The problem that Alper Karagöl and Taner Karagöl of Istanbul University set out to solve is familiar to anyone who works on membrane proteins. Human transporters are fragile, unstable and expensive to produce in quantities suitable for structural studies. Bacterial cousins, by contrast, are often far more tractable. The Escherichia coli transporter DtpA, for example, is a stable and well-behaved structural surrogate for PepT1 — but it is still a bacterial protein. The binding pocket inside DtpA differs in crucial ways from the human version, so drug-screening results obtained with the bacterial surrogate do not always translate faithfully to human pharmacology. The obvious fix is to graft the human binding-site residues onto the bacterial scaffold, a strategy sometimes described as humanizing the transporter. In practice, that fix has repeatedly failed, because swapping in non-native side chains disrupts the finely co-evolved packing of the surrounding protein, producing what the authors describe as severe thermodynamic frustration.

The scale of the difficulty is staggering. A transporter’s binding microenvironment involves dozens of residues whose interactions are interdependent: change one, and the energetic consequences ripple through its neighbors. Traditional structure-based engineering confronts a combinatorial explosion — the number of possible compensatory mutations grows exponentially with each humanizing substitution — and laboratory directed evolution, which mutates proteins randomly and screens for improved variants, is slow, expensive and poorly suited to membrane proteins that are hard to express and assay. The Karagöls’ answer was to move the entire evolutionary search into silicon, building what they call a fully in silico co-evolutionary pipeline powered by deep contextual protein language models.

Protein language models, or PLMs, are neural networks trained on vast databases of natural protein sequences. Like large language models that learn the statistical grammar of human text, PLMs learn the grammar of amino acid sequences — which residues tend to appear where, and which combinations are evolutionarily tolerated. The researchers used ESM-2, a 650-million-parameter model developed by Meta researchers and described in Science in 2023, to perform zero-shot mutational profiling. Zero-shot means the model predicts the effect of a mutation it has never explicitly been trained on, simply by asking how likely the mutated sequence is under the statistical patterns it has internalized. The team mapped 19 pharmacologically critical residues from human PepT1 onto the corresponding positions in the DtpA sequence and asked ESM-2 to score each humanizing substitution.

The results were sobering but informative. Fourteen of the primary humanizing mutations received very low likelihood scores — below 10 to the power of minus 5 — indicating that the model considered them essentially incompatible with the rest of the bacterial protein. In evolutionary terms, these substitutions would be deleterious on their own. This is precisely the thermodynamic frustration that has doomed previous humanization attempts: the human residues carry the pharmacological information researchers want, but the bacterial scaffold cannot accommodate them without help. The question became whether that help could be found computationally, without running a single experiment.

To find it, the authors deployed an automated epistatic rescue algorithm. Epistasis refers to the phenomenon in which the effect of one mutation depends on the presence of another — a deleterious change can be buffered, or rescued, by a compensatory mutation elsewhere in the protein. The algorithm swept through the flanking microenvironment of each high-risk substitution, systematically testing nearby positions for secondary mutations that would restore the model’s confidence in the humanized sequence. The heuristic proved computationally efficient and, remarkably, succeeded for all 14 high-risk targets, identifying localized non-native compensatory mutations for each one — all without recourse to traditional molecular dynamics simulations, which are far more expensive.

Two examples illustrate the power of the approach. The substitution Q41R — replacing glutamine with arginine at position 41 — was flagged as highly deleterious, but pairing it with a secondary mutation, Y38D, increased the sequence likelihood assigned to the primary substitution by a factor of 5,215. Even more dramatic was R305F, an arginine-to-phenylalanine change, which was buffered by a companion I304K mutation yielding an 11,614-fold increase in likelihood. The authors are careful to note an important technical caveat: these figures describe changes in sequence likelihood as assigned by the language model, not calculated free energies. The likelihood gains indicate that the model considers the rescued sequences evolutionarily plausible, but they are not direct thermodynamic measurements.

With the rescued variants in hand, the team turned to structural validation. Binding dynamics were assessed through docking calculations, using tools including SwissDock, and through all-atom molecular dynamics simulations of the transporter embedded in a realistic membrane environment, built with the CHARMM-GUI Membrane Builder and run with the GROMACS simulation package using the CHARMM36m force field. These simulations allowed the researchers to examine whether the humanized binding pocket maintained a stable, transport-competent architecture in a lipid bilayer — a critical check, since membrane proteins can misbehave badly when their environment is not modeled faithfully.

The practical payoff could be substantial for drug development. A humanized DtpA surrogate, if it works, would give pharmacologists a stable bacterial scaffold carrying the human PepT1 binding pocket, enabling more faithful screening of peptide-mimetic drugs and inhibitors of the SLC15 transporter family. By computationally rationalizing which humanizing mutations are viable and which compensatory changes make them viable, the framework dramatically reduces the experimental screening space that laboratory researchers would need to explore. The authors also point toward applications beyond drug screening: engineered peptide transporters are relevant to live biotherapeutic products, including probiotic strains of E. coli Nissle 1917 that are being developed as delivery vehicles for therapeutic peptides.

The team is careful about what has and has not been achieved. The humanized surrogate exists only as a computational construct so far, and the authors state explicitly that experimental validation of folding, binding and transport is required before the design can be used in pharmacological screening. All the statistical and computational analyses, including AlphaFold calculations and the code to regenerate them, have been made publicly available on GitHub to ensure reproducibility. Still, the study offers a striking preview of how protein engineering may be done in the years ahead: rather than mutating and screening in the lab, researchers may first ask a language model which mutations evolution would reject, then let an epistatic rescue algorithm find the partners that make the impossible tolerable — and only then head to the bench, with a shortlist of designs that artificial intelligence has already vetted against billions of years of evolutionary constraint.

Subject of Research: Computational humanization of the bacterial peptide transporter DtpA using protein language models and epistatic rescue to create a surrogate of human PepT1 for drug screening

Article Title: In silico directed evolution of humanized peptide transporters via computational epistatic rescue

Article References: In silico directed evolution of humanized peptide transporters via computational epistatic rescue. (n.d.). https://doi.org/10.1007/s11030-026-11710-3

Image Credits: AI Generated

DOI: 10.1007/s11030-026-11710-3

Keywords: PepT1, SLC15A1, DtpA, protein language models, ESM-2, epistasis, directed evolution, membrane transporters, drug bioavailability, molecular dynamics, protein engineering, in silico mutagenesis