The cystic fibrosis transmembrane conductance regulator (CFTR) is an ATP-gated chloride channel whose opening is driven by ATP binding and hydrolysis at two cytosolic nucleotide-binding domains (NBDs). A 2024 preprint (Saad-Falcon et al., arXiv:2404.09281) introduced an ensemble electrostatic analysis - the molecular ambiguity function (MAF), a cross-correlation of the protein's electrostatic potential with a translated ATP probe over molecular dynamics trajectories - and reported a favorable electrostatic potential gradient of order tens of kT drawing a free ATP toward the competent binding site when that site is unoccupied, while explicit solvent interactions largely wash out this signal in the net interaction. The study leaves open the central question formulated here: whether and how CFTR's internal conformational state - occupancy of the ATP-binding site, gating state, and phosphorylation state - quantitatively alters that external free-energy landscape, and whether such state-dependent electrostatic cues translate into reproducible ATP approach pathways and altered binding kinetics under physiological ionic screening.

Background

CFTR (cystic fibrosis transmembrane conductance regulator) is a membrane protein of the ATP-binding cassette (ABC) transporter superfamily that functions as an epithelial chloride channel; its dysfunction causes cystic fibrosis. Unlike most ABC proteins, CFTR is an ion channel: its two cytosolic nucleotide-binding domains (NBDs) - NBD1 and NBD2 - form two ATP-binding pockets at their dimer interface, a "degenerate" site (site 1) that binds but barely hydrolyzes ATP and a "consensus" or "competent" site (site 2, ABS2) at which ATP binding and hydrolysis drive the channel's gating cycle (the programmed sequence of closed and open pore states). Channel activation additionally requires phosphorylation - the enzyme-mediated attachment of phosphate groups - of the intrinsically disordered regulatory (R) domain by protein kinase A (PKA). High-resolution cryo-electron microscopy structures (structures determined from images of frozen protein samples), such as the ATP-bound phosphorylated structure determined by Zhang, Liu and Chen (PNAS 2018), deposited in the Protein Data Bank as entry 6MSM, made all-atom molecular dynamics (MD) simulation of CFTR feasible.

The nucleotide-binding sites sit buried at the NBD dimer interface, and how cytosolic ATP molecules actually reach them is a long-standing open problem in CFTR and ABC-transporter biophysics: the field's standard review (Csanády, Vergani and Gadsby, Physiological Reviews 2019) states that the pathways ATP takes to arrive at the consensus binding sites are unknown, and records a postulated "ATP reentry pathway" involving separation of the NBD dimer interface around site 2 uncoupled from pore closure. Most kinetic treatments assume ATP reaches the sites by diffusion with random collisions.

Saad-Falcon et al. (arXiv:2404.09281, 2024) tested a complementary hypothesis: that long-range electrostatics steer ATP toward CFTR at the mesoscale of tens of ångströms. They simulated CFTR (built from PDB entry 6MSM, with the E1371Q catalytic-inactivation mutation - a replacement of the glutamate at position 1371 that abolishes ATP hydrolysis - reverted to the wild-type residue, in a membrane of POPC, a standard phospholipid used to build model bilayers, with explicit water and 150 mM NaCl) with a free ATP placed in solution near the intracellular face, in two ensembles of ten trajectories each - one with the competent site occupied, one unoccupied - and evaluated the molecular ambiguity function (MAF), an electrostatic cross-correlation measure developed by the same group (Allen et al., NAECON 2018) that, by analogy with the radar ambiguity function, maps the interaction energy between the protein's charges and an ATP probe translated over a grid, averaged over the MD ensemble. They found that with the site unoccupied the CFTR-ATP MAF shows a clear favorable gradient toward the active site (about −30 kT-30\,kT at larger distances, versus values tending toward 0 kT0\,kT when the site is occupied), that solvent-ATP interactions are noisier but are the dominant energetic "backdrop", and that in the net (protein + solvent + membrane) interaction the solvent largely washes out the CFTR-directed gradient. The Debye screening length of water (the distance over which dissolved ions screen out a static electrostatic field, about 7-10 Å under physiological conditions) is derived for static fields at experimental timescales, so the transmission of protein-originated electrostatic cues under physiological ionic conditions and timescales remains unsettled. The authors explicitly flag as an open question "whether the interior state of the protein influences the external free energy landscape (MAF), which may alter affinity for ATP or its path to the (competent) binding site". The study itself is computational: it observes more generally that because CFTR function can be studied with high resolution using the patch-clamp technique (electrophysiological recording of single-channel currents through a membrane patch, which resolves CFTR gating kinetics), linking such functional analysis with molecular-dynamics structural analysis and with methods such as the MAF positions the field to answer these questions.

