Interactions between non-prion and prion domains of Rnq1 direct formation of amyloid vs liquid-like aggregates and create transmission barriers

This study demonstrates that the T27P mutation in the non-prion domain of the yeast protein Rnq1 creates a transmission barrier by preventing the propagation of wild-type prion conformations and promoting the formation of liquid-like aggregates, thereby revealing how interactions between non-prion and prion domains dictate amyloid versus liquid-like assembly and influence protein misfolding disease mechanisms.

The Gist

The Big Picture: Protein "Bad Habits" and the Yeast Detective

Imagine your body (or a yeast cell) is a bustling city. In this city, proteins are the workers. Most of the time, they fold into the perfect shape to do their jobs. But sometimes, a worker gets confused, folds the wrong way, and starts sticking to other confused workers. They form a giant, sticky clump called an amyloid.

In humans, these clumps cause diseases like Alzheimer's. In yeast, they cause "prions" (like a bad habit that spreads). One specific yeast protein, Rnq1, is famous for being a "prion starter." When Rnq1 gets stuck in a clump, it acts like a match that can accidentally light other proteins on fire, causing them to clump up too.

This paper is about a team of scientists who found a tiny "glitch" in the Rnq1 protein that stops it from spreading its bad habit to other versions of itself, but also changes how it clumps up.

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The Characters: The "Head" and the "Tail"

Think of the Rnq1 protein as a two-part robot:

1. The Head (Non-Prion Domain): This is the front part. Scientists didn't really know what it did, so they called it the "Non-Prion Domain" (NPD). It's like the robot's helmet or control panel.
2. The Tail (Prion Domain): This is the back part. It's sticky and full of "glue" (amino acids Q and N). This is the part that actually does the clumping. It's the "Prion Domain" (PD).

The Discovery: The scientists found a tiny mutation (a typo in the genetic code) in the Head of the robot. Specifically, one letter changed, turning an amino acid called Threonine into Proline at position 27 (let's call this the T27P glitch).

The Experiment: The "Transmission Barrier"

The scientists wanted to see what happens when a "Normal" Rnq1 robot (with a healthy head) tries to infect a "Glitchy" Rnq1 robot (with the T27P head).

The Analogy: The Dance Floor
Imagine a dance floor where the "Normal" robots are already dancing in a synchronized, rigid line (this is the amyloid fiber). When a new robot joins, it usually has to copy the dance moves exactly to join the line.

• What they expected: The Glitchy robot should be able to jump onto the dance floor and copy the dance moves, joining the line.
• What happened: The Glitchy robot could jump onto the dance floor, but it couldn't copy the dance moves perfectly. It was like trying to dance a rigid line dance while wearing a heavy, awkward helmet. It couldn't keep the rhythm.

The Result:

1. The Barrier: Because the Glitchy robot couldn't copy the dance, it couldn't "inherit" the prion state from the Normal robots. The Normal robots couldn't turn the Glitchy ones into clumps. This is called a Transmission Barrier. It's like a language barrier; the Normal robots speak "Amyloid," but the Glitchy ones can't understand the dialect.
2. The Adaptation: However, every once in a while, a Glitchy robot did manage to join the line. But at first, it was wobbly and unstable. Over time (many generations of yeast), these Glitchy robots "learned" a new, slightly different dance. They adapted and found a new, stable way to clump up that worked for them.

The Surprise: Liquid Droplets vs. Sticky Fibers

Here is the most interesting part. The scientists noticed that the Glitchy robots behaved differently when they were alone.

• Normal Rnq1: When alone, it stays dissolved in the cell soup. It only clumps if it sees a "seed" (another clump) to join.
• Glitchy Rnq1 (T27P): When alone, it spontaneously turns into liquid-like droplets.

The Analogy: Honey vs. Water

• Normal Rnq1 is like water. It flows freely. If you add a drop of oil (a seed), the water might mix with it, but it doesn't suddenly turn into oil.
• Glitchy Rnq1 is like honey. It's thick and sticky. Even without a seed, it starts to form globs. But here's the twist: these globs aren't hard, rigid rocks (amyloid fibers); they are soft, squishy, liquid droplets.

The scientists found that the "Head" of the protein (the NPD) acts like a regulator. In the Normal robot, the Head keeps the Tail from getting too sticky. In the Glitchy robot, the Head is broken, so the Tail gets too excited and turns into liquid droplets instead of waiting to form hard fibers.

