Design and characterization of SAKe, a new building block for protein self-assembly

This paper introduces SAKe, a thermally stable, kelch-like designer protein engineered to form large, well-defined, pH-dependent two-dimensional assemblies on solid surfaces while maintaining structural integrity, thereby providing a versatile platform for the nanofabrication of functional protein-based materials.

The Gist

The Big Picture: Building a Protein City

Imagine you want to build a perfect, flat city on a table (the table is a microscopic surface like mica). You want to use tiny, living bricks (proteins) to build this city.

The problem is that most natural proteins are like jumbled piles of LEGOs. If you dump them on a table, they stick together in messy clumps, or they stick to the table upside down, or they fall apart. They are too sticky, too wobbly, or just too confused to form a neat, organized grid.

The scientists in this paper wanted to solve this. They asked: "Can we design a custom protein brick that knows exactly how to snap together with its neighbors to form a perfect, flat sheet?"

The answer is SAKe.

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1. The Blueprint: The "Kelch" Shape

To build their new brick, the scientists looked at a natural protein shape called a Kelch domain.

• The Analogy: Think of a Kelch protein like a six-pointed star-shaped plate (or a pizza with six slices).
• The Problem: In nature, these plates are a bit wobbly and the edges (loops) are different lengths, making them hard to stack perfectly.
• The Fix: The team used a "time machine" for DNA (called Ancestral Sequence Reconstruction). They looked at the ancient ancestors of these proteins to find the "perfect" version that nature used millions of years ago. They then copied this perfect shape six times and stitched them together into a super-stable, symmetrical hexagon.

They call this new, super-stable brick SAKe. It's so stable that it can survive boiling water (well, almost—over 95°C!), whereas the original natural version falls apart at a lukewarm 44°C.

2. The Magic Glue: The "Bottom" and the "Top"

A protein brick has two sides:

1. The Top: This is where you can attach tools, sensors, or medicines later. The scientists made this side very flexible so they could change it easily without breaking the brick.
2. The Bottom: This is the side that touches the table. This is where the magic happens.

The scientists realized that for the bricks to line up perfectly on the table, they needed a special "glue." They decided to use Histidine (a specific amino acid) as the glue.

• The Analogy: Imagine the table (mica) is negatively charged, like a magnet with a negative pole. The scientists added "magnetic feet" (Histidines) to the bottom of the SAKe bricks.
• The Trick: These magnetic feet only work when the water around them is slightly acidic (low pH). When the pH is right, the feet grab the table, and the bricks snap together side-by-side.

3. The Experiment: From Soup to Sheet

The team tested their new bricks in the lab:

• The Soup: When they put the SAKe bricks in a liquid with the right pH, they didn't just clump into a mess. They started dancing and snapping together.
• The Sheet: Within minutes, they formed a giant, flat sheet that looked like a honeycomb. These sheets were huge—up to 5 micrometers long (which is like a whole city block if the bricks were the size of a house!).
• The Control: When they removed the "magnetic feet" (the Histidines), the bricks just floated around or made messy piles. The feet were essential.

4. Why This Matters

Why do we care about a protein that makes a flat sheet?

• Biosensors: Imagine a security camera that can detect a virus. If you can lay down a perfect, flat carpet of these protein bricks, you can attach a "trap" to the top of every single brick. This makes the sensor incredibly sensitive because there are no gaps where the virus could hide.
• Catalysis: You could turn this protein sheet into a factory floor where chemical reactions happen efficiently.
• Stability: Because the SAKe brick is so strong (thanks to the "time machine" design), it won't fall apart when you try to use it in real-world applications.

Summary

The scientists took a wobbly, natural protein, used computer design and ancient DNA to make a super-stable, six-sided brick, and added magnetic feet to the bottom. When they dropped these bricks into a slightly acidic bath, they automatically snapped together into a perfect, flat honeycomb sheet.

This is a major step forward because it gives us a reliable "Lego brick" for building complex, functional surfaces out of proteins, which could revolutionize how we make medical sensors and new materials.

Technical Summary

1. Problem Statement

The nanofabrication of functional protein-based surfaces faces significant challenges:

• Unpredictable Interface Behavior: Proteins often undergo adsorption-induced conformational changes at solid-liquid interfaces, leading to loss of activity and structural heterogeneity.
• Lack of Control: Traditional methods like covalent linkage (e.g., via thiols or amines) result in random protein orientations, low surface coverage, and hinder the dynamic rearrangement necessary for defect-free crystalline growth.
• Denaturation: Physisorption on hydrophobic surfaces often causes protein denaturation.
• Current Limitations: Existing scaffolds (like S-layer proteins) or engineered assemblies often rely on complex modifications, intermediate molecules, or lack the ability to combine high-density 2D assembly with functional versatility (e.g., binding sites or catalytic activity).

There is a critical need for a modular, symmetric protein scaffold that can self-assemble into large, ordered 2D arrays on surfaces while maintaining structural integrity and offering a platform for further functionalization.