Problem Statement

Determine whether and how the internal conformational state of CFTR - including the occupancy of the competent ATP-binding site, the gating state (whether the channel pore is closed or open), and the phosphorylation state of the regulatory (R) domain - influences the external, mesoscale (tens of ångströms) electrostatic free-energy landscape that a cytosolic ATP molecule experiences while approaching the protein. The question, as posed by the source study and left untouched by later literature, has two jointly linked parts:

  1. Compute or measure, across distinct interior states (at minimum the occupied-versus-unoccupied contrast of the source study, and additionally phosphorylation and gating states), the ensemble electrostatic interaction landscape between cytosolic ATP and CFTR, with quantified uncertainties, and establish which state-dependent differences in that landscape are statistically robust.

  2. Determine whether such state-dependent electrostatic differences translate into physical consequences for ATP access: specific, reproducible approach pathways for ATP to the consensus nucleotide-binding site, and altered ATP binding kinetics - where the electrostatic cue must be shown to survive physiological solvent and ion screening (the source study's net interaction is dominated and largely washed out by explicit solvent, and the Debye screening picture applies to static fields at experimental timescales) rather than being an artifact of short-timescale, low-concentration simulation.

The open issue is whether CFTR's interior state regulates ATP access through its external electrostatic field, connecting the internal gating machinery to the external-facing energetics of its ligand.

References

  1. Alex Saad-Falcon, Mark Bolding, James Dee, Ryan S. Westafer, Douglas R. Denison, Nael McCarty, William D. Hunt, "Applying an electrostatic cross-correlation to the CFTR-ATP interaction", arXiv:2404.09281 (2024), DOI: 10.48550/arXiv.2404.09281, https://arxiv.org/abs/2404.09281
  2. László Csanády, Paola Vergani, David C. Gadsby, "Structure, Gating, and Regulation of the CFTR Anion Channel", Physiological Reviews 99(1):707-738 (2019), DOI: 10.1152/physrev.00007.2018, https://doi.org/10.1152/physrev.00007.2018
  3. Kenneth W. Allen, William D. Hunt, Jonathan D. Andreasen, John D. Farnum, Alex Saad-Falcon, Ryan S. Westafer, Douglas R. Denison, "Rigorous Approach to Simulate Electromagnetic Interactions in Biological Systems", NAECON 2018 - IEEE National Aerospace and Electronics Conference, pp. 491-495 (2018), DOI: 10.1109/NAECON.2018.8556724, https://doi.org/10.1109/NAECON.2018.8556724
  4. Zhe Zhang, Fangyu Liu, Jue Chen, "Molecular structure of the ATP-bound, phosphorylated human CFTR", Proceedings of the National Academy of Sciences USA 115(50):12757-12762 (2018), DOI: 10.1073/pnas.1815287115, https://doi.org/10.1073/pnas.1815287115
  5. Ahmad Elbahnsi, Jean-Paul Mornon, Isabelle Callebaut, "Mechanistic Insights into Cystic Fibrosis Transmembrane Conductance Regulator Function from Molecular Dynamics Analysis of Electrostatic Interactions", Journal of Chemical Information and Modeling 66(10):5990-6010 (2026), DOI: 10.1021/acs.jcim.5c02661, https://doi.org/10.1021/acs.jcim.5c02661
  6. Karol Fiedorczuk, Iordan Iordanov, Csaba Mihályi, András Szöllősi, László Csanády, Jue Chen, "The structures of protein kinase A in complex with CFTR: Mechanisms of phosphorylation and noncatalytic activation", Proceedings of the National Academy of Sciences USA 121(46):e2409049121 (2024), DOI: 10.1073/pnas.2409049121, https://doi.org/10.1073/pnas.2409049121