Cryo-EM structures and structure-guided design of Atlas virus capsids

This study identifies the Atlas virus family in nematodes, characterizes their unique structural and functional properties via cryo-EM, and demonstrates their potential as engineered, scalable vehicles for tissue-specific RNA delivery.

The Gist

Imagine your body is a vast library, and inside the books of your DNA, there are thousands of ancient, dusty pages written by viruses that infected our ancestors millions of years ago. For a long time, scientists thought these viral leftovers were just junk—parasitic stains on the genetic page. But recently, we've discovered that some of these ancient viruses have been repurposed by nature to do helpful jobs, like helping form the placenta or even helping our brains form memories.

Now, a team of scientists has found a brand-new family of these "ancient viral leftovers" living inside tiny worms called nematodes. They've named them Atlas Viruses. Think of them as the "Swiss Army Knives" of the viral world: they have a weird mix of parts from different viruses, and they have a superpower that could revolutionize how we deliver medicine to our cells.

Here is the story of the Atlas Virus, broken down into simple concepts:

1. The Genetic Mosaic: A "Frankenstein" Virus

Most viruses are like standard cars; they have a specific engine and a specific body. The Atlas virus is different. It's like a car built with a Ford engine, a Ferrari body, and a Toyota transmission.

• The Engine: It has a genome that looks like a retrovirus (the family that includes HIV).
• The Body: But instead of the usual viral "coat," it uses a glycoprotein (a surface protein) borrowed from a completely different type of virus called a phlebovirus.
• The Result: This unique mix creates a virus that doesn't fit into any existing category. It's a genetic chimera that nature accidentally built inside worms.

2. The Capsid: The "Pop-Up Tent"

The most exciting part of this discovery is the capsid—the shell that holds the virus's genetic material.

• The Shape: When the scientists looked at these shells under a super-powerful microscope (Cryo-EM), they saw they were perfect icosahedrons (20-sided shapes), like soccer balls.
• The Size: They come in different sizes, ranging from tiny (20 nanometers) to quite large (60 nanometers).
• The Analogy: Imagine a pop-up tent. Most viral tents are thick and heavy. The Atlas tent is surprisingly thin and lightweight, made of a single layer of protein sheets. It's so flexible that the same protein can build a small tent, a medium tent, or a giant tent depending on the conditions. This flexibility is rare and very useful.

3. The Delivery Truck: Picking Up and Dropping Off

The scientists wanted to know: Can we use these shells to deliver medicine?

• Picking Up Cargo: When they made these shells in a lab, they didn't need a special "loader" to put cargo inside. The shells spontaneously grabbed onto any RNA or DNA floating nearby. It's like a magnet that just picks up metal shavings as it forms. They can hold huge loads—big enough to carry the massive instructions needed for CRISPR gene editing.
• The Delivery Route: When they put these shells into human cells, the cells didn't reject them. Instead, the cells swallowed them (a process called endocytosis), thinking they were food.
• The Drop-Off: Here is the magic trick. These shells are designed to fall apart when they get into the acidic "stomach" of the cell (the endosome). Once the shell dissolves, it releases its cargo right where it needs to be. It's like a time-release capsule that only opens when it hits a specific temperature or pH.

4. Why This is a Game-Changer

Currently, we have trouble delivering gene therapies (like CRISPR) to specific parts of the body.

• The Problem: The current delivery trucks (like Lipid Nanoparticles) mostly only go to the liver. The viral trucks we use (like AAVs) are too small to carry big cargo, or they might accidentally insert themselves into the wrong part of your DNA.
• The Atlas Solution:
  • Big Enough: These shells are huge compared to current options, so they can carry the massive CRISPR tools.
  • Safe: Because they come from worms, our immune systems have never seen them before. They won't trigger an allergic reaction or be blocked by antibodies.
  • Customizable: The scientists showed they can "plug and play" with these shells. They took the N-terminus (the tip of the shell) and swapped it out for a "GPS" (a nanobody) that specifically targets T-cells. This means in the future, we could paint these shells to go only to cancer cells or only to heart cells.

The Bottom Line

The scientists have discovered a new family of viral shells that are:

1. Self-assembling (they build themselves in a test tube).
2. Huge (they can carry big genetic payloads).
3. Smart (they enter cells and release their cargo at the right time).
4. Tunable (we can change their surface to target specific tissues).

Think of the Atlas virus not as a disease, but as a pre-made, empty delivery truck that nature built for worms, which we can now hijack to deliver life-saving gene therapies to humans. It's a new, safer, and more versatile way to fix the broken parts of our DNA.

Technical Summary

1. Problem Statement

Current gene and cell therapies face significant bottlenecks regarding delivery vehicles:

• Limitations of Lipid Nanoparticles (LNPs): Poor tissue specificity (primarily accumulate in the liver) and limited ability to target specific cell types.
• Limitations of Viral Vectors (AAVs, Lentiviruses):
  • Cargo Capacity: Adeno-associated viruses (AAVs) are too small to accommodate large payloads like CRISPR-Cas9 components.
  • Safety: Lentiviruses and retroviruses pose risks of random genomic integration and persistent expression.
  • Immunogenicity: Pre-existing neutralizing antibodies in the human population limit repeated dosing.
  • Scalability: Production of viral vectors at scale is difficult and costly.
• Knowledge Gap: While endogenous viral elements (EVEs) make up a large portion of genomes, the structural and functional properties of most retroelement-derived proteins remain uncharted, representing an untapped resource for novel delivery platforms.

