Molecular architecture of Influenza A virions

By combining structural, compositional, and integrative modelling approaches, this study defines the complete molecular architecture of Influenza A filamentous virions, revealing distinctive features such as selective lipid incorporation and a viral cytoskeleton containing cofilactin that drives filament morphogenesis.

The Gist

Imagine the Influenza A virus not as a tiny, static ball, but as a shape-shifting, microscopic factory that can build itself into two very different forms: a compact, round bubble (like a marble) or a long, slender tube (like a piece of uncooked spaghetti).

For decades, scientists have studied the "marble" version because it's easier to catch and look at in a lab. But the "spaghetti" version is what actually causes the virus to spread effectively in real-world infections, especially in humans. The problem? These spaghetti-like viruses are incredibly fragile. Trying to study them with standard tools is like trying to examine a wet noodle by squeezing it through a colander; it just gets destroyed.

This paper is the first time scientists successfully took a high-resolution "3D selfie" of these fragile, long viruses without breaking them, revealing a hidden world of architecture and machinery inside.

Here is the story of what they found, broken down into simple analogies:

1. The Shape-Shifting Problem

Think of the virus as a construction crew. Sometimes they build a small, round house (spherical virus). Other times, they build a long, narrow apartment complex (filamentous virus).

• The Round House: This is what we usually see in labs. It's short and stout.
• The Apartment Complex: This is the long, thin version found in sick people. It's much longer (sometimes 10 times longer than the round version) and is crucial for the virus to survive and spread in the body.

The researchers realized that to understand how the virus works, they had to stop looking at the "marbles" and start studying the "spaghetti."

2. The Secret Ingredients (The Recipe)

Before looking inside, the scientists checked the "ingredients list" of both the round and long viruses. They found some surprising differences:

• The Skin (Lipids): The round viruses have a skin rich in specific fats that help them curve into a ball. The long viruses have less of these fats. It's like the long viruses are wearing a straighter, stiffer suit that prevents them from curling up, allowing them to stretch out.
• The Tools (Proteins): The long viruses are missing some of the usual tools (like the Neuraminidase protein) that help the virus escape. Instead, they seem to have swapped these for something else entirely.

3. The Hidden Skeleton (The "Spaghetti" Inside)

This is the biggest discovery. When they looked inside the long, thin viruses, they didn't just see the genetic code (the virus's instruction manual). They found a structural skeleton that the round viruses don't have.

• The Inner Tube: Inside the long virus, there is a second, inner spiral layer of protein (M1) running parallel to the outer shell. Imagine a long, thin tube inside a larger tube. This inner tube acts like a reinforcing rod, giving the long virus the strength to stay straight and not collapse.
• The "Cofilactin" Cables: The most exciting find was a bundle of twisted fibers running down the center of the virus. The scientists identified these as cofilactin.
  • What is it? It's a mix of Actin (a protein that acts like the steel rebar in concrete) and Cofilin (a protein that acts like a construction manager).
  • The Metaphor: Think of the virus as a long, inflatable tube. Without support, it would flop over. The virus hijacks the host cell's "rebar" (actin) and adds a "manager" (cofilin) to twist it into a tight, strong cable. This cable runs down the middle of the virus, acting as a spine that keeps the long shape rigid and stable.

4. The Construction Site (How it Happens)

The study also looked at how the virus builds this structure.

• The virus infects a cell and tells the cell to make more of this "construction manager" (cofilin).
• Crucially, it tells the manager to turn off its safety switch (dephosphorylation). When the switch is off, the manager becomes hyper-active, grabbing onto the steel rebar (actin) and twisting it into those strong cables.
• The virus then wraps its outer shell around these cables, creating that long, sturdy filament.

5. The Big Picture: Why Does This Matter?

For a long time, we thought the virus was just a bag of genetic code wrapped in a protein coat. This paper shows that the long version of the virus is a sophisticated, multi-layered machine.

• The "Spine": The cofilactin cables act as a spine, allowing the virus to grow long without breaking.
• The "Reinforcement": The inner spiral layer adds extra strength.
• The Strategy: By changing its shape and building this internal skeleton, the virus might be better at evading the immune system or moving through the mucus in our lungs.

The Takeaway

Imagine the Influenza virus as a master builder. When it needs to be stealthy and compact, it builds a round bubble. But when it needs to travel long distances or survive harsh conditions, it switches to a "construction mode," recruiting the host cell's own steel beams (actin) and twisting them into a super-strong spine (cofilactin) to build a long, durable tube.

This discovery changes how we see the virus. It's not just a simple blob; it's a complex, shape-shifting structure that actively remodels the cell's own machinery to build its own internal skeleton. Understanding this "spine" could help scientists design new drugs that stop the virus from building these long tubes, effectively trapping it and stopping the spread of the flu.

Technical Summary

1. Problem Statement

Influenza A viruses (IAV) are pleomorphic, producing both spherical/bacilliform virions (common in lab strains) and long filamentous virions (characteristic of clinical isolates). While the atomic structures of individual IAV proteins (HA, NA, M1, RNPs) are known, the complete molecular architecture of the intact virion, particularly the fragile filamentous forms, remains poorly defined.

• Challenges: Filamentous virions are fragile, easily destroyed by standard purification methods, and highly heterogeneous.
• Knowledge Gap: It is unclear how host and viral components are organized within filaments, what drives their elongated morphology, and whether they possess unique structural features distinct from spherical particles.

