Recombinant Saccharomyces cerevisiae [PSI+] induction protein 2 (PIN2)

What is the relationship between PIN2 and [PIN+] in S. cerevisiae?

Methodologically, this distinction can be verified through:

• Genetic knockouts of PIN2 while monitoring [PIN+] status

• Expression of PIN2 variants lacking prion-inducing domains

• Biochemical fractionation to separate PIN2 and Rnq1 aggregates

How does [PIN+] facilitate [PSI+] induction in S. cerevisiae?

[PIN+] enhances the de novo induction of [PSI+] through a mechanism known as heterologous prion cross-seeding. Different [PIN+] variants promote the appearance of different [PSI+] variants to dramatically different extents . The molecular basis for this interaction involves the ability of [PIN+] aggregates to serve as templates or nucleation sites for the initial misfolding of Sup35 protein into the [PSI+] prion form.

To experimentally study this relationship:

• Express PIN2/Rnq1 and Sup35 with different fluorescent tags

• Use confocal microscopy to monitor colocalization during [PSI+] induction

• Employ protein cross-linking assays to capture transient interactions

• Design genetic screens where [PSI+] induction is the readout for [PIN+] activity

What phenotypic assays are available to detect [PSI+] in yeast cells?

[PSI+] detection relies on its ability to cause translational read-through of nonsense mutations. The standard experimental approach utilizes the ade1-14 (UGA) nonsense mutation system:

• Colorimetric assay: [psi-] cells accumulate a red pigment when grown on adenine-limited media, while [PSI+] cells appear white/pink due to nonsense suppression

• Growth assay: [PSI+] cells can grow on media lacking adenine due to nonsense suppression

• Fluorescence microscopy: Using Sup35 fused to fluorescent proteins (YFP/GFP) to visualize aggregation patterns; [psi-] cells show diffuse fluorescence while [PSI+] cells display punctate patterns

• Biochemical detection: Through SDS-resistant aggregates in semi-denaturing detergent-agarose gel electrophoresis (SDD-AGE)

For robust experimental design, researchers should combine at least two different detection methods to confirm [PSI+] status.

How do different [PIN+] variants influence [PSI+] induction efficiency and variant specificity?

Different [PIN+] variants (high, medium, low, and very high) exhibit distinct efficiencies in [PSI+] induction. Research has demonstrated that:

• High [PIN+] forms rings and induces [PSI+] with the highest frequency while containing the least soluble Rnq1

• Low [PIN+] forms rings and induces [PSI+] with the lowest efficiency and contains more soluble Rnq1

• Very high [PIN+] exhibits the highest efficiency of ring formation and [PSI+] induction despite containing the most soluble Rnq1

This variant-specific behavior creates a complex relationship between Rnq1 aggregation state and [PSI+] induction efficiency.

Methodological approach for variant characterization:

Researchers should consider using separation-of-function mutants to dissect the mechanisms underlying variant-specific cross-seeding.

What roles do chaperone proteins play in maintaining and regulating [PIN+] variants?

Chaperone proteins critically influence [PIN+] variant selection, maintenance, and phenotypic expression. Deletion of specific chaperone genes alters established [PIN+] variants in S. cerevisiae, affecting [PSI+] induction efficiency, Rnq1 aggregate morphology/size, and variant dominance .

Experimental evidence shows:

• Deletion of HSC82, CPR6, CPR7, or TAH1 in [PIN+] low strains increases [PSI+] induction efficiency to levels similar to wild-type [PIN+] high strains

• Deletion of HSC82, AHA1, CPR6, CPR7, or SSE1 in [PIN+] medium strains similarly increases [PSI+] induction

• Deletion of SBA1 in [PIN+] medium and high backgrounds decreases [PSI+] induction efficiency to levels matching wild-type [PIN+] low strains

These phenotypic shifts are inherited in a non-Mendelian manner, confirming they represent bona fide changes in [PIN+] variants rather than simple loss of function effects.

To experimentally manipulate chaperone-prion interactions:

• Use temperature-sensitive chaperone mutants for temporal control

• Apply small molecule inhibitors of specific chaperones

• Design protein-fragment complementation assays to detect direct interactions

• Employ cross-linking mass spectrometry to identify binding interfaces

How can we design separation-of-function mutations to study distinct PIN2/[PIN+] activities?

