BTF3

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.^[5]

BTF3
Available structures PDB Ortholog search: PDBe RCSB List of PDB id codes 3LKX, 3MCB
Identifiers
Aliases	BTF3, BETA-NAC, BTF3a, BTF3b, NACB, basic transcription factor 3
External IDs	OMIM: 602542; MGI: 1202875; HomoloGene: 37453; GeneCards: BTF3; OMA:BTF3 - orthologs
Gene location (Human) Chr. Chromosome 5 (human)[1] Band 5q13.2 Start 73,498,408 bp[1] End 73,505,667 bp[1]
Gene location (Mouse) Chr. Chromosome 13 (mouse)[2] Band 13|13 D1 Start 98,446,404 bp[2] End 98,453,514 bp[2]
RNA expression pattern Bgee Human Mouse (ortholog) Top expressed in mucosa of sigmoid colon lactiferous duct human penis left ovary embryo vulva ganglionic eminence oral cavity trabecular bone cartilage tissue Top expressed in epiblast lens ventricular zone ganglionic eminence yolk sac embryo embryo thymus blastocyst uterus More reference expression data BioGPS n/a
Gene ontology Molecular function protein binding RNA binding Cellular component cytoplasm nucleus cytosol nascent polypeptide-associated complex polysomal ribosome Biological process protein transport in utero embryonic development regulation of transcription, DNA-templated transcription by RNA polymerase II transcription, DNA-templated transport Sources:Amigo / QuickGO
Orthologs Species Human Mouse Entrez 689 218490 Ensembl ENSG00000145741 ENSMUSG00000021660 UniProt P20290 Q64152 RefSeq (mRNA) NM_001207 NM_001037637 NM_001393652 NM_001393653 NM_001170540 NM_145455 RefSeq (protein) NP_001032726 NP_001198 NP_001164011 NP_663430 Location (UCSC) Chr 5: 73.5 – 73.51 Mb Chr 13: 98.45 – 98.45 Mb PubMed search [3] [4]
Wikidata
View/Edit Human View/Edit Mouse

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.^[5]

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.^[6] 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.^[7]

Properties

Structure

BTF3 is a compact α-helical protein of about 180-210 amino acids, which depend on the isoform produced.^[8] 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.^[7]

In humans, there are two major isoforms, which are BTF3a and BTFb.^[9] The BTF3a is the longer protein that is transcriptionally active, while the BTFb is the shortened one that is transcriptionally inactive.^[5][6][9]

Composition

BTF3's amino acid composition is has α-helical residues. In humans, BTF3 has leucine (9.9%), alanine (9.9%), and lysine (9.9%).^[10] 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.^[11]

Solubility

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.^[12] 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.^[10]

The ribosome-associated β-NAC component inhibits nascent chain targeting errors and remains soluble in physiological buffer conditions, contributing to the stability of NAC complexes, including BTF3's high solubility.^[13] BTF3 is a cytosolic/nuclear, soluble protein that lacks transmembrane helices and is distributed similarly to freely soluble transcription and chaperone proteins.

Biological role

BTF3 has many important activities in eukaryotic cells, such as transcription, translation, and cellular growth regulation. Its most fundamental biological role was discovered in 1987 that showed that BTF3 forms a stable complex with RNA polymerase II and is important for proper transcription initiation. This meant that it helped in facilitating both the construction of the pre-initiation complex and the shift to early elongation. This discovery confirmed BTF3 as a fundamental universal transcription factor and gave the first molecular understanding of how BTF3 regulates gene expression at the promoter level.^[8]

The structural and biochemical investigations that has been done showed that β-NAC forms a stable heterodimer with α-NAC and dynamically associates with translating ribosomes to regulate protein maturation. So, in this function BTF3 connects transcriptional regulation to translational quality assurance. This works both upstream and downstream in gene expression pathways. This means that BTF3 also plays an important role in cell proliferation and cancer biology.^[8]

BTF3 affects the expression of cell cycle elated genes, for example the FOXM1, Ki-67, PCNA, and P27. When BTF3 was suppressed in gastric cancer cells, it lowered proliferation, delayed S- and G2/M-phases. It also increased apoptosