Discovery of Plasmodium falciparum SR12 as a GOLD-Domain seven transmembrane protein regulating GPCR trafficking in mammalian cells

This study identifies *Plasmodium falciparum* SR12 as a GOLD-domain seven-transmembrane protein that structurally resembles mammalian GOST proteins and functions as a chaperone to promote GPCR trafficking to the plasma membrane in mammalian cells.

The Gist

The Big Picture: A Malaria Parasite's "Traffic Cop"

Imagine the malaria parasite (Plasmodium falciparum) as a tiny, invasive construction crew trying to build a house inside your red blood cells. To do this, they need to move their tools and materials around efficiently.

For years, scientists have been trying to find new ways to stop malaria because the old medicines (like chloroquine) aren't working as well as they used to. The researchers in this paper decided to look at a specific protein inside the parasite called SR12. They wanted to know: What does this protein actually do?

The Mystery: Is it a Radio or a Delivery Truck?

When scientists first looked at SR12, they saw it had a shape that looked very much like a GPCR (G-Protein Coupled Receptor).

• The Analogy: Think of a GPCR as a radio antenna on the outside of a cell. Usually, when a signal (like a hormone) hits the antenna, it sends a message inside the cell to start an action (like "release calcium!" or "move this muscle!").

Because SR12 looked like an antenna, scientists thought, "Maybe this parasite protein is a radio that listens to signals."

However, the researchers found something surprising. SR12 doesn't actually have the right "wiring" to act as a radio. It lacks the specific switches needed to send messages. So, if it's not a radio, what is it?

The Discovery: The "Gold" Delivery Driver

Using advanced computer modeling (like a 3D printer for proteins), the team discovered that SR12 actually looks like a member of a special family of proteins called GOST proteins.

• The Analogy: Instead of being a radio antenna, SR12 is more like a delivery truck driver or a traffic controller.
• These "GOST" proteins have a special badge on their front called a GOLD domain. Think of the GOLD domain as a GPS navigation system that knows exactly where to drop off packages inside the cell.

The researchers found that SR12 is essentially a delivery driver that helps move other proteins from the "factory" (inside the cell) to the "storefront" (the cell surface).

The Experiment: Helping the Neighbors

To prove this, the scientists put the malaria protein (SR12) into human cells (HEK293 cells) and watched what happened to two specific human "radios" (receptors called PAR1 and M3R).

• Without SR12: The human radios were getting lost. They were stuck in the factory or the warehouse, never making it to the storefront where they could do their job.
• With SR12: When the malaria protein was added, it acted like a helpful chaperone. It grabbed the human radios, put them on the delivery truck, and drove them straight to the cell surface.

The Result: The human radios were now 25% to 39% more likely to be found on the surface of the cell. Because they were in the right place, they could send signals much more effectively.

The "Aha!" Moment: Why Does This Matter?

The paper suggests two main possibilities for why the malaria parasite has this protein:

1. The Self-Help Theory: The parasite uses SR12 to help its own internal proteins get to the right places so the parasite can survive and grow.
2. The Hijacking Theory: The parasite might use SR12 to mess with the human cell's traffic. By shoving human receptors to the surface, the parasite might be "hijacking" the human cell's communication system to make the cell more friendly to the parasite's needs.

The Takeaway

This paper is a detective story. The scientists solved the mystery of a protein that looked like a radio but turned out to be a delivery driver.

• The Problem: Malaria is becoming resistant to drugs.
• The Clue: A protein called SR12.
• The Solution: SR12 isn't a signal receiver; it's a traffic manager. It helps move proteins around the cell.
• The Future: Now that we know SR12 is a "traffic cop" essential for the parasite's logistics, scientists might be able to design new drugs that jam the traffic. If the delivery trucks stop moving, the parasite can't build its house, and the infection could be stopped.

In short: The malaria parasite has a secret delivery driver (SR12) that keeps its operations running smoothly. If we can fire that driver, we might finally win the war against malaria.

Technical Summary

1. Problem Statement

• Context: Malaria, caused by Plasmodium falciparum, remains a critical global health threat due to increasing resistance to conventional antimalarials (e.g., artemisinin, chloroquine). New drug targets are urgently needed.
• Knowledge Gap: While P. falciparum possesses several G protein-coupled receptor (GPCR)-like proteins (SR1, SR10, SR12, SR25), the parasite lacks canonical heterotrimeric G proteins. Consequently, the signaling mechanisms and physiological roles of these 7-transmembrane (7TM) proteins are poorly understood.
• Specific Focus: Serpentine Receptor 12 (SR12) shares sequence homology with human orphan GPCRs (like GPR180) but lacks canonical GPCR signaling motifs (e.g., DRY, PIF, NPY). Its structural classification and functional role in protein trafficking or signaling remain undefined.

2. Methodology

The study employed a multi-disciplinary approach combining computational biology, structural modeling, and cell-based functional assays in mammalian systems.