2. Methodology

The authors employed an integrative multidisciplinary approach combining structural biology, multi-omics, and computational modeling:

• Sample Preparation:
  • Used MDCK cells infected with A/Udorn/72 (H3N2).
  • Purification: Utilized iso-osmotic iodixanol density gradients to separate virions by length, enriching for "bacilliform" (short) vs. "filamentous" (long) populations without damaging them.
  • Cryo-ET: Propagated virus directly on EM grids to image budding filaments in a near-native state, preserving ultrastructure better than purified samples.
• Multi-Omics Analysis:
  • Lipidomics: Electrospray ionization mass spectrometry (ESI-MS) to compare lipid profiles between fractions.
  • Proteomics: LC-MS/MS to quantify viral and host proteins in different morphological fractions.
  • Glycomics: GC-MS analysis of carbohydrate content.
• Structural Imaging & Analysis:
  • Cryo-Electron Tomography (Cryo-ET): Collected tilt-series of budding filaments.
  • Sub-tomogram Averaging (STA): Used RELION-5 to reconstruct 3D densities of internal fibrils at high resolution (12 Å for cofilactin, 24 Å for actin).
  • Fourier Analysis: Used SPIDER to determine helical parameters (pitch, rise) of internal structures.
• Molecular Modeling:
  • AlphaFold Integration: Predicted canine actin and cofilin structures (100% identity with human) and docked them into STA maps.
  • Integrative Modeling: Built comprehensive 3D models of spherical, bacilliform, and filamentous virions using signed distance fields, M1 helical lattices, and host protein distributions.
• Cellular Validation: Confocal microscopy and Western blotting to assess cofilin expression and phosphorylation status (activity) during infection.

3. Key Results

A. Compositional Differences (Omics)

• Lipids: Filamentous virions are significantly depleted (to ~60%) in Phosphatidylethanolamine (PE) and Phosphatidylinositol (PI) compared to bacilliform virions. These lipids promote membrane curvature; their absence supports the formation of long, single-axis curved filaments.
• Proteins:
  • Viral Proteins: HA and M1 are consistent across morphologies. Neuraminidase (NA) and viral genome proteins (PB2, PB1, PA, NP) are depleted in filaments. NS1 is also depleted and correlates with genome proteins, suggesting specific packaging.
  • Host Proteins: CD9 (tetraspanin) is enriched in filamentous fractions.
  • Cofilactin: The presence of actin-binding proteins was confirmed structurally.

B. Structural Architecture (Cryo-ET & STA)

• Internal Fibrils: Filaments contain distinct internal fibrillar densities absent in spherical virions.
  • Identification: STA revealed these are cofilactin (actin filaments decorated with cofilin).
  • Parameters: The helical pitch is ~27.6 nm (shortened from bare actin's ~37 nm), consistent with cofilin binding.
  • Organization: Cofilactin fibrils often assemble into tightly packed bundles. Membrane-proximal fibrils show curvature (suggesting long-range helical organization), while central fibrils remain straight.
• Secondary Inner Helix: ~36% of filaments possess a second concentric helical layer inside the primary M1 matrix.
  • This inner helix has a larger pitch (~12 nm) and diameter than the M1 lattice.
  • It is likely a secondary M1 layer or an M1-host complex, distinct from the RNPs.
• Heterogeneity: Filaments are structurally diverse; some contain RNPs, some contain only cofilactin, and some contain both cofilactin and the secondary inner helix.

C. Regulation of Cofilin

• Expression & Activity: IAV infection in MDCK cells leads to a 1.9-fold increase in total cofilin and a significant reduction in phosphorylated cofilin (p-cofilin).
• Implication: This shift indicates an accumulation of active, dephosphorylated cofilin, which promotes actin severing and the formation of cofilactin bundles necessary for filament elongation.

D. Integrative Model

• The authors constructed the first comprehensive molecular models of IAV filaments, visualizing the "7+1" RNP arrangement, the M1 scaffold, the secondary inner helix, and bundled cofilactin fibrils within a lipid envelope depleted of curvature-promoting lipids.

4. Key Contributions

1. First Complete Architecture: Provided the first high-resolution, integrative molecular model of the filamentous IAV virion, moving beyond isolated protein structures to a whole-virion view.
2. Discovery of Cofilactin: Identified cofilactin as a structural component of IAV filaments, demonstrating that host cytoskeletal elements are actively recruited and organized into bundles to support virion elongation.
3. Lipidome Insight: Established a link between the depletion of specific curvature-promoting lipids (PE/PI) and the filamentous morphology.
4. Secondary Helix: Characterized a native, secondary inner helical layer in budding filaments, challenging previous assumptions that such structures only form under artificial conditions.
5. Mechanistic Regulation: Demonstrated that IAV actively modulates host cofilin activity (via dephosphorylation) to facilitate filament assembly.

5. Significance

• Viral Persistence & Immune Evasion: Understanding the unique architecture of filaments is crucial for explaining how IAV persists in hosts and evades immune detection, as filaments are the predominant form in clinical isolates.
• Therapeutic Targets: The study identifies specific host-virus interactions (e.g., cofilin activation, lipid composition) and structural features (secondary helix) that could serve as novel targets for antiviral drugs aimed at disrupting filament formation or budding.
• Methodological Advance: The combination of native cryo-ET (imaging on grids) with multi-omics and integrative modeling sets a new standard for characterizing fragile, pleomorphic viruses.

In summary, the paper reveals that filamentous Influenza A virions are not merely elongated spheres but possess a distinct, complex molecular architecture driven by specific lipid depletion, a secondary M1 helix, and the active recruitment and bundling of host cofilactin to drive morphogenesis.