Full length TECPR1 displays cis Dysferlin domain architecture

This study presents the first cryo-EM structure of full-length TECPR1, revealing an elongated hook-shaped architecture with cis-oriented Dysferlin domains stabilized by a novel tectonin-PH interface that provides a structural framework for its role in noncanonical autophagy.

The Gist

Imagine your cell is a bustling city. Sometimes, parts of this city get damaged—like a pothole in a road or a leak in a water pipe. To fix these problems, the cell has a specialized repair crew. One of the most important tools in this crew is a protein called TECPR1.

For a long time, scientists knew TECPR1 was a hero, but they didn't know what it actually looked like or how it held its tools together. This paper is like taking a high-resolution 3D photo of TECPR1 for the first time, revealing its secret shape and how it works.

Here is the story of what they found, explained simply:

1. The Shape: A Giant Fishing Hook

Think of TECPR1 not as a tiny ball, but as a long, flexible fishing hook.

• The Handle: The middle of the hook is a sturdy, twisted structure that holds everything together.
• The Bait: At the very tip of the hook, there are two special "claws" (called Dysferlin domains). These claws are designed to grab onto a specific type of fat (sphingomyelin) that is only exposed when a cell membrane is damaged.

2. The Big Discovery: The "Cis" Claws

Before this study, scientists were arguing about how TECPR1 grabs onto damaged membranes.

• Theory A: Maybe it uses just one claw to grab the membrane, like a one-handed handshake.
• Theory B: Maybe it uses both claws at the same time, like a two-handed hug.

The new photo proves Theory B. The two claws are positioned right next to each other on the same side of the hook. Scientists call this a "cis" arrangement. It's like a pair of tongs. This means TECPR1 can grab the damaged membrane with both claws simultaneously, making it much harder to let go. This "double grip" ensures the repair crew stays exactly where it's needed to fix the leak.

3. The Secret Glue: The TR1-PH Bridge

How does this long, floppy hook stay in this perfect shape? It wouldn't work if it just fell apart.

• The researchers found a hidden internal bridge connecting two parts of the protein's body (the TR1 and PH domains).
• Imagine a drawbridge on a castle. This bridge is currently down, locking the two halves of the protein together. This lock keeps the "claws" in the perfect position to grab the membrane.
• Interestingly, this bridge might also be a safety lock. It might be hiding a part of the protein (the PH domain) that usually helps with other tasks, keeping it "asleep" until the right moment.

4. The Simulation: Testing the Hook in Water

The scientists didn't just take a picture; they also ran a computer simulation (a digital movie) to see how TECPR1 behaves when it hits a membrane.

• They dropped the digital TECPR1 onto a digital membrane.
• Result: The hook landed perfectly. The two claws grabbed the membrane and held on tight. Even as the protein wiggled and rocked, the "bridge" holding the shape together didn't break.
• However, the part of the protein that might grab other types of fats (the PH domain) stayed in the air, not touching the membrane. This suggests that TECPR1 needs a specific "key" (another protein called ATG5-ATG12) to unlock that part of itself later in the repair process.

Why Does This Matter?

Think of the cell's repair system as a construction site.

1. Damage happens: A hole appears in the cell wall, exposing the "forbidden" fats inside.
2. TECPR1 arrives: It uses its double-claw grip (the two Dysferlin domains) to latch onto the hole securely.
3. The Bridge holds: The internal bridge keeps the protein stable so it doesn't fall off.
4. The Repair begins: Once latched, TECPR1 calls in the rest of the repair crew (the ATG proteins) to patch the hole and seal the cell.

In summary: This paper solved the mystery of TECPR1's shape. It's a long, hook-shaped protein that uses a secret internal bridge to keep its two "claws" perfectly aligned to grab onto damaged cell membranes with a strong, two-handed grip. This discovery helps us understand how our cells heal themselves, which could be crucial for treating diseases where this repair system fails.

Technical Summary

1. Problem Statement

Tectonin Beta-Propeller Repeat-containing 1 (TECPR1) is a critical regulator of a non-canonical autophagy pathway known as Sphingomyelin TECPR1-induced LC3 lipidation (STIL). This pathway is activated during cellular stress, specifically when lysosomal membranes rupture and expose sphingomyelin (SM) to the cytosol. TECPR1 functions as an E3-like ligase, replacing ATG16L1 to conjugate LC3 proteins onto single-membrane compartments.

Despite its established role, the structural basis for TECPR1 function remained unresolved. Key questions included:

• How are the six structural domains of full-length TECPR1 arranged in 3D space?
• Do the two Dysferlin (DysF) domains, responsible for SM binding, engage the membrane individually (trans-like) or cooperatively (cis-like)?
• How do intramolecular interactions regulate membrane recruitment and LC3 conjugation?
• Previous studies offered conflicting models regarding DysF domain cooperation, and AI-predicted structures lacked experimental validation for the full-length protein's conformation.

