Myristoylation licenses disordered viral VP4 protein to anchor to and perforate the membrane through phase separation

This study reveals that myristoylation enables the intrinsically disordered viral VP4 protein to anchor to, undergo liquid-liquid phase separation on, and mechanically perforate host membranes through the formation of dynamic condensates, thereby establishing a novel paradigm for how a single lipid modification orchestrates membrane breach during viral entry.

The Gist

The Big Picture: A Viral "Trojan Horse" with a Magic Key

Imagine a non-enveloped virus (like Coxsackievirus B3) as a tiny, hard-shelled delivery truck trying to get into a fortified castle (your cell). The truck has a secret compartment containing a small, shape-shifting protein called VP4.

To get inside, this truck needs to punch a hole in the castle wall. But here's the problem: The VP4 protein is naturally floppy and messy (scientists call it "intrinsically disordered"). It's like a wet noodle. A wet noodle can't punch a hole in a brick wall on its own.

However, this noodle comes with a special attachment: a myristoyl group. Think of this as a greasy, sticky key attached to the end of the noodle.

This paper discovers exactly how that "greasy key" turns a useless wet noodle into a powerful hole-puncher. It turns out the key does three amazing things:

---

1. The "Velcro" Effect: Getting to the Wall

The Problem: The floppy VP4 noodle floats around in the cell's fluid. It doesn't know where the wall is, and even if it bumps into the wall, it bounces off because it's too slippery and loose.

The Solution: The myristoyl "greasy key" acts like Velcro.

• Without the key: The noodle bounces off the wall.
• With the key: The greasy end sticks firmly into the oily surface of the cell membrane.
• The Catch: The noodle must be floppy to work. If you stiffened the noodle (made it rigid), it couldn't twist and turn enough to let the greasy key dig deep into the wall. The flexibility is actually required for the Velcro to stick!

2. The "Crowd Surfing" Effect: Forming a Super-Group

The Problem: One single noodle stuck to the wall isn't strong enough to break the wall. It's like one person trying to push down a heavy door; they just slip.

The Solution: The greasy key doesn't just stick the noodle to the wall; it makes the noodles stick to each other.

• Once the first noodle sticks, its greasy key attracts other floating noodles.
• They all clump together on the surface of the wall, forming a liquid droplet or a "condensate."
• The Analogy: Imagine a group of people on a trampoline. If one person jumps, the trampoline barely moves. But if 20 people jump together in a tight, bouncy cluster, they create a massive wave.
• In the virus's case, this "clump" of proteins is so heavy and active that it warps the cell membrane, bending it and making it thin and weak, just like a trampoline stretching under a heavy load.

3. The "Transformation" Effect: Turning into a Drill Bit

The Problem: Even with the wall bent, the floppy noodles still can't punch through. They need to become rigid to pierce the membrane.

The Solution: The moment the clump pushes the membrane down, the floppy noodles undergo a magical transformation.

• They snap from being "wet noodles" into stiff, helical drills (like a corkscrew).
• The greasy key helps hold these drills in place, locking them into a six-sided ring (a hexagon).
• The Result: This ring of six stiff drills creates a permanent hole (a pore) in the membrane. The virus can now dump its genetic cargo into the cell.

---

Why Does This Matter? (The "Why Should I Care?" Part)

You might wonder, "Why do some viruses need this greasy key and others don't?"

• The "Rigid" Viruses: Some viruses have VP4 proteins that are already stiff and structured (like a pre-made drill bit). They don't need the greasy key to help them fold or stick; they can just punch through on their own.
• The "Floppy" Viruses (like CVB3): These viruses have messy, floppy proteins. They desperately need the greasy key. The key acts as a license that allows the messy protein to organize itself, stick to the wall, gather a crowd, and finally transform into a drill.

The Takeaway:
This research solves a long-standing mystery in virology. It shows that the greasy key isn't just a simple "glue." It is a multi-tool that:

1. Anchors the protein to the wall.
2. Gathers the proteins into a powerful crowd.
3. Stabilizes the final hole-punching structure.

By understanding this "crowd-surfing" mechanism, scientists might be able to design new drugs that jam the greasy key or stop the proteins from clumping together, effectively disarming the virus before it can break into your cells.

Technical Summary

1. Problem Statement

Non-enveloped viruses, such as Coxsackievirus B3 (CVB3), must breach the host cell membrane to deliver their genome. This process is mediated by the VP4 protein, a small, intrinsically disordered protein (IDP) released from the viral capsid.

• The Paradox: While VP4 is essential for infectivity across many picornaviruses, its dependence on N-terminal myristoylation (a lipid modification) varies. In some viruses (e.g., CVB3, EV71), myristoylation is strictly required; in others (e.g., HAV, AiV), it is dispensable.
• The Knowledge Gap: The precise biophysical mechanism by which a lipid anchor enables a disordered protein to efficiently breach a membrane remains unresolved. Specifically, it is unclear how myristoylation compensates for the lack of inherent structure and hydrophobicity in disordered VP4 proteins to facilitate membrane insertion and pore formation.

2. Methodology

The study employs a multi-scale approach integrating computational modeling with experimental validation, using CVB3 VP4 as the model system.

