findings indicate that peroxisome fission and inheritance are the primary mechanisms for peroxisome maintenance in wild-type cells, while de novo biogenesis serves as a backup system .

What experimental approaches can resolve conflicting data about INP2 function and regulation?

Researchers have employed several sophisticated approaches to address complex questions about INP2 function:

• Conditional mutants: Using temperature-sensitive or chemically-inducible mutants to temporally control INP2 function

• Cell-cycle synchronization: Analyzing INP2 dynamics in synchronized cell populations to dissect cell-cycle related events

• Specific point mutations: Creating targeted mutations in Myo2p that specifically disrupt peroxisome binding without affecting other cargo

• Multi-color live cell imaging: Simultaneously tracking multiple organelles and proteins to dissect temporal relationships

• Quantitative phosphoproteomics: Identifying specific phosphorylation events that regulate INP2 function

A particularly powerful approach has been the use of myo2 mutants (e.g., myo2-Y1483A) that specifically disrupt peroxisome inheritance without affecting cell cycle progression . These mutants enabled researchers to artificially dissociate peroxisome partitioning from cell cycle progression, revealing that INP2 levels on peroxisomes are controlled by peroxisome inheritance status rather than simply by cell cycle stage. Similarly, the use of integrative vectors for multi-color imaging has enabled researchers to simultaneously track peroxisomes, the vacuole, and the nucleus during cell division, demonstrating that peroxisome inheritance occurs in parallel with vacuole inheritance but before nuclear migration .

How do peroxisome-organelle interactions influence INP2 function and peroxisome inheritance?

Peroxisomes interact with multiple cellular compartments through membrane contact sites and functional interplay, which influences peroxisome positioning and inheritance:

• Peroxisome-ER contacts: Peroxisomes form membrane contact sites with the endoplasmic reticulum, which may influence peroxisome growth and lipid transfer

• Peroxisome-mitochondria relationships: Functional interactions related to metabolism and ROS homeostasis

• Interactions with the cytoskeleton: Beyond INP2-Myo2p, other proteins may link peroxisomes to cytoskeletal elements

• Coordination with other inheritance pathways: Peroxisome inheritance occurs in parallel with some organelles (e.g., vacuoles) but not others

Recent research has highlighted the importance of membrane contact sites between peroxisomes and other organelles for various cellular functions . In yeast, peroxisomes are associated with the actin/myosin cytoskeleton through proteins like INP1 and INP2, while in mammals, peroxisomes associate with microtubules through different mechanisms . These species-specific differences in cytoskeletal associations may reflect evolutionary adaptations to different cellular architectures and division strategies.

What are the implications of INP2 research for understanding human peroxisomal disorders?

While INP2 itself appears to be specific to fungi, research on peroxisome inheritance in yeast has broader implications for understanding human peroxisomal disorders:

• Fundamental mechanisms of peroxisome biogenesis and maintenance are conserved from yeast to humans

• Insights into peroxisome quality control and inheritance may help explain clinical variability in peroxisomal disorders

• Yeast models enable high-throughput screening approaches to identify compounds affecting peroxisome function

• Understanding peroxisome-organelle contact sites may reveal new therapeutic targets

Peroxisomes are essential organelles with roles in lipid metabolism, ROS detoxification, and signaling across eukaryotes . In humans, peroxisomal dysfunction leads to a spectrum of disorders with neurological, developmental, and metabolic manifestations . The study of peroxisome inheritance in yeast provides valuable insights into how cells maintain peroxisome populations and quality control, which may inform our understanding of disease mechanisms and potential therapeutic approaches for human peroxisomal disorders.

What genetic tools are most effective for studying INP2 in recombinant S. cerevisiae?

