The variant details are as follows: Protein change: p.I294T (Ile294Thr) DNA change: c.881T>C Zygosity: HeterozygousPopulation Frequency: < 0.01% Structural & In-Silico Context:AlphaFold2: The residue ILE 294 is located in a highly confident region (pLDDT > 90) within a hydrophobic/transmembrane-like helix segment. The wild-type residue is a branched, hydrophobic Isoleucine, which is substituted by a polar Threonine (p.I294T).Predictive Tools: The in-silico predictors show highly discordant/uncertain results overall (REVEL: 0.47, AlphaMissense: 0.58, MetaLR: 0.22/Benign). However, conservation- and evolutionary-based tools flag it as deleterious (DANN: 0.99, MutationTaster: 1.0, PrimateAI: 0.9). SpliceAI shows a score of 0.Given that this is a single amino acid substitution (hydrophobic to polar) in a well-conserved structural domain, but the protein length remains intact and the other allele is wild-type:How likely is this specific structural disruption (Ile to Thr) to cause a severe local destabilization versus a mild conformational shift?Based on the discordant in-silico metrics and the AlphaFold model, would you lean towards predicting a partial loss of function (attenuated binding/stability) or is a complete localized loss of function (e.g., total failure to interact with structural partners like GABBR) plausible for the mutated protein subunit itself?I would highly appreciate your thoughts on the structural physics of this specific residue exchange or any experience with similar discordant predictor profiles in AJAP1.
Thank you!
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I don't think anyone can answer your question with a high degree of certainty, nor will you be able to do it without an experiment. You pretty much laid out all the arguments why that is the case. BLOSUM62 matrix gives -1 to an I -> T substitution, which is slightly worse than neutral but not obviously deleterious. I would not expect any protein to functionally fall apart because of this mutation, but its exact effect will depend on the location. That's always the case.
Here is what AI says, and I agree with all the points.
Physical and Chemical Differences
- Hydrophobicity: Isoleucine is deeply hydrophobic and prefers the protein core; threonine carries a polar hydroxyl group (—OH) that prefers aqueous, solvent-exposed environments or hydrogen-bonding networks.
- Steric bulk/Structure: Both share a branched beta-carbon structure with two chiral centers, meaning their spatial footprints are somewhat similar, but introducing a polar hydroxyl into a buried hydrophobic pocket can destabilize protein folding.
- Functional impact: Replacing a buried isoleucine with threonine can cause local structural strain or create an unwanted cavity/water- trapping site, whereas replacing a surface threonine with isoleucine removes potential phosphorylation or hydrogen-bonding sites.
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