The trimeric structures of the extracellular domains of FAM171A1 and FAM171A2 neuronal proteins belong to a novel structural superfamily

This study reveals that the extracellular domains of the neuronal proteins FAM171A1 and FAM171A2 adopt a novel structural architecture and form equilateral trimers, providing critical structural insights into their potential roles in ligand binding, signaling, and human diseases such as cancer and neurodegeneration.

The Gist

The Big Picture: Unlocking the Shape of "Brain Keys"

Imagine your body is a bustling city, and your cells are the buildings. To keep the city running, the buildings need to talk to each other. They do this using "keys" on their doors—proteins that stick out of the cell surface to send messages or grab onto things.

Two of these keys, called FAM171A1 and FAM171A2, are very important. They are found mostly in the brain. Scientists know that if these keys get broken or act weird, it can lead to serious problems like Alzheimer's, Parkinson's, and even some cancers. But until now, nobody knew what these keys actually looked like. It's like trying to fix a lock without ever seeing the key.

This paper is the first time scientists have taken a high-resolution "3D photo" of these keys and figured out how they work together.

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1. The Shape: A New Kind of Lego Brick

For a long time, scientists thought these proteins were made of standard parts found in other proteins. But when the researchers looked closely, they realized these proteins are unique.

• The Analogy: Imagine you have a standard Lego brick (Domain 1) and a standard Duplo block (Domain 2). Usually, you find them separately. But FAM171A1 and FAM171A2 are like a brand new, custom-made Lego piece where these two different shapes are fused together into one weird, wonderful unit.
• The Discovery: This is a completely new architectural style in the protein world. It's like finding a new type of gear in a clock that nobody has ever seen before.

2. The Teamwork: The "Three-Legged Stool"

The most exciting discovery is how these proteins behave when they are floating around in the cell fluid. They don't like to be alone.

• The Analogy: Think of these proteins as people at a party.
  • FAM171A1 is the social butterfly that always wants to be in a group of three. It instantly grabs two friends to form a perfect three-legged stool. Once they are linked, they are very hard to pull apart. They are a tight-knit trio.
  • FAM171A2 is a bit more shy. It also likes to form groups of three, but it's easier to break them up. If you dilute the crowd (lower the concentration), the trio falls apart, and you get single people wandering around. It's a "wobbly" stool that only holds together when the room is crowded.

3. The Giant Snow Globe: The "60-mer"

Here is where it gets really cool. When the researchers looked at FAM171A1 at very high concentrations (a very crowded room), something magical happened.

• The Analogy: The three-legged stools didn't just sit there; they started holding hands with other three-legged stools. They arranged themselves into a giant, hollow soccer ball (or a snow globe) made of 20 of those trios.
• The Science: This giant sphere is made of 60 individual protein pieces (20 groups of 3). The researchers proved this wasn't a mistake caused by their lab equipment. They built "mutant" versions of the protein (like taking a screw out of the Lego) that couldn't hold hands, and the giant ball disappeared. This suggests that FAM171A1 has a natural ability to build these massive structures, which might be how it organizes signals on the surface of a real brain cell.

4. Why Does This Matter?

Why should you care about the shape of a protein?

• The Lock and Key: If you know what the key looks like, you can design a better key (or a fake key) to open the door. Since these proteins are linked to diseases like Parkinson's and Alzheimer's, knowing their shape helps drug companies design medicines that can stop them from causing trouble.
• The Alpha-Synuclein Connection: The paper mentions that FAM171A2 helps spread a toxic protein called "alpha-synuclein" (the villain in Parkinson's disease). Now that we know FAM171A2 is a three-legged stool, scientists can try to figure out exactly how that stool grabs the toxic protein. Maybe if we break the stool, we stop the disease from spreading.

Summary

In short, this paper is like an architectural blueprint for two mysterious brain proteins.

1. They are made of a new, unique shape never seen before.
2. They naturally clump together in groups of three (trimers).
3. One of them (FAM171A1) can even build giant soccer-ball structures out of those groups.

This gives scientists the map they need to understand how these proteins communicate in the brain and how to fix them when they go wrong in diseases like Alzheimer's and Parkinson's.

Technical Summary

Title

The trimeric structures of the extracellular domains of FAM171A1 and FAM171A2 neuronal proteins belong to a novel structural superfamily.

1. Problem Statement

The FAM171 family (FAM171A1, FAM171A2, and FAM171B) consists of type-I transmembrane proteins enriched in the brain and implicated in severe human pathologies, including various cancers (FAM171A1) and neurodegenerative diseases like Alzheimer's and Parkinson's (FAM171A2). Despite their clinical relevance and known roles in cell signaling, cytoskeletal regulation, and $\alpha$-synuclein endocytosis, the structural biology of these proteins remained completely unknown. No experimentally determined structures existed, and computational homology modeling (SWISS-Model) failed to identify structural homologs covering more than 10% of the sequence. Consequently, the mechanisms of their oligomerization, ligand binding, and signal transduction were undefined.