Researchers employ various genetic approaches to study INP2 function in recombinant S. cerevisiae:

• Genomic integration of fluorescent tags: C-terminal tagging of INP2 with GFP or other fluorescent proteins enables visualization of its dynamics

• Promoter replacement: Substituting the endogenous INP2 promoter with regulatable promoters allows controlled expression

• Point mutation introduction: Site-directed mutagenesis of specific INP2 domains helps identify functional regions

• Creation of deletion strains: Complete deletion of INP2 reveals its necessity for peroxisome inheritance

• Multi-color marker integration: Simultaneous tagging of peroxisomes, other organelles, and cytoskeletal elements

A particularly useful approach is the pDK vector series, which allows efficient integration of up to 8 markers, enabling simultaneous visualization of various cellular compartments . These vectors contain integrative modules flanked by split markers and can be used with constitutive, inducible, or daughter-specific promoters. The system enables researchers to precisely track peroxisomes in relation to other organelles during cell division, revealing that peroxisome inheritance happens in parallel with vacuole inheritance in the first 15 minutes of cell division .

How can researchers quantitatively assess peroxisome inheritance defects?

Quantitative assessment of peroxisome inheritance involves several complementary approaches:

For example, researchers have used live cell imaging to analyze multiple budding events (e.g., forty budding events captured in four separate movies) to quantitatively determine that in pex11 inp2 double mutant cells, no peroxisomes moved to the bud, confirming a complete block in peroxisome inheritance . Similarly, growth curve analysis on methanol media has been used to demonstrate that pex11 inp2 double mutants have enhanced doubling times compared to wild-type or single mutant strains, reflecting the functional consequences of defective peroxisome inheritance .

What are the experimental considerations for studying INP2 phosphorylation?

Studying INP2 phosphorylation requires specialized techniques to detect and characterize these post-translational modifications:

• Phospho-specific antibodies: Development of antibodies that specifically recognize phosphorylated forms of INP2

• Phosphatase treatments: Treating samples with phosphatases to confirm phosphorylation status

• Mass spectrometry: Identifying specific phosphorylation sites on INP2

• Phosphomimetic mutations: Creating mutations that mimic permanent phosphorylation (e.g., Ser/Thr to Asp/Glu)

• Phospho-null mutations: Creating mutations that prevent phosphorylation (e.g., Ser/Thr to Ala)

• Cell cycle synchronization: Analyzing phosphorylation status at different cell cycle stages

Research has shown that INP2 is a phosphoprotein whose level of phosphorylation is coupled to the cell cycle irrespective of peroxisome positioning in the cell . This phosphorylation likely plays a role in regulating INP2 function and stability during the cell cycle. Experimental approaches to study these phosphorylation events must carefully consider the dynamic nature of these modifications and their temporal relationship to cell cycle progression and peroxisome inheritance events.

What are the emerging areas of investigation regarding INP2 and peroxisome inheritance?

Several promising research directions are emerging in the field of peroxisome inheritance:

• Structural biology of INP2-Myo2p interactions: Determining the atomic structure of this complex

• Systems biology approaches: Comprehensive mapping of the peroxisome inheritance interactome

• Single-molecule tracking: Following individual peroxisomes and INP2 molecules in real-time

• Computational modeling: Predicting inheritance patterns based on molecular interactions

• Integration with cell cycle regulation: Understanding how peroxisome inheritance is coordinated with other cell cycle events

As new technologies emerge, researchers will be able to address increasingly sophisticated questions about the molecular mechanisms of peroxisome inheritance and the role of INP2 in this process. Advanced imaging techniques, such as super-resolution microscopy, may enable visualization of INP2-Myo2p interactions at unprecedented resolution, while CRISPR-based approaches could facilitate more precise genetic manipulations to dissect INP2 function.

How might INP2 research contribute to biotechnological applications?

Understanding INP2 and peroxisome inheritance has potential applications in biotechnology:

• Improved yeast cell factories: Engineering peroxisome inheritance to enhance metabolic processes

• Protein expression systems: Using peroxisome targeting for production of recombinant proteins

• Biofuel production: Optimizing peroxisomal β-oxidation pathways for biodiesel production

• Vaccine development: Utilizing yeast-based systems for antigen presentation