TaHL-PTM: Post-Translational Modification Prediction in Proteins via Target-Hooked Discriminative Fine-Tuning of Decoder-only Protein Language Models

Post-translational modifications (PTMs) regulate protein function, making accurate residue-level PTM prediction essential for understanding cellular mechanisms and disease pathways. While decoder-only protein language models (PLMs) pretrained with the causal language modeling (CLM) objective have driven breakthroughs across various bioinformatics tasks, their potential for PTM prediction remains largely underexplored. CLM-based PLMs that rely on Byte-Pair Encoding (BPE) for tokenization, such as ProtGPT2, introduce intra-token label collision by merging multiple amino acids with conflicting labels into a single token, creating a major bottleneck for residue-level tasks. To overcome this, we propose TaHL-PTM (Target-Hooked Low-rank adaptation for PTM prediction), a novel framework that integrates target-hooked tokenization with site-directed discriminative LoRA fine-tuning. Target-hooked tokenization constrains tokenization around the candidate residue using dedicated marker tokens to eliminate intra-token label collision while preserving the surrounding sequence context, whereas the proposed discriminative objective repurposes the standard generative CLM objective for residue-level PTM classification by directly optimizing the separation between modified and unmodified sites. We benchmark TaHL-PTM across six distinct PTM tasks on ProtGPT2 and ProGen2 models. TaHL-PTM consistently improves MCC, with the largest gain of up to +0.11 for tyrosine phosphorylation (0.34 to 0.45), alongside improvements in F1, AUROC, and AUPR. Performance gains are more pronounced for collision-affected samples, validating the effectiveness of target-hooked tokenization, while consistent improvements across both BPE-based and per-residue-based causal PLMs demonstrate that the proposed framework generalizes across models with different pretraining tokenization schemes.