Structural insights into the inactive state of the adhesion GPCR ADGRV1

This study presents the first high-resolution cryo-EM structure of the inactive adhesion GPCR ADGRV1, revealing that its unique structural features, including a divergent tethered agonist and a closed intracellular loop 3, enable weak constitutive Gi signaling through an activation mechanism distinct from the canonical tethered agonist model.

The Gist

The Big Picture: A Broken Switch in the Body's "Smart Home"

Imagine your body is a giant, high-tech smart home. Inside the walls of your cells, there are millions of tiny doorbells (called receptors). When someone rings the doorbell, it sends a signal inside to tell the house what to do—like turning on the lights, locking the doors, or playing music.

One specific doorbell, called ADGRV1, is a very important one. It's located in the "hearing and vision" wing of the house. If this doorbell is broken, the lights (vision) and the speakers (hearing) stop working, leading to a condition called Usher Syndrome.

For years, scientists knew this doorbell existed, but they didn't know how it worked. They thought it had a specific "key" (a tiny peptide called the Stachel peptide) that fit into the lock to ring the bell. But this new study shows that ADGRV1 is actually a very strange, unique doorbell that doesn't work like the others.

The Discovery: Taking a 3D Snapshot

To figure out how this doorbell works, the scientists had to take a super-high-resolution 3D photograph of it. This is like trying to take a photo of a hummingbird's wings while it's flying; it's incredibly fast and blurry.

To solve this, they used a special tool called a Nanobody (let's call it a "Stabilizer Glove"). They put this glove on the doorbell to hold it perfectly still. Then, they used a powerful microscope (Cryo-EM) to snap a picture.

What they found:

1. The "Off" Position: They captured the doorbell in its "resting" or "off" state.
2. The Missing Key: They looked for the "Stachel key" inside the lock, but it wasn't there. The lock was empty.
3. The Self-Blocking Mechanism: The most surprising discovery was a part of the doorbell itself (a loop called ICL3) that was folded over the inside of the mechanism, effectively jamming the doorbell from the inside. It's like someone having taped the "ON" switch from the inside of the wall, making it very hard to turn on.

Why the "Key" Doesn't Work

In most other doorbells (receptors), the "Stachel key" is a standard shape (like a square peg). It fits perfectly into a square hole, turns the mechanism, and rings the bell.

However, the ADGRV1 doorbell is weird:

• The Wrong Shape: The "key" on this doorbell is shaped like a triangle, not a square. It doesn't fit the lock properly.
• The Stiff Hinge: The part of the doorbell that needs to bend to ring the bell has a stiff screw (a Serine amino acid) instead of a flexible hinge (a Glycine amino acid). It can't bend, so it can't ring.

Because of these two flaws, the doorbell doesn't ring when it's supposed to. It stays mostly silent.

The "Weak Hum" (Constitutive Activity)

Even though the doorbell is jammed and the key doesn't fit, the scientists noticed something strange: the doorbell wasn't completely silent. It was making a very faint, weak "hum."

In scientific terms, this is called weak constitutive activation. It means the doorbell is slightly "on" all the time, even without a key. It's not ringing loudly, but it's not totally off either. This explains why people with mutations in this gene get sick—the system is stuck in a low-power, confused state rather than a clear "on" or "off" state.

The "Self-Jamming" Loop (ICL3)

The biggest surprise was the ICL3 loop. Imagine the inside of the doorbell has a long, floppy tail. In most doorbells, this tail hangs down loosely, waiting for a signal.

In ADGRV1, this tail is long and sticky. It curls up and hugs the inside of the mechanism tightly, blocking the place where the "signal messenger" (the G-protein) is supposed to attach. It's like a security guard standing in the doorway, refusing to let anyone in.

The scientists used computer simulations to prove that even without the "Stabilizer Glove" (the nanobody), this tail stays curled up and blocking the door. This suggests the doorbell is designed to be hard to activate.

