A sterol-binding pocket in iRhom1 underlies paralog-specific regulation of the sheddase ADAM17

This study reveals that a paralog-specific sterol-binding pocket in iRhom1, identified through cryo-EM, is essential for stabilizing the iRhom1/ADAM17 complex and regulating its shedding activity, distinguishing its mechanism from iRhom2 and linking mutations in this region to cardiac disease.

The Gist

Imagine your cells are like a busy city, and ADAM17 is the city's most important "delivery truck." This truck's job is to pick up important messages (like growth signals and inflammation alerts) from the cell's surface and drop them off inside the city to keep everything running smoothly. But this truck can't drive on its own; it needs a special mechanic to get it ready for the road.

For a long time, scientists knew about one mechanic called iRhom2. This mechanic is well-studied and knows exactly how to hook up to the truck and get it working. However, there is a second, very similar mechanic called iRhom1 that works in almost every cell in the body, but nobody knew exactly how it did its job or what made it tick.

This paper acts like a high-resolution blueprint (a 3D map made with a powerful microscope) that finally shows us how iRhom1 and the ADAM17 truck fit together. Here is what the researchers discovered:

1. The "Gas Station" Discovery
The scientists found a hidden pocket inside the iRhom1 mechanic. Think of this pocket like a specialized gas station built right into the mechanic's engine. This station is designed to hold a specific type of fuel called sterol (a type of fat molecule found in our bodies).

2. How the Fuel Works
The paper shows that for iRhom1 to do its job, this "gas station" must be filled with sterol fuel. When the fuel is there, the mechanic and the truck lock together tightly, and the delivery truck can start working. If you block this pocket or remove the fuel, the mechanic and the truck fall apart, and the delivery service stops.

3. The Twin Paradox
Here is the twist: Even though iRhom1 and iRhom2 are like twin brothers, they work in completely different ways.

• iRhom1 needs that external "sterol fuel" to stay connected to the truck.
• iRhom2 doesn't have a gas station at all. Instead, it has a built-in "seatbelt" (a direct internal connection) that holds the truck in place without needing any outside fuel.

4. The Broken Blueprint
The researchers also looked at two specific versions of the iRhom1 mechanic found in humans that are linked to heart disease. They discovered that these broken versions have a crack right next to the "gas station." Because of this damage, the fuel can't get in, the mechanic and truck can't lock together, and the whole system fails.

In Summary
This study reveals that while the two mechanics (iRhom1 and iRhom2) do the same job, they use totally different strategies to hold the delivery truck (ADAM17) together. iRhom1 relies on a special pocket to catch a fat molecule (sterol) to stay stable, while iRhom2 holds on by itself. This discovery explains why certain heart conditions happen when the iRhom1 "gas station" is broken and suggests that if we ever want to fix just one of these twins without affecting the other, we might be able to target that specific fuel pocket.

Technical Summary

Technical Summary: A Sterol-Binding Pocket in iRhom1 Underlies Paralog-Specific Regulation of ADAM17

1. Problem Statement

ADAM17 (a disintegrin and metalloproteinase 17) is the primary sheddase in mammalian cells responsible for releasing membrane-tethered EGFR ligands and inflammatory cytokines, making it a critical regulator of cell signaling. Its function is strictly dependent on its interaction with rhomboid pseudoproteases, specifically iRhom1 and iRhom2, which act as essential cofactors for ADAM17 maturation and activation.

While the regulatory mechanisms of iRhom2 are well-characterized, the ubiquitously expressed paralog iRhom1 remains poorly understood. Key gaps in knowledge included:

• The structural basis of the iRhom1/ADAM17 complex.
• The specific molecular mechanisms governing iRhom1-mediated ADAM17 activation.
• The functional divergence between iRhom1 and iRhom2, particularly regarding how they differentially regulate the same target enzyme.
• The pathogenic mechanisms of human iRhom1 variants linked to cardiac diseases.

2. Methodology

The study employed a multi-disciplinary approach combining structural biology, biochemistry, and pharmacology:

• Cryo-Electron Microscopy (Cryo-EM): The researchers determined the high-resolution (2.5 Å) structure of the full-length human iRhom1/ADAM17 complex. This allowed for the visualization of the transmembrane architecture and interfacial interactions.
• Structure-Guided Mutagenesis: Based on the cryo-EM data, specific residues within the identified binding pockets were mutated to test their functional necessity.
• Pharmacological Perturbation: Small molecules and sterol-binding inhibitors were used to disrupt specific interactions within the complex to observe downstream effects on stability and activity.
• Biochemical Assays: Shedding activity assays were conducted to measure the functional output of ADAM17 under various conditions (wild-type vs. mutants, with vs. without sterol binding).
• Clinical Variant Analysis: Human iRhom1 variants associated with cardiac disease were mapped onto the structure to assess their impact on the protein's integrity and function.

3. Key Contributions

• Structural Breakthrough: The paper provides the first high-resolution (2.5 Å) cryo-EM structure of the full-length human iRhom1/ADAM17 complex.
• Discovery of a Novel Binding Site: Identification of a previously unrecognized sterol-binding pocket located between transmembrane domains 2 (TMD2) and 5 (TMD5) of iRhom1.
• Mechanistic Divergence: The study establishes that iRhom1 and iRhom2 utilize fundamentally different mechanisms to stabilize ADAM17, resolving the mystery of their paralog-specific regulation.
• Clinical Correlation: The research links specific cardiac disease-associated variants to the disruption of this newly identified sterol-binding pocket.

4. Key Results

• Sterol Binding is Essential for iRhom1: The structural analysis revealed that a sterol molecule binds within the pocket between TMD2 and TMD5. Mutagenesis and pharmacological disruption of this site demonstrated that sterol binding is strictly required to stabilize the iRhom1/ADAM17 complex and sustain its shedding activity. Without sterol binding, the complex is unstable and inactive.
• Paralog-Specific Regulation: A striking divergence was found between the paralogs:
  • iRhom1: Relies on an exogenous sterol-binding pocket to stabilize ADAM17.
  • iRhom2: Lacks this pocket (it is structurally precluded) and instead stabilizes ADAM17 through direct intramolecular interactions within the protein complex.
• Pathogenic Mechanism: Two human iRhom1 variants linked to cardiac disease were found to localize adjacent to the sterol-binding pocket. These mutations disrupt the pocket's integrity, leading to failed ADAM17 maturation and loss of shedding activity, providing a molecular explanation for the associated pathology.

5. Significance

• Fundamental Biology: This work redefines the understanding of ADAM17 regulation, shifting the paradigm from a uniform mechanism to a paralog-specific model where iRhom1 is uniquely dependent on lipid (sterol) interactions.
• Therapeutic Potential: The identification of the sterol-binding pocket offers a unique, paralog-selective target. Since iRhom2 does not possess this pocket, drugs designed to modulate sterol binding could selectively inhibit or modulate iRhom1/ADAM17 activity without affecting iRhom2, potentially reducing off-target effects in treating inflammatory diseases or cancer.
• Clinical Insight: The findings provide a structural basis for understanding specific cardiac diseases caused by iRhom1 mutations, opening avenues for targeted genetic therapies or diagnostic markers based on the sterol-binding interface.