2. Methodology

The authors employed a multi-disciplinary approach combining structural biology, biochemistry, and computational simulations:

• Protein Expression and Purification: Full-length human TECPR1 (residues 1–1,165) with an N-terminal Twin-Strep-Flag tag was transiently expressed in suspension HEK293F cells. The protein was purified via Strep-Tactin affinity chromatography followed by Size Exclusion Chromatography (SEC) to ensure homogeneity.
• Cryo-Electron Microscopy (CryoEM):
  • Sample Prep: To mitigate severe preferred orientation bias, the sample was treated with 0.01 mM Lauryl Maltose Neopentyl Glycol (LMNG) and data was collected at a 30° stage tilt.
  • Data Acquisition: Imaging was performed on a Titan Krios G4 Cryo-TEM (300 kV) with a Gatan K3 detector.
  • Processing: Data was processed using CryoSPARC. After iterative 2D classification and 3D reconstruction, a final map was generated at a nominal resolution of 3.26 Å (global), though local resolution varied (3.5–12 Å) due to flexibility in specific domains.
• Model Building: Initial domain structures were derived from AlphaFold2 predictions. Rigid-body docking and real-space refinement were performed using UCSF ChimeraX, Coot, and Phenix.
• Molecular Dynamics (MD) Simulations: All-atom simulations were conducted using NAMD3 and the CHARMM36 force field. The system included TECPR1 docked onto a lipid bilayer mimicking damaged membranes (7:2:1 POPC:POPE:DPSM ratio) to observe stability and domain behavior over a 1-microsecond trajectory.

3. Key Contributions and Results

A. Elongated Hook Architecture

The study presents the first high-resolution structure of full-length human TECPR1. The protein adopts an elongated, hook-shaped conformation.

• Domain Arrangement: The structure reveals a central core formed by Tectonin Repeat 1 (TR1) and Tectonin Repeat 2 (TR2), flanked by the Plekstrin Homology (PH) domain and the two Dysferlin (DysF1 and DysF2) domains.
• Dynamic Linkers: The ATG5-interacting region (AIR) helix was not resolved in the density map, suggesting it is highly dynamic or disordered in the absence of the ATG5-ATG12 conjugate.

B. The TR1-PH Intramolecular Bridge

A novel finding is a stable intramolecular interface between the TR1 and PH domains.

• Structure: This interface acts as a structural bridge, linking the two folded halves of the protein. It involves a hydrophobic core (residues V610, F648, Y650 in PH; I329, L367 in TR1) and electrostatic contacts.
• Function: This interaction blocks the predicted lipid-binding pocket of the PH domain. The authors propose this acts as an autoinhibitory mechanism, keeping the PH domain inaccessible until a conformational change (potentially triggered by ATG5-ATG12 binding) occurs.

C. "Cis" Dysferlin Domain Arrangement

The most significant functional insight is the spatial orientation of the DysF domains.

• Cis Conformation: Both DysF1 and DysF2 domains protrude from the central core and align along the same molecular face (a "cis" arrangement).
• Spacing: The hydrophobic tips responsible for SM binding are positioned 98 Å apart.
• Implication: This geometry supports a cooperative, dual-membrane binding mode. It allows TECPR1 to simultaneously engage multiple sphingomyelin headgroups on a damaged membrane, increasing avidity and retention compared to a single-domain (trans) engagement model.

D. Molecular Dynamics Validation

MD simulations confirmed the stability of this architecture in a membrane environment:

• The TR1-PH bridge remained intact (RMSD < 1 Å) throughout the simulation.
• The DysF domains maintained contact with the membrane surface, with DysF1 showing some diffusion but DysF2 remaining stably docked.
• The PH domain did not engage the membrane in the absence of the ATG5-ATG12 conjugate, supporting the hypothesis that the TR1-PH bridge keeps it sequestered.

4. Significance

This work provides the first structural framework for understanding how TECPR1 regulates non-canonical autophagy:

1. Mechanism of Membrane Recruitment: It resolves the debate on DysF domain function, confirming that TECPR1 utilizes a cooperative "cis" mechanism to bind damaged membranes, likely enhancing the efficiency of LC3 lipidation on single-membrane compartments.
2. Regulatory Insight: The discovery of the TR1-PH bridge suggests a regulatory switch where the protein exists in an inhibited state. The disruption of this bridge (likely by ATG5-ATG12 binding) may be the trigger that exposes the PH domain for PI3P/PI4P binding and facilitates downstream conjugation.
3. Structural Biology Benchmark: It demonstrates the utility of CryoEM combined with tilt-series data to resolve large, flexible multi-domain proteins that are difficult to crystallize or predict accurately with AI alone.

In summary, the paper establishes that TECPR1 functions as a rigid, elongated scaffold that positions two lipid-binding domains in a "cis" configuration to maximize membrane engagement, while utilizing an intramolecular bridge to regulate its activity until the appropriate autophagy partners are present.