Why Does This Matter?

1. It's a New Rulebook: For a long time, scientists thought all these doorbells worked the same way (Key fits in lock -> Bell rings). This paper proves that ADGRV1 is an exception. It has its own unique rules.
2. Understanding Disease: Since this doorbell is broken in people with Usher Syndrome (deafness and blindness), knowing exactly how it is jammed helps doctors and researchers design better medicines. Instead of trying to force the "wrong key" in, they might need to find a way to un-jam the "sticky tail" or fix the "stiff hinge."
3. Future Treatments: Now that we have a 3D map of the broken doorbell, we can start looking for the right tools to fix it.

Summary in One Sentence

Scientists took a 3D photo of a broken "hearing and vision" doorbell and discovered it doesn't work because it has the wrong keyhole, a stiff hinge, and a sticky tail that jams the switch from the inside, explaining why it causes deafness and blindness when mutated.

Technical Summary

1. Problem Statement

Adhesion G protein-coupled receptors (aGPCRs) are a large family of receptors involved in cell adhesion, migration, and sensory functions. While recent cryo-electron microscopy (cryo-EM) studies have elucidated the activation mechanisms of several aGPCRs (e.g., ADGRE5, ADGRL3), these mechanisms rely heavily on a tethered agonist (Stachel) peptide released upon autoproteolytic cleavage. This peptide binds to an orthosteric pocket, inducing conformational changes (kinks in transmembrane helices VI and VII) that allow G-protein coupling.

ADGRV1 (also known as GPR98 or VLGR1), the largest known GPCR, is a member of the unique IX sub-family. Mutations in ADGRV1 cause Usher syndrome type IIC, leading to deafness and blindness. Despite its clinical importance, the molecular mechanism of ADGRV1 activation remains controversial. Previous studies suggested it couples to Gi proteins, but conflicting data exists regarding whether its Stachel peptide acts as a functional agonist. Furthermore, no high-resolution structure of ADGRV1 existed, leaving its inactive state, intracellular loop conformations, and activation pathway unknown.

2. Methodology

The authors employed a multidisciplinary approach combining structural biology, biophysics, and cellular functional assays:

• Protein Engineering & Expression:
  • Focused on the C-terminal fragment (CTF) of ADGRV1 (ADGRV1β), comprising the 7-transmembrane domain (7TMD), intracellular loops, and a short N-terminal Stachel peptide, excluding the massive extracellular domain.
  • Expressed murine ADGRV1β in Sf9 insect cells and purified it in LMNG/CHS detergent micelles.
• Nanobody (VHH) Development:
  • Immunized alpacas with purified ADGRV1β and transfected HEK cells to generate a phage-display library.
  • Selected RE02, a nanobody with high affinity ($K_D \approx 3$ nM) that binds the intracellular interface of ADGRV1β without cross-reacting with the isolated intracellular domain.
  • Used RE02 to stabilize the receptor and provide extra density for cryo-EM, avoiding the need for fusion proteins (like T4L or BRIL) that would alter native loop structures.
• Biophysical Characterization:
  • Analytical Ultracentrifugation (AUC) and Small-Angle X-ray Scattering (SAXS) confirmed ADGRV1β exists as a monomer in solution with a flexible C-terminal domain.
  • Bio-Layer Interferometry (BLI) confirmed the high-affinity interaction between RE02 and ADGRV1β.
• Cryo-EM Structure Determination:
  • Prepared the ADGRV1β/RE02 complex for cryo-EM.
  • Solved the structure at 3.8 Å resolution using single-particle analysis (SPA) in CryoSPARC.
  • Refined the model to resolve the 7TMD, extracellular loops (ECLs), and intracellular loops (ICLs), specifically capturing the native conformation of the large ICL3.
• Functional Assays:
  • Performed Bioluminescence Resonance Energy Transfer (BRET) assays in HEK293 cells to measure G-protein coupling (Gi, Go, Gq, Gs).
  • Tested wild-type (WT) and mutant (Y5886A/Y5889A) Stachel peptides to assess agonist dependency.
• Computational Simulations:
  • Conducted 2 µs Molecular Dynamics (MD) simulations (total 12 µs) to assess the stability of the ICL3 conformation in the presence and absence of the nanobody.
  • Compared experimental data with AlphaFold3 predictions.