• Structural Prediction & Modeling:
  • Used AlphaFold2 (AF2) and AlphaFold3 (AF3) to predict the 3D structure of SR12.
  • Performed Molecular Dynamics (MD) simulations (1 µs) in a native-like POPC lipid bilayer to assess conformational stability and the impact of histidine protonation (mimicking acidic organelle environments).
  • Conducted structural homology searches using Foldseek against PDB and AlphaFold-DB to identify structural relatives.
• Cellular Localization (BRET & Microscopy):
  • Utilized Bioluminescence Resonance Energy Transfer (BRET) assays in HEK293 cells. SR12 was tagged with rLuc (donor), and organelle-specific markers (PM, ER, Golgi, Endosomes, Mitochondria) were tagged with rGFP (acceptor) to quantify subcellular distribution.
  • Validated localization via Confocal Microscopy in U2OS cells using GFP10-tagged SR12 and mKO2-tagged organelle markers.
• Functional Trafficking Assays:
  • Co-expressed SR12 with canonical GPCRs (PAR1, M3R, D2R) in HEK293 cells.
  • Measured surface expression and intracellular distribution of these receptors using BRET and ELISA (with HA-tagged receptors).
• Signaling Pathway Analysis:
  • Employed BRET-based biosensors to detect Diacylglycerol (DAG) production and Protein Kinase C (PKC) activation.
  • Stimulated cells with thrombin (PAR1 agonist) or carbamoylcholine (muscarinic agonist).
  • Used Gαq/11 Knockout (KO) cell lines and rescue experiments (re-expression of Gαq/11) to confirm pathway dependence.
  • Measured intracellular calcium ($[Ca^{2+}]_i$) using an Obelin-based biosensor.

3. Key Contributions

• Structural Classification: Identified SR12 as a member of the GOLD-domain seven-transmembrane helix (GOST) protein family. It possesses an N-terminal Golgi Dynamics (GOLD) domain fused to a 7TM bundle, structurally homologous to human proteins like GPR180, GPR107, GPR108, and TMEM87A.
• Mechanism of Action: Demonstrated that SR12 acts as a chaperone-like protein that enhances the trafficking of GPCRs to the plasma membrane and secretory pathway compartments (Golgi, endosomes), rather than functioning as a canonical GPCR itself.
• Structural Dynamics: Revealed that protonation of luminal-exposed histidines induces conformational changes in SR12, specifically expanding the extracellular pocket via ECL2 reorientation, suggesting a pH-dependent regulatory mechanism for cargo binding.

4. Key Results

A. Structural Insights

• AlphaFold Models: Both AF2 and AF3 predicted a high-confidence structure (pLDDT > 70) comprising an N-terminal globular GOLD domain (residues ~88–230) and a C-terminal 7TM bundle (residues ~231–470).
• Homology: Foldseek identified human GPR180 as the closest structural homolog. The GOLD domain adopts a $\beta$-sandwich fold.
• MD Simulations:
  • The structure remained stable, but secondary structure content in low-confidence regions evolved during simulation.
  • Protonation Effect: Simulating protonated histidines (acidic conditions) caused increased flexibility in the GOLD domain and extracellular loops (ECL1, ECL2). Crucially, this caused a rotation of bulky residues (Leu361, Tyr362, Leu365) in ECL2, expanding the extracellular pocket volume, potentially facilitating ligand or cargo entry.

B. Subcellular Localization

• SR12 is predominantly localized in the Endoplasmic Reticulum (ER), Golgi apparatus, and mitochondria, with lower abundance at the plasma membrane compared to canonical GPCRs (PAR1, M3R).
• Confocal microscopy confirmed colocalization with Golgi and ER markers.

C. Regulation of GPCR Trafficking

• Co-expression of SR12 significantly increased the abundance of PAR1 and M3R in the secretory pathway:
  • Plasma Membrane: +13% (PAR1) and +15% (M3R).
  • Golgi: +18% (PAR1) and +20% (M3R).
  • Endosomes: Significant increases in early and recycling endosomes.
• ELISA confirmed that SR12 co-expression boosted surface expression of HA-tagged M3R by ~58%.

D. Potentiation of Signaling

• DAG and PKC: SR12 expression significantly potentiated thrombin-induced DAG production and PKC activation in HEK293 cells.
• Dependence on Host GPCRs:
  • In PAR-KO cells, SR12 expression failed to induce thrombin-stimulated DAG production, proving SR12 is not a thrombin receptor itself.
  • In PAR-KO cells re-expressing PAR1, SR12 restored and enhanced the signaling response.
• Mechanism: The potentiation is Gαq/11-dependent. The effect was blocked by the Gαq/11 inhibitor YM-254890 and absent in Gαq/11 KO cells.
• Specificity: SR12 also potentiated signaling for other endogenous GPCRs (e.g., muscarinic receptors responding to carbamoylcholine), suggesting a general chaperone role for 7TM proteins.
• Steric Constraints: Structural overlay suggests SR12 cannot directly couple to G-proteins due to steric clashes between its Helix 8 and the G-protein, and the absence of canonical switch motifs.

5. Significance and Implications

• Parasite Biology: This study identifies the first GOST protein in the Plasmodium falciparum genome. It suggests SR12 may function as a specialized chaperone to assist in the trafficking of other parasite 7TM proteins (like SR1, SR10, SR25) through the complex secretory pathway of the parasite (crossing multiple membranes: plasma membrane, parasitophorous vacuole, erythrocyte membrane).
• Host-Parasite Interaction: SR12 may interfere with host cell GPCR trafficking, potentially hijacking host signaling pathways (e.g., Ca²⁺, cAMP) to promote parasite survival and development within hepatocytes and erythrocytes.
• Human Medicine: The findings provide new insights into the function of human orphan GPCRs (GPR180, GPR107, GPR108) within the GOST family, linking them to cargo transport and Golgi dynamics.
• Therapeutic Potential: Understanding the unique trafficking mechanisms of Plasmodium proteins could reveal novel drug targets to disrupt parasite development without affecting human GPCR signaling.

Limitations: The study relies on heterologous expression in mammalian cells. The authors acknowledge the need for future experiments in Plasmodium-infected cells to confirm these functions in the native parasite environment, as P. falciparum lacks canonical GPCRs. Additionally, the lack of experimental structural data (CryoEM/X-ray) for Plasmodium proteins limits the absolute confirmation of the AlphaFold models.