Computational Approaches:

• All-Atom Molecular Dynamics (AA-MD): Used to characterize the conformational ensemble of VP4 (with and without myristoylation) in solution, confirming its disordered nature and analyzing flexibility.
• Coarse-Grained Molecular Dynamics (CG-MD): Utilized the Martini 3.0 force field to simulate:
  • VP4-membrane interactions and anchoring mechanisms.
  • The formation of biomolecular condensates (phase separation) on the membrane surface.
  • Membrane remodeling and curvature generation.
  • Umbrella Sampling: Calculated the Potential of Mean Force (PMF) to determine the energy barrier for a single VP4 protein translocating from a condensate into the membrane.
• Well-Tempered Metadynamics (WT-MetaD): Investigated the folding free energy of the N-terminal 20 residues of VP4 to determine if they transition from a coil to an $\alpha$-helix upon membrane insertion.
• 2D Umbrella Sampling: Mapped the correlation between helical content and membrane embedding depth.
• Pore Stability Simulations: Constructed a six-fold symmetric pore model using VP4 N-terminal helices to assess the stability of myristoylated vs. non-myristoylated pores.

Experimental Approaches:

• Circular Dichroism (CD): Verified the intrinsically disordered nature of purified CVB3 VP4 in solution.
• Live-Cell Imaging (Confocal Microscopy): Visualized the localization of EGFP-tagged wild-type (WT) VP4 and a non-myristoylated G2A mutant in HEK-293T cells.
• Phase Separation Assays:
  • Time-lapse Imaging: Observed the fusion and dynamics of VP4 puncta.
  • FRAP (Fluorescence Recovery After Photobleaching): Confirmed the fluidity of VP4 condensates.
  • 1,6-Hexanediol Treatment: Used to disrupt weak hydrophobic interactions and test the stability of condensates.

3. Key Contributions & Results

A. VP4 is Intrinsically Disordered and Flexible

• CD spectra and AA-MD simulations confirmed that CVB3 VP4 is an IDP with a random coil conformation.
• Myristoylation induces a moderate compaction of the protein ensemble but does not induce a stable folded structure. High backbone flexibility (RMSF) is retained in both modified and unmodified forms.

B. Myristoylation Enables Anchoring via Flexibility

• Mechanism: Myristoylation acts as a hydrophobic anchor, inserting $\sim$10 Å into the lipid bilayer.
• Role of Flexibility: The study demonstrated that high conformational flexibility is a prerequisite for anchoring. Artificially rigidifying VP4 (via elastic restraints) abolished membrane binding. The flexible protein allows the myristoyl group to find the optimal insertion angle and allows the C-terminal hydrophobic residues to assist in membrane contact.

C. Myristoylation Drives Liquid-Liquid Phase Separation (LLPS)

• Observation: In cells, myristoylated VP4 forms dynamic, fluid condensates (puncta) on the membrane, whereas the non-myristoylated mutant remains diffuse in the cytoplasm.
• Simulation: CG-MD revealed that myristoylated VP4 monomers aggregate on the membrane surface. The myristoyl groups form a hydrophobic core, while the flexible protein chains wrap around it.
• Driver: The myristoyl group serves a dual role: anchoring to the membrane and nucleating the condensate by driving the formation of a hydrophobic core. Non-myristoylated VP4 fails to form stable condensates.

D. Condensates Remodel Membranes to Lower Energy Barriers

• Membrane Curvature: Large condensates (e.g., 20-mers) induce significant membrane curvature (both positive and negative), unlike monomers or small 6-mers which leave the membrane flat.
• Energy Barrier Reduction: PMF calculations showed that translocating a VP4 protein from a condensate into the membrane is energetically favorable compared to a monomer.
  • Monomer barrier: $\sim$48.3 kcal/mol.
  • 20-mer condensate barrier: $\sim$18.4 kcal/mol.
• Mechanism: The pre-remodeled, curved membrane beneath the condensate provides an optimal environment for VP4 insertion, effectively "priming" the membrane for penetration.

E. Myristoylation Stabilizes the Transmembrane Pore

• Disorder-to-Helix Transition: Metadynamics simulations showed that the N-terminal 20 residues of VP4 fold into an $\alpha$-helix upon entering the membrane. Myristoylation enhances this folding propensity and allows for deeper membrane embedding.
• Pore Stability: A simulated six-fold symmetric pore formed by VP4 helices remained stable only when myristoylated. The non-myristoylated pore fluctuated significantly and collapsed due to lipid occlusion. Myristoylation stabilizes the supramolecular assembly, ensuring a functional water channel.

4. Significance

This study proposes a novel paradigm for how disordered viral proteins breach membranes:

1. Unified Mechanism: It explains the conditional requirement for myristoylation. Viruses with disordered VP4s require the lipid anchor not just for attachment, but to drive phase separation.
2. Collective Behavior: The transition from individual protein action to collective condensate formation is the key to overcoming the high energy barrier of membrane penetration.
3. Therapeutic Implications: By identifying myristoylation as a critical driver of condensate formation and pore stabilization, the study highlights the myristoylated N-terminus as a potential drug target. Disrupting this phase separation or the subsequent pore stabilization could inhibit viral entry.
4. Biophysical Insight: The work bridges the gap between intrinsic disorder, lipid modifications, and phase separation, demonstrating how a single modification can orchestrate a multi-step mechanical process (anchoring $\to$ condensation $\to$ remodeling $\to$ pore formation).

In summary, the paper establishes that myristoylation "licenses" the disordered VP4 protein to function by enabling it to form dynamic condensates that mechanically remodel the host membrane, thereby facilitating the formation of a stable, functional transmembrane pore.