2. Methodology

The authors employed a multi-stage computational and experimental approach to design and characterize the SAKe (Symmetric Assembly kelch-like) protein:

• Computational Design & Ancestral Sequence Reconstruction (ASR):

  • Template: The Kelch domain of the human Keap1 protein (PDB: 1ZGK) was selected due to its high symmetry and conserved core.
  • Strategy: Using the RE3Volutionary design procedure, the authors generated fully symmetric C6-symmetric backbones by repeating ancestral sequences derived from the six blades of the Keap1 $\beta$-propeller.
  • Optimization: Models were filtered using Rosetta symmetry docking energy scores and RMSD deviations. Two primary backbones were created: Type A (based on the 2nd blade, 10 AA loops) and Type B (based on the 6th blade, 6 AA loops).
  • Loop Engineering: To assess modularity, loop variants (L1, L2, L3) were generated using randomized sequences based on nanobody CDR motifs, ensuring the inclusion of Tyrosine and Histidine residues.

• Experimental Characterization:

  • Expression & Purification: Proteins were expressed in E. coli BL21 (DE3) and purified via Ni-NTA affinity chromatography followed by size-exclusion chromatography (SEC).
  • Stability Analysis: Circular Dichroism (CD) spectroscopy was used to determine melting temperatures ($T_m$). X-ray crystallography confirmed the structural accuracy of the designs.
  • Self-Assembly Engineering:
    • pH Sensitivity: pH titration experiments identified the isoelectric point (pI) and conditions for spontaneous assembly.
    • Surface Anchoring: To prevent vertical stacking (crystallization) and promote 2D monolayers, bis-histidine clamps were engineered into the bottom loops. These residues facilitate metal coordination (Zn$^{2+}$) and interaction with the negatively charged mica surface.
    • Variants: Key variants included S6BE (wild-type-like), S6BE-3HH (histidines on blades 2, 4, 6), and S6BE-6HH (histidines on all 6 blades).

• Imaging & Analysis:

  • AFM: Amplitude-modulated Atomic Force Microscopy (in liquid) was used to visualize assembly on muscovite mica.
  • DLS: Dynamic Light Scattering assessed solution-phase aggregation.
  • X-ray Diffraction: Used to solve crystal structures of the self-assembled variants.

3. Key Contributions

• SAKe Scaffold: Introduction of a new, highly stable, pseudo-symmetric protein building block based on the Kelch fold.
• Modular Design: Demonstration that the top loops can be extensively mutated (up to 15 AA) while retaining the core structural integrity, enabling the creation of functional binding interfaces.
• Surface-Mediated Assembly Mechanism: Discovery that engineering specific histidine residues on the protein's "bottom" face allows for controlled, pH-dependent self-assembly into 2D monolayers on mica, preventing the formation of 3D crystals.
• Reversibility: The assembly process is reversible and tunable via pH, allowing for dynamic control over the material state.

4. Key Results

• Thermal Stability: The ancestral reconstruction strategy yielded exceptional stability. The S6BE variant exhibited a melting temperature ($T_m$) of >95 °C, significantly higher than the natural Keap1 template ($T_m$ ~44.1 °C). Even variants with longer, more flexible loops retained stability above 50 °C.
• Structural Integrity: All designed proteins crystallized rapidly (days) with high resolution (1.3–1.95 Å), confirming that the computational models accurately predicted the folded structures.
• Solution Assembly:
  • Wild-type S6BE formed crystals at pH < pI (around pH 4) but required a week to grow.
  • Histidine-engineered variants (S6BE-3HH, S6BE-6HH) assembled faster (within a day) and showed increased stability against pH variations (up to pH 7 for S6BE-6HH).
  • Metal coordination (Zn$^{2+}$) was tested but found to be unnecessary for assembly; in fact, it led to non-ordered aggregates at high concentrations.
• Surface Assembly (Mica):
  • S6BE-6HH formed large, well-defined, hexagonal 2D arrays (up to 5 $\mu$m in length) on muscovite mica at pH 4 and 5.
  • Mechanism: The positively charged, protonated histidine residues at the bottom loops anchor the protein to the negatively charged mica surface. This anchoring, combined with lateral protein-protein interactions, drives the formation of a monolayer.
  • Symmetry: High-resolution AFM confirmed a C6 symmetry lattice with a repeat distance matching the protein dimensions.
  • pH Dependence: As pH increased (deprotonating histidines), the protein-surface interaction weakened, leading to disassembly. S6BE-6HH assemblies disappeared completely at pH 7.
  • Control: Variants lacking the full bis-histidine clamp (S6BE-3EH, S6BE-3HR) failed to form ordered surface assemblies, proving the necessity of the specific interface design.

5. Significance

• Platform for Functional Materials: SAKe provides a robust, modular platform for creating functional 2D protein materials. The top loops remain exposed to the solvent, allowing for the attachment of enzymes, antibodies, or catalytic sites without disrupting the assembly.
• Overcoming Interface Challenges: By utilizing physisorption driven by specific electrostatic interactions (His-Mica) rather than random covalent binding, the method preserves protein orientation and activity.
• Scalability: The ability to form large (micrometer-scale) ordered arrays suggests potential for scalable nanofabrication.
• Applications: This technology opens avenues for advanced biosensors (high surface coverage, oriented binding sites), on-surface catalysis, and the creation of bio-hybrid nanomaterials with tunable periodicity.

In conclusion, the paper successfully demonstrates that rational design combined with ancestral sequence reconstruction can yield a protein scaffold (SAKe) that overcomes traditional limitations of protein surface assembly, offering a versatile tool for the next generation of nanomaterials.