Basic transcription factor 3 is a eukaryotic protein that in humans is encoded by the BTF3 gene. They are very important to the development of many eukaryotic organisms such as in humans, plants, fungi, and protists. Some of the functions it plays a part in are gene expression regulation, cell proliferation control, protein homeostasis maintenance, and stress response modulation. BTF3 can be produced in both transcriptionally active and transcriptionally inactive forms through alternative splicing. This helps it to work in multiple cellular compartments and regulatory pathways.

BTF3 evolutionary conservation shows how important its involvement in gene control and cellular homeostasis is. Overtime, it's known for its role in cancer progression and metastasis. This includes gastric cancer, colorectal cancer, pancreatic ductal adenocarcinoma, and nasopharyngeal carcinoma.

More current studies has enhanced our understanding of BTF3 beyond its original designation as a generic transcription factor. Early biochemical studies indicated that BTF3 forms a stable, functional complex with RNA polymerase II, which is required for proper promoter binding and transcription initiation. BTF3 is the β-subunit of the Nascent Polypeptide-Associated Complex (NAC). It attaches to ribosomes at the nascent polypeptide exit tunnel, preventing premature interactions and protein misfolding. This establishes BTF3 as a key regulator of co-translational protein quality regulation.

BTF3 is a compact α-helical protein of about 180-210 amino acids, which depend on the isoform produced. The protein has a three-dimensional structure and can be roughly classified into two functional regions. The first is the N-terminal region. This has about the first 90 amino acids and is responsible for BTF3's traditional classification as a transcription factor. Using cDNA sequencing and biochemical research, it was shown that BTF3 is needed for promoter binding and contributes to the development of the pre-initiation complex. The second is BTF3's C-terminal region, this has the β-NAC domain. It's an important structural and functional component of the Nascent Polypeptide-Associated Complex and it ranges from 90-200. The NAC complex binds to ribosomes near the polypeptide exit tunnel, which prevents fold error and promotes correct nascent-chain targeting.

In humans, there are two major isoforms, which are BTF3a and BTFb. The BTF3a is the longer protein that is transcriptionally active, while the BTFb is the shortened one that is transcriptionally inactive.

BTF3's amino acid composition is has α-helical residues. In humans, BTF3 has leucine (9.9%), alanine (9.9%), and lysine (9.9%). There's a significant concentration of basic amino acids, lysine and arginine, especially in the N-terminal region. It allows interactions with DNA, transcriptional cofactors, and the acidic surfaces of the transcription machinery.

The C-terminal NAC region has hydrophobic and aromatic residues that are important for ribosome docking and stabilizing interactions with α-NAC. Post-translational changes to the protein affect its nuclear place as well as its interaction with partner proteins and having these changes likely affects the BTF3's actions.

BTF3 is a soluble, α-helical protein that is stable under physiological buffer conditions. Its solubility was first shown during early purification operations, when BTF3 was successfully identified as a soluble transcription factor that remains in solution while building complexes with RNA polymerase II. A structural and biochemical investigation on NAC found that the β-NAC subunit is soluble and interacts to the ribosomal exit tunnel to avoid protein folding errors.