Designing separation-of-function mutations requires structure-based approaches similar to those used for other prion proteins, such as Srs2 . For PIN2/[PIN+] research, the following methodological framework can be applied:

• Structure analysis: Identify functional domains through bioinformatics and structural prediction

• Targeted mutagenesis: Generate mutations that selectively disrupt specific functions:

  • Prion domain mutations that affect aggregation but not normal protein function

  • Mutations in interaction interfaces that disrupt cross-seeding with [PSI+] but maintain [PIN+] propagation

  • Mutations affecting chaperone interactions while preserving prion formation

• Phenotypic validation: Test mutants for:

  • [PIN+] formation capacity

  • [PSI+] induction efficiency

  • Protein trafficking functions (for PIN2)

  • Interactions with other cellular components

For example, the T27P mutation in Rnq1 dramatically reduces the Pin+ phenotype but does not affect de novo [PIN+] prion formation, maintenance, or facilitation of [PSI+] formation, suggesting it might disrupt a specific aspect of [PIN+]-[PSI+] interaction .

What approaches can be used to study the interaction between [PIN+] and other prions like [SWI+]?

Research has revealed that [PIN+] interacts with the [SWI+] prion to create novel heritable traits in S. cerevisiae. While [PIN+] does not cause nonsense suppression by itself, it strongly enhances the effect of [SWI+] . This interaction causes inactivation of the SUP45 gene, leading to nonsense suppression.

Methodological approaches to study prion-prion interactions:

• Proteomic screening for prions: Apply methods to identify protein determinants of multi-prion interactions

• Genetic analysis:

  • Create strains carrying different combinations of prions

  • Use prion curing agents to selectively eliminate specific prions

  • Apply synthetic genetic array analysis to identify genetic modifiers of prion interactions

• Biochemical characterization:

  • Co-immunoprecipitation of prion proteins

  • Amyloid co-aggregation assays in vitro

  • Native gel electrophoresis to detect prion complexes

• Functional readouts:

  • Nonsense suppression assays

  • Growth phenotypes on selective media

  • Changes in gene expression profiles

When designing such experiments, researchers should control for potential confounding effects like competition for cellular machinery or indirect effects mediated by other proteins.

How can photoconvertible fluorescent proteins be used to study PIN2 dynamics?

Photoconvertible fluorescent proteins like Dendra2 provide powerful tools for studying protein turnover in living cells. While initially developed for PIN2 auxin transporters in plants , these approaches can be adapted for yeast PIN2 studies:

• Construct design: Create PIN2-Dendra2 fusion by inserting Dendra2 into a loop region that doesn't disrupt protein function

• Validation: Confirm proper localization and functionality through:

  • Complementation of pin2 mutant phenotypes

  • Immunohistochemistry with Dendra2-specific antibodies

  • Colocalization with known trafficking markers

• Experimental protocol:

  • Photoconvert a subset of PIN2-Dendra2 from green to red fluorescence

  • Track both populations simultaneously using time-lapse imaging

  • Quantify signal intensities to measure:

    • Delivery rate of newly synthesized protein (green signal)

    • Removal rate of existing protein (red signal)

    • Half-life and turnover dynamics

• Applications for PIN2 in yeast:

  • Measure effects of prion states on PIN2 trafficking

  • Analyze PIN2 dynamics during stress responses

  • Assess impact of mutations on protein stability and localization

This approach provides quantitative data on protein dynamics that cannot be obtained through conventional methods.

What glycoproteomic approaches can identify post-translational modifications of PIN2?

PIN2 and other yeast proteins undergo various post-translational modifications, including glycosylation, which can be studied using advanced glycoproteomic approaches:

• Protein microarray screening with lectins:

  • Purify PIN2 along with other yeast proteins

  • Create protein microarrays

  • Probe with fluorescently-labeled lectins like Concanavalin A (ConA, for mannose) and Wheat-Germ Agglutinin (WGA, for GlcNAc)

  • Analyze binding patterns to identify glycosylated proteins

• Validation of glycosylation:

  • Mobility shift assays with glycosidases (EndoH, PNGase F)

  • Mass spectrometry to identify glycosylation sites

  • Site-directed mutagenesis of predicted glycosylation sites

• Functional analysis:

  • Assess impact of glycosylation on PIN2 localization

  • Test effects on prion-inducing properties

  • Evaluate interactions with trafficking machinery

This methodological approach has successfully identified 534 glycoproteins in yeast, many associated with cellular compartments like the ER, vacuole, and cell wall . Similar approaches could reveal important modifications of PIN2 that influence its function.

What genomic approaches can identify strain-specific variations in PIN2 and their effects on [PSI+] induction?