2. Methodology

The authors employed a multidisciplinary approach combining bioinformatics, structural biology, and protein engineering:

• Bioinformatics & Phylogenetics:
  • Utilized a two-step BLAST pipeline to identify novel viruses. First, a PSI-BLAST search against the non-redundant protein database using the full-length Atlas virus polyprotein (focusing on phlebovirus-like Env). Second, a rescreening using the structured Capsid (CA) core as a query to identify homologs.
  • Identified 11 new Atlas family viruses in nematodes.
• Protein Expression & Purification:
  • Expressed recombinant CA proteins from Atlas virus (Ancylostoma ceylanicum), Atlas-Oos1 (Ostertagia ostertagi), and Atlas-Nbr (Nippostrongylus brasiliensis) in E. coli.
  • Purified proteins using GST-affinity chromatography followed by size-exclusion chromatography (SEC) to isolate assembled particles.
• Cryo-Electron Microscopy (Cryo-EM):
  • Performed single-particle cryo-EM imaging on assembled particles.
  • Reconstructed structures at resolutions ranging from 2.2 Å to 4.4 Å for four distinct icosahedral assemblies (T=1, T=4, T=7, T=12).
  • Used AlphaFold 3 for initial model building and rigid-body fitting into cryo-EM density maps.
• Functional Assays:
  • Nucleic Acid Packaging: Used EMSA (Electrophoretic Mobility Shift Assay) and nuclease protection assays to test spontaneous RNA/DNA packaging.
  • Cellular Entry: Tracked fluorescently labeled particles in HEK293T, HeLa, and iBMDM cells using confocal microscopy to determine uptake mechanisms and subcellular localization.
  • pH Stability: Tested particle disassembly at endosomal pH (4.5) via cryo-EM.
• Protein Engineering:
  • Designed point mutations (e.g., K162C) to introduce intermolecular disulfide bonds for pH resilience.
  • Created fusion proteins by replacing the disordered N-terminal tail with an H11 anti-CTLA-4 nanobody to demonstrate modular targeting.

3. Key Contributions

• Discovery of the Atlas Virus Family: Identified a new family of 11 endogenous viruses in nematodes characterized by retrovirus-like genomes but containing phlebovirus-like glycoproteins (class II fusion proteins) and spumavirus-like capsid assemblies.
• Structural Characterization: Solved the first high-resolution cryo-EM structures of Atlas capsids, revealing a unique architecture distinct from orthoretroviruses.
• Mechanistic Insight: Elucidated the assembly mechanism, showing that Atlas capsids lack the Major Homology Region (MHR) and N-myristoylation sites typical of retroviruses, relying instead on specific hydrophobic cores and hydrogen-bond networks.
• Proof of Concept for Engineering: Demonstrated that Atlas capsids can be rationally engineered for enhanced stability (pH resistance) and specific cell targeting (CTLA-4) without compromising assembly.

4. Key Results

• Structural Plasticity & Diversity:
  • Atlas CA proteins spontaneously assemble into icosahedral particles of varying sizes (20–60 nm) with different triangulation numbers: T=1 (60 protomers), T=4 (240 protomers), T=7 (420 protomers), and T=12 (720 protomers).
  • The capsid shell is thin and single-layered, with the N-terminal domain (CANTD) and C-terminal domain (CACTD) arranged laterally rather than radially stacked. This geometry resembles Prototype Foamy Virus (PFV) and Drosophila Arc (dArc) capsids.
• Unique Assembly Interactions:
  • No MHR: Unlike orthoretroviruses, Atlas capsids lack the MHR. Instead, assembly is driven by a hydrophobic core formed by helix $\alpha$1 interactions and specific hydrogen bonds (e.g., His121-Asn172) at the icosahedral twofold axes.
  • Pore Structure: Pentameric capsomeres feature a central pore lined with a ring of basic residues (Lys41), but unlike HIV-1, they do not bind inositol hexaphosphate (IP6) as a cofactor.
• Functional Properties:
  • Spontaneous Packaging: Atlas particles spontaneously package nucleic acids (ssRNA, dsRNA, dsDNA) in a sequence-independent manner driven by electrostatic interactions with the positively charged inner surface. They can accommodate payloads up to ~10 kb.
  • Endocytosis & Disassembly: Particles enter cells via endocytosis, accumulate in acidic endolysosomes, and rapidly disassemble at pH 4.5, releasing their cargo.
• Engineering Success:
  • Stability: The K162C mutation introduced a disulfide bond at the icosahedral twofold interface, rendering particles resistant to disassembly at pH 4.5 while retaining the ability to reassemble.
  • Targeting: Fusion of an anti-CTLA-4 nanobody to the N-terminus resulted in functional particles that retained assembly capability and displayed the targeting domain on the surface.

5. Significance

• Novel Delivery Platform: Atlas capsids offer a "best-of-both-worlds" solution for gene therapy: they possess the large cargo capacity of viral vectors (suitable for CRISPR-Cas9 and CAR-T constructs) without the risks of genomic integration or pre-existing immunity (due to their nematode origin).
• Scalability: Unlike complex viral vectors, Atlas capsids are produced recombinantly in bacteria, allowing for cost-effective, large-scale manufacturing.
• Tunability: The ability to produce particles of different sizes (T=1 to T=12) allows for "tunable" packaging based on cargo size. Smaller particles (T=1) may offer better tissue penetration, while larger ones (T=12) accommodate larger payloads.
• Rational Design: The high-resolution structures provide a blueprint for "plug-and-play" engineering. The N-terminus is exposed and dispensable for assembly, making it an ideal site for attaching targeting ligands, endosomal escape modules, or other functional domains.
• Future Impact: This work establishes a new class of protein-based vectors that could overcome current limitations in RNA/DNA delivery, potentially revolutionizing the field of in vivo gene editing and cell therapy.