2. Methodology

The authors employed a multi-faceted structural biology approach to characterize the ectodomains (extracellular regions) of human FAM171A1 and FAM171A2:

• Protein Production: Ectodomains were cloned with IgG1-Fc or Alkaline Phosphatase (AP) tags and expressed in Expi293 cells (transient) and HEK293S GnTI- cells (stable) to ensure homogeneous Man5GlcNAc2 glycosylation.
• Biophysical Characterization:
  • SEC & SEC-MALS: Size-exclusion chromatography coupled with Multi-Angle Light Scattering to determine oligomeric states and molecular weights in solution.
  • Analytical Ultracentrifugation (AUC): Sedimentation velocity and equilibrium experiments to calculate precise molecular masses and stoichiometry.
  • Mass Photometry: To assess particle mass distributions at low concentrations.
  • ELISA: To measure self-association affinity ($K_D$) and test for hetero-oligomerization.
• Cryo-Electron Microscopy (Cryo-EM):
  • Samples were prepared at varying concentrations (1 mg/mL and 10 mg/mL) with and without NP-40 detergent.
  • Data was collected on a Titan Krios G4 microscope.
  • Processing involved 3D reconstruction with C3 symmetry (for trimers) and Icosahedral symmetry (for higher-order assemblies).
  • De novo model building was performed using ModelAngelo and refinement in Phenix.
• Small Angle X-ray Scattering (SAXS): SEC-SAXS experiments were conducted at the Australian Synchrotron to validate quaternary structures in solution and measure dimensions ($R_g$ and $D_{max}$).
• Mutagenesis: Site-directed mutagenesis introduced N-linked glycosylation sites (e.g., F110N, S122N, Y241S, L255N) to disrupt specific interfaces and validate the biological relevance of observed oligomers.

3. Key Contributions and Results

A. Novel Protein Architecture

• The monomeric ectodomains of both FAM171A1 and FAM171A2 possess a novel structural architecture composed of two distinct domains:
  • Domain 1 (N-terminal): A carboxypeptidase-like regulatory domain (CATH class 2.60.40.1120), structurally similar to human carboxypeptidase D.
  • Domain 2 (C-terminal): An immunoglobulin-like domain (CATH class 2.60.220) related to Chondroitinase proteins and the ZU5 domain, though with low sequence identity.
• This specific combination of domains represents a new structural superfamily not previously characterized.

B. Oligomeric States in Solution

• FAM171A1: Exists as a stable, constitutive trimer across a wide concentration range. At high concentrations (~10 mg/mL), it further assembles into a hollow icosahedral 60-mer (20 trimers arranged in a sphere).
• FAM171A2: Exists in a concentration-dependent equilibrium between monomers and trimers. It forms a stable trimer at high concentrations but dissociates significantly at lower concentrations.
• Affinity: FAM171A1 self-association is ~40-fold stronger ($K_D \approx 9$ nM) than FAM171A2 ($K_D \approx 389$ nM). No hetero-oligomerization (A1/A2) was detected.

C. Structural Details of the Trimer

• The trimers form equilateral triangles with a buried surface area of ~4,500–4,750 Ų.
• Interface: The primary interaction interface involves $\beta$-strands 7 and 8 and their flanking loops.
• Stabilization: FAM171A1 trimers are stabilized by specific hydrogen bonds (K37-L163'/L165') and a salt bridge (R106-D130') that are absent in FAM171A2, explaining the higher stability of the A1 trimer.
• 60-mer Assembly: The icosahedral 60-mer of FAM171A1 is formed via interactions between Domain 2 of adjacent trimers (specifically $\beta$-strands 13–15). Mutagenesis (Y241S, L255N) abolished 60-mer formation, confirming the interface.

D. Validation

• SAXS: Confirmed the trimeric and 60-mer structures in solution, with $R_g$ values of 37.9 Å (trimer) and 101.9 Å (60-mer) for FAM171A1.
• Mutagenesis: Introduction of glycosylation sites at the trimer interface (F110N/S122N) successfully disrupted trimerization, shifting the protein to a monomeric state in SEC, validating the structural model.

4. Significance

• Structural Breakthrough: This study provides the first experimental structures for the FAM171 family, defining a novel protein superfamily.
• Mechanistic Insight: The discovery of stable homo-trimerization suggests that these receptors likely function via clustering on the cell surface to transduce signals to the cytoskeleton.
• Disease Relevance:
  • The high-affinity trimerization of FAM171A1 may be critical for its role in tumor invasion and cell motility.
  • The concentration-dependent behavior of FAM171A2 and its interaction with $\alpha$-synuclein (a known binder of Domain 1) suggest that oligomeric state changes could regulate $\alpha$-synuclein pathology in Parkinson's disease.
• Therapeutic Potential: The identification of specific interfaces (e.g., the salt bridge in A1) and the 60-mer assembly mechanism provides potential targets for therapeutic intervention in cancer and neurodegenerative disorders.
• Limitations: The study was conducted in vitro on ectodomains only; the full-length protein behavior on the cell membrane and the identification of endogenous ligands remain to be determined.

In conclusion, the paper establishes that FAM171A1 and FAM171A2 are novel, trimeric cell surface receptors with distinct oligomerization stabilities, providing a structural foundation for future research into their roles in neurological diseases and cancer.