Different S. cerevisiae strains exhibit variations in PIN2 sequence and [PSI+] induction properties. Researchers can employ genomic approaches to characterize these strain-specific differences:

• Whole genome sequencing and comparative analysis:

  • Sequence PIN2 loci across diverse strain collections

  • Compare with reference strains like S288C

  • Identify natural variants and polymorphisms

  • Correlate with [PSI+] induction phenotypes

• Strain replacement experiments:

  • Replace PIN2 alleles between strains using CRISPR/Cas9

  • Measure effects on [PSI+] induction and propagation

  • Identify strain backgrounds that modify PIN2 function

• Transcriptomic analysis:

  • Compare PIN2 expression levels across strains

  • Identify co-expressed genes that might influence PIN2 function

  • Analyze expression changes upon [PSI+] induction

The Peterhof genetic collection of S. cerevisiae strains represents a valuable resource for such studies, containing strains extensively used for translation termination and prion research .

How can PIN2/[PIN+] research contribute to understanding protein-based inheritance mechanisms in higher organisms?

Research on PIN2/[PIN+] in S. cerevisiae provides valuable insights into protein-based inheritance mechanisms that may be applicable to higher organisms:

• Evolutionary conservation of prion-like domains:

  • Compare PIN2/Rnq1 prion domains with those in other organisms

  • Identify conserved sequence features that promote prion formation

  • Test mammalian prion proteins in yeast to assess cross-species prion mechanisms

• Translational research applications:

  • Use yeast as a screening system for anti-prion drug candidates

  • Study mammalian prion protein (PrP) expression in yeast

  • Map epitopes of anti-prion antibodies using yeast expression systems

• Methodological approach for cross-species analysis:

  • Express mammalian proteins with prion-like domains in yeast

  • Create chimeric proteins combining yeast and mammalian prion domains

  • Use PIN2/[PIN+] as a reporter system for prion-like behavior of heterologous proteins

The finding that yeast prions share physical mechanisms underlying amyloid formation with mammalian prions suggests conservation of these processes across evolution . This makes S. cerevisiae an valuable model system for studying fundamental aspects of protein-based inheritance.

What are the current challenges in establishing a standardized system for measuring [PIN+]-mediated [PSI+] induction?

Despite extensive research, standardizing measurements of [PIN+]-mediated [PSI+] induction remains challenging. Researchers face several methodological issues:

• Variability in prion variants:

  • Different [PIN+] variants show distinct [PSI+] induction efficiencies

  • Environmental conditions affect variant selection and stability

  • Laboratory strains may harbor uncharacterized prion states

• Detection sensitivity limitations:

  • Current assays vary in sensitivity and specificity

  • [PSI+] induction rates can be low, requiring large sample sizes

  • False positives from spontaneous [PSI+] formation

• Standardization framework proposal:

• Implementation strategy:

  • Establish repository of reference strains and plasmids

  • Develop standardized protocols with defined metrics

  • Create community standards for reporting experimental conditions

  • Implement quality control measures for reproducibility

Addressing these challenges will improve reproducibility across laboratories and accelerate progress in understanding the complex interactions between PIN2/[PIN+] and [PSI+].

How does PIN2/[PIN+] research contribute to understanding stress responses and adaptation in yeast?

PIN2/[PIN+] prion dynamics are increasingly recognized as potential mechanisms for stress adaptation in yeast:

• Role in thermotolerance:

  • Similar to Hsp70 orthologs like Ssa2, prions may contribute to stress response

  • PIN2/[PIN+] might modulate protein homeostasis during heat stress

  • Experimental approach: Compare survival rates of [pin-] and [PIN+] strains under thermal stress

• Environmental adaptation:

  • [PIN+] and [PSI+] can create phenotypic diversity through nonsense suppression

  • This diversity might provide selective advantages under changing conditions

  • Methodology: Evolution experiments under fluctuating environments to test adaptive potential

• Stress granule interactions:

  • PIN2/Rnq1 may participate in stress granule formation during cellular stress

  • These interactions could regulate translation and protein synthesis

  • Analytical approach: Proximity labeling to identify stress-dependent interactors

• Human microbiome relevance:

  • S. cerevisiae has been detected in human microbiota

  • Prion states might influence colonization and persistence

  • Research direction: Compare prion frequencies in clinical versus environmental isolates

Understanding these connections requires integrating prion biology with stress response pathways, opening new avenues for investigating how protein-based inheritance contributes to cellular adaptation.