3. Key Contributions & Results

A. Structural Insights: A Unique Inactive State

• Inactive Conformation: The structure reveals ADGRV1β in a canonical inactive state. Unlike active aGPCRs, there are no kinks in transmembrane helices VI (TM6) and VII (TM7).
• The Stachel Peptide Paradox: The orthosteric pocket is empty; no density for the Stachel peptide was observed.
  • Sequence Divergence: The Stachel peptide sequence of ADGRV1 is Y3xxY6A7, diverging significantly from the conserved hydrophobic consensus motif (F3xxL6M/L7) found in other aGPCRs.
  • Critical Residue Substitution: ADGRV1 lacks the conserved Glycine at position 6.50 (G6.50), which is essential for helix kinking in other aGPCRs. Instead, it possesses a Serine (S6109), which likely restricts the flexibility required for activation.
• ICL3 as a "Lid": The most striking feature is the conformation of the large Intracellular Loop 3 (ICL3).
  • In active aGPCR structures (often solved with BRIL insertions), ICL3 is disordered or replaced. Here, the native 17-residue ICL3 is resolved.
  • It adopts a closed, lid-like conformation, tightly packed against the intracellular face of the 7TMD via hydrophobic interactions (W6082, V6088, F6089).
  • This conformation physically blocks the G-protein binding site.
  • MD simulations confirmed this "closed" state is stable even in the absence of the nanobody, suggesting it is an intrinsic regulatory feature.

B. Functional Insights: Constitutive Activity Independent of Stachel

• Gi Coupling: BRET assays confirmed ADGRV1β exhibits weak constitutive activity coupled specifically to Gi proteins (Gi1, Gi2, Gi3), but not Go, Gq, or Gs.
• Stachel Independence: Mutating the critical tyrosine residues in the Stachel peptide (Y5886A/Y5889A) did not abolish Gi coupling. The mutant retained activity comparable to the wild type.
• Conclusion: ADGRV1 activation is independent of the tethered agonist peptide. The weak constitutive activity observed is likely driven by the receptor's intrinsic properties rather than Stachel-mediated activation.

4. Significance

• Redefining aGPCR Activation: This study challenges the "canonical" model where the Stachel peptide is the universal trigger for all aGPCRs. ADGRV1 represents a distinct subclass that likely operates via an alternative activation mechanism.
• Mechanism of Regulation: The discovery of the ICL3 "lid" provides a novel regulatory mechanism. By occluding the G-protein binding site, the ICL3 acts as an intrinsic negative regulator, potentially explaining the weak constitutive activity and the difficulty in activating the receptor.
• Disease Relevance: Understanding the unique structural features of ADGRV1 (specifically the non-consensus Stachel sequence and the S6109 substitution) is crucial for understanding Usher syndrome. It suggests that pathological mutations may disrupt this unique regulatory balance rather than simply preventing Stachel cleavage.
• Therapeutic Implications: The findings imply that developing agonists for ADGRV1 will require strategies distinct from those used for other aGPCRs, as the standard tethered agonist approach may be ineffective.

Summary

This paper presents the first high-resolution structure of the inactive ADGRV1 receptor, revealing a unique regulatory mechanism where a closed ICL3 loop blocks G-protein access and a divergent Stachel peptide sequence (lacking key hydrophobic residues and the critical G6.50) prevents canonical activation. Consequently, ADGRV1 exhibits weak, constitutive Gi signaling independent of its tethered agonist, establishing a new paradigm for aGPCR biology.