A LoRA adapter that converts the generative (decoder-only) protein language model hugohrban/progen2-base into a bidirectional sequence encoder for producing fixed-length protein embeddings, following the LLM2Vec recipe adapted to proteins.

The adaptation has three ingredients:

Bidirectional attention: the causal triangular attention mask is removed so every residue attends to the whole sequence.
Two-stage training: masked next-token prediction (MNTP) first, then a separate SimCSE contrastive stage (same sequence encoded twice under dropout = a positive pair), resuming the MNTP adapter.
LoRA: only low-rank adapters are trained; the base weights are frozen.

Results: full benchmark (frozen encoder + linear probe)

Evaluated on all 9 tasks from the TAPE (Rao et al. 2019) and ProteinBERT (Brandes et al. 2022) benchmark suites. Protocol: freeze the encoder, take a fixed representation (mean-pooled for sequence-level tasks, per-residue for token-level), fit a closed-form linear probe on the train split, and score the test split. "Baseline" = the same bidirectional ProGen2 without this adapter.

Validation vs. held-out. One of the nine tasks, Stability (a protein-stability regression benchmark from TAPE), was used to pick the recipe: across a 10-config sweep, the hyperparameters scoring highest on Stability were kept. The other 8 tasks were then evaluated once on that fixed configuration, so their numbers are held-out generalization, not values the recipe was tuned to maximize.

Sequence-level

Task	metric	baseline	this adapter	Δ
Stability	Spearman ρ	0.403	0.661	+0.258
Fluorescence	Spearman ρ	0.157	0.367	+0.210
Remote homology	accuracy	0.063	0.102	+0.039
Fold class.	accuracy	0.099	0.210	+0.112
Signal peptide	accuracy	0.838	0.931	+0.093
Neuropeptide	accuracy	0.691	0.932	+0.240

Token-level (per-residue)

Task	metric	baseline	this adapter	Δ
Secondary structure (SS3)	Q3 accuracy	0.535	0.590	+0.056
PTM (phosphosite)	ROC-AUC	0.908	0.920	+0.012
Disorder	ROC-AUC	0.745	0.822	+0.077

The adapter improves the frozen representation on every task, at both the sequence and per-residue level. PTM and disorder are reported as ROC-AUC because they are heavily imbalanced (majority class ≈ 0.98), making raw accuracy uninformative. Absolute values are those of a frozen linear probe (an embedding-quality measurement), not task-specific fine-tuning.

Usage

python

import torch, torch.nn.functional as F
from transformers import AutoModelForCausalLM, AutoTokenizer
from peft import PeftModel

BASE = "hugohrban/progen2-base"
ADAPTER = "ratishsp/progen2-base-bidirectional-llm2vec"  # this repo

tok = AutoTokenizer.from_pretrained(BASE, trust_remote_code=True)
if tok.pad_token_id is None:
    tok.pad_token = tok.eos_token

model = AutoModelForCausalLM.from_pretrained(
    BASE, trust_remote_code=True, torch_dtype=torch.bfloat16,
    attn_implementation="eager",   # required: the bidirectional mask edit needs eager attention
)

# 1) make attention bidirectional: flip each attention module's causal `bias` buffer to all-True
for m in model.modules():
    b = getattr(m, "bias", None)
    if isinstance(b, torch.Tensor) and b.dtype == torch.bool:
        b.fill_(True)

# 2) load the LoRA adapter
model = PeftModel.from_pretrained(model, ADAPTER)
model.eval().cuda()

@torch.no_grad()
def embed(seqs):
    enc = tok(seqs, padding=True, truncation=True, max_length=512, return_tensors="pt").to("cuda")
    h = model(input_ids=enc.input_ids, attention_mask=enc.attention_mask,
              output_hidden_states=True).hidden_states[-1]
    mask = enc.attention_mask.unsqueeze(-1).float()
    pooled = (h * mask).sum(1) / mask.sum(1).clamp(min=1)   # mean pool
    return F.normalize(pooled.float(), dim=-1)

emb = embed(["MKTAYIAKQR", "MVLSPADKTNVKAAW"])

The full, tested implementation is bundled in this repo under code/: training (pretrain.py), the bidirectional conversion and objectives (src/bidir_progen.py: make_bidirectional, mean_pool), and all benchmark evals (eval_protein.py, eval_token.py).

Training details


Base model	`hugohrban/progen2-base` (764M, BSD-3-Clause)
Pretraining data	UniRef50 (`agemagician/uniref50`), streamed
Stage 1 (MNTP)	2,000 steps, max-len 512
Stage 2 (SimCSE)	2,000 steps, temperature 0.10, dropout 0.1, max-len 256
LoRA	r=16, α=32, targets: `qkv_proj`, `out_proj`, `fc_in`, `fc_out`
Optimizer	AdamW, lr 1e-4, warmup 50
Precision	bf16

This was the best of a 10-configuration sweep. Key findings: the MNTP stage is necessary (SimCSE-only transfers far worse), SimCSE temperature 0.10 beat 0.05/0.02, and the short recipe outperformed longer / larger-data runs.

Limitations

Numbers come from a frozen linear probe, not task fine-tuning; they measure embedding quality, so absolute values trail task-specific SOTA (e.g. SS3 Q3 ≈ 0.59 vs ~0.8+ for dedicated predictors).
Sequence-level uses a single mean-pooled vector; no specialised pooling head.
No per-task hyperparameter tuning: the recipe was tuned once, on Stability (see Results).

License & provenance

Adapter weights: BSD-3-Clause, matching the progen2-base base model.
Bundled code (code/): MIT; see code/LICENSE.

Research artifact produced on the DCAI Gefion cluster. The base model and all benchmark datasets (TAPE via AI4Protein/GleghornLab mirrors; ProteinBERT signal-peptide/neuropeptide via GrimSqueaker mirrors; PTM/disorder from the ProteinBERT data repo) are publicly available, so the results are reproducible from public sources.

Framework versions

PEFT 0.12.0
transformers 4.44.2