Why Does This Matter?

1. Understanding Disease: This helps us understand why some protein misfolding diseases are hard to catch. Just like the Glitchy robot, a slight change in a protein's structure can create a "barrier" that stops a disease from spreading from one person (or species) to another.
2. The "Head" Matters: For a long time, scientists thought only the "sticky tail" (the Prion Domain) mattered for clumping. This paper proves that the "head" (the non-sticky part) is crucial. It controls how the protein clumps, whether it forms hard fibers or soft droplets, and whether it can spread its bad habit to others.
3. Evolution of Prions: The study shows that prions aren't static. If a barrier stops them, they can sometimes "adapt" and find a new way to survive, just like a virus mutating to bypass a vaccine.

The Takeaway

The Rnq1 protein is like a complex machine with a control panel (Head) and a sticky engine (Tail). The scientists found that breaking the control panel (the T27P mutation) doesn't just stop the engine; it changes the type of engine it becomes. It stops the engine from copying the old, rigid dance moves (creating a barrier), forces it to invent a new, softer dance (liquid droplets), and eventually, it might learn a new, stable dance routine (conformational adaptation).

This teaches us that in the world of proteins, the "boring" parts (the heads) are just as important as the "sticky" parts (the tails) in deciding whether a protein becomes a harmless worker or a dangerous, clumping disease.

Technical Summary

1. Problem Statement

The study addresses the mechanisms governing protein misfolding, prion transmission barriers, and the structural determinants of prion variants (strains). Specifically, it investigates the yeast prion [PIN+], formed by the Rnq1 protein. Rnq1 consists of a C-terminal Prion Domain (PD) rich in QN (Glutamine-Asparagine) repeats, which is sufficient for prion formation, and an N-terminal Non-Prion Domain (NPD) of unknown function.

While prion transmission barriers are typically attributed to sequence differences within the PD, this study explores how mutations in the NPD (outside the amyloid-forming region) can influence the conformational flexibility of the PD, create transmission barriers, and alter the physical state of aggregates (amyloid vs. liquid-like droplets). Understanding these mechanisms is crucial for elucidating how prion strains arise and how barriers to interspecies or inter-strain transmission are overcome, with implications for human neurodegenerative diseases.

2. Methodology

The authors employed a combination of genetic screens, molecular biology, biophysics, and microscopy:

• Genetic Screen: A random mutagenesis screen was conducted on the RNQ1 gene in a [PIN+] yeast strain. The screen sought mutants that lost the ability to transmit the [PIN+] state to a wild-type (WT) recipient or failed to facilitate the de novo induction of the [PSI+] prion (a readout for [PIN+] activity).
• Plasmid Shuffle Assays: To test transmission barriers, the authors used a plasmid shuffle system where a chromosomal rnq1Δ strain maintained [PIN+] via a URA3-marked RNQ1WT plasmid. This was replaced by a LEU2-marked mutant RNQ1 plasmid. The retention of the [PIN+] state (Pin+ phenotype) was scored by the ability to induce [PSI+] upon overexpression of Sup35NM-YFP.
• Protein Overexpression & Tagging: Rnq1 variants (WT and T27P mutant) were expressed with fluorescent tags (YFP, CFP) to visualize aggregation dynamics, co-localization, and the formation of liquid-like droplets.
• In Vitro Amyloid Assays: Purified bacterial recombinant Rnq1 fragments (WT and T27P) were tested for amyloid formation using Thioflavin T (ThT) fluorescence kinetics and Transmission Electron Microscopy (TEM).
• Liquid-Like Droplet Analysis: The sensitivity of Rnq1 aggregates to 1,6-hexanediol (Hex) and ThT staining was used to distinguish between amyloid fibers and liquid-liquid phase separation (LLPS) droplets.
• Deletion Analysis: Various Rnq1 fragments lacking specific QN-rich regions (QN1, QN2, QN3, QN4) were used as donors or recipients to map the interaction between the NPD mutation and specific PD regions.

