The GFP tag, the molecular staple, and the conduction pore.
In cell-surface assays, the channel is tagged at its N-terminus with Green Fluorescent Protein to separate trafficking from conduction. Here is the 1.41-million-atom molecular dynamics test of whether that tag distorts the channel or alters the Brugada variant mechanism.
Is the sodium channel broken, or just missing from the cell surface?
When cells carrying the SCN5A R104Q variant are measured in an electrical patch-clamp assay, sodium current drops to 68.3 percent of normal (O'Neill et al. 2022, PMID 35305865). But standard electrical testing records the total flow through the whole cell. It cannot distinguish between two opposite biological realities:
To measure how many channels reach the surface, researchers fuse Green Fluorescent Protein (GFP, a 27 kDa fluorescent tag from jellyfish) to the channel's N-terminal tail. Flow cytometry lasers then count surface glow independently from electrical current. But does attaching a 238-amino-acid fluorescent tag onto the channel tail pinch the pore shut or alter the mutation's behavior?
How do you tell whether a channel is broken, or simply missing?
When a laboratory measures patch-clamp sodium current in cells expressing SCN5A R104Q, current drops to 68.3 percent of normal (O'Neill et al. 2022, PMID 35305865). Standard electrophysiology records the sum of all ions crossing the cell membrane. It cannot distinguish between two fundamentally different causes:
- "Broken Doors" (Pore Disruption): The channels reach the cell membrane normally, but the pore is structurally defective or collapsed and cannot conduct sodium ions.
- "Missing Doors" (Trafficking Failure): The conduction pore is completely healthy and functional, but the mutated protein fails intracellular quality control in the endoplasmic reticulum and never reaches the surface.
To settle this experimentally, researchers fuse an N-terminal Green Fluorescent Protein (GFP, ~27 kDa, 238 amino acids) to Nav1.5. Flow cytometry can then count surface fluorescence independently from whole-cell current. But this introduces a new scientific concern: does fusing a bulky fluorescent protein directly to the cytoplasmic N-terminal domain perturb the channel or artificially distort the R104Q phenotype?
1.41 million atoms, in a real membrane bilayer.
Rather than assuming the tag is inert, I reconstructed the full chimeric fusion down to atomistic resolution:
Proof of "Missing Doors": The channel can conduct if delivered.
The central finding of this matched simulation is definitive:
Even as the N-terminal Gln104 sidechain unhinges from Asp84 and the N-terminal domain RMSD expands past 3.1 Å, the DEKA selectivity filter remains open at 5.18-5.37 Å and fully hydrated with 12-14 waters. The pore is identical to Wild-Type.
This decisively supports the trafficking defect ("Missing Doors") hypothesis over pore collapse ("Broken Doors"). If a therapeutic intervention, such as a pharmacological chaperone, temperature rescue, or molecular staple stabilizer, enables the mutant channel to pass endoplasmic reticulum quality control and reach the sarcolemma, the channel will conduct physiological sodium current.
Furthermore, the presence of the N-terminal GFP tag does not perturb the transmembrane core or selectivity filter. Experimentalists using GFP-tagged constructs in surface flow cytometry can trust that their optical readout reflects genuine trafficking behavior, not tag-induced pore collapse.
How this finding could be disproved in the laboratory:
- If an experimental assay successfully rescues R104Q channels to the cardiomyocyte plasma membrane (verified by surface biotinylation or high-resolution confocal microscopy), but whole-cell patch-clamp records zero or severely reduced single-channel conductance, the "Missing Doors" hypothesis is refuted.
- If longer microsecond-scale atomistic simulations reveal that N-terminal unhinging subsequently transmits an allosteric closing torque through the DI-S1 linker into the pore domain, this finding of pore independence would be limited to short-timescale states.