3. Key Contributions

• Identification of the T27P Mutation: The study identified a specific missense mutation, T27P, in the N-terminal Non-Prion Domain (NPD) of Rnq1. This mutation creates a potent transmission barrier, preventing the propagation of the [PIN+] state from WT Rnq1 to the mutant Rnq1T27P.
• NPD Regulation of PD Conformation: The paper demonstrates that the NPD is not merely a passive linker but actively regulates the conformational flexibility of the PD. The T27P mutation restricts the PD to specific conformations, preventing it from adopting the specific amyloid fold required to be seeded by WT [PIN+].
• Dual Aggregation Pathways: The study reveals that the T27P mutation shifts the aggregation propensity of Rnq1. While WT Rnq1 forms amyloid fibers, Rnq1T27P readily forms liquid-like droplets (LLPS) that are ThT-negative and Hex-sensitive, even at physiological expression levels.
• Conformational Adaptation: The research provides a detailed model of how rare transmission events across a barrier lead to unstable prion variants that undergo conformational adaptation over mitotic generations to become stable, heritable strains.

4. Key Results

• Transmission Barrier: When RNQ1WT was shuffled out and replaced by RNQ1T27P in a [PIN+] strain, >98% of cells lost the [PIN+] state (became Pin-). Only ~1.5–2% of cells retained the prion state, indicating a severe transmission barrier.
• Mechanism of the Barrier:
  • Co-localization: Rnq1T27P efficiently joins pre-existing [PIN+WT] amyloid aggregates (co-localizes with Rnq1WT-CFP), ruling out a failure to seed.
  • Propagation Defect: The barrier arises because the initial [PIN+T27P] variants formed are mitotically unstable. They fail to propagate efficiently until they undergo conformational adaptation.
  • Adaptation: Over ~60 cell divisions, unstable [PIN+T27P] variants evolve into stable strains with distinct mitotic stabilities (some highly stable, some moderately stable), mimicking the "strain" selection seen in mammalian prion transmission.
• Structural Determinants:
  • The transmission barrier is mediated through the first two QN-rich regions (QN1 and QN2) of the PD.
  • Deleting the B1C2 region (encompassing QN1 and QN2) from the recipient Rnq1T27P eliminates the barrier when the donor is also a B1C2-deleted fragment.
  • However, deleting B1C2 from the recipient does not eliminate the barrier if the donor is full-length WT, suggesting the NPD mutation constrains the conformational space of these specific regions.
• In Vitro Differences: TEM analysis showed that while both WT and T27P form amyloid fibers, T27P fibers are wider (30–50 nm vs. ~20 nm) and less twisted, indicating a distinct amyloid core structure.
• Liquid-Like Droplets: In [pin-] cells, Rnq1T27P-YFP spontaneously forms dot-like aggregates that are ThT-negative and dissolve upon 1,6-hexanediol treatment, characteristic of liquid-like droplets. WT Rnq1-YFP does not form these droplets. This suggests the NPD mutation alters the aggregation pathway from amyloid to LLPS.
• Prion-Prion Interactions: The newly formed stable [PIN+T27P] variants can still induce [PSI+], but the spectrum of induced [PSI+] variants differs slightly (a shift toward weaker variants) compared to WT.

5. Significance

• Role of Non-Prion Domains: The study challenges the view that prion transmission barriers are solely determined by the sequence of the Prion Domain. It establishes that the NPD acts as a critical regulator of the PD's conformational landscape, effectively acting as a "gatekeeper" for specific prion strains.
• Mechanism of Strain Selection: The paper provides a live-cell model for how transmission barriers are crossed. It shows that rare transmission events result in unstable "proto-strains" that must undergo conformational adaptation (evolution) to become stable, heritable prion variants. This mirrors the adaptation of mammalian prions (e.g., vCJD) when crossing species barriers.
• Amyloid vs. Liquid Droplets: The findings highlight the plasticity of Rnq1 aggregation. A single mutation in the NPD can switch the protein's behavior from forming toxic amyloid fibers to forming liquid-like droplets. This has broad implications for understanding the pathogenesis of human protein misfolding diseases, where the balance between amyloid and liquid phases is increasingly recognized as critical.
• Regulation of Aggregation: The authors propose that the cellular function of the Rnq1 NPD is to regulate the aggregation properties of the PD, potentially controlling the aggregation of other heterologous proteins. This suggests a physiological role for Rnq1 in managing protein homeostasis, rather than just being a "selfish" prion element.

In summary, this paper elucidates how a mutation outside the amyloid core can dictate the structural outcome of a prion, creating transmission barriers and altering the physical state of protein aggregates, thereby offering new insights into the origins and evolution of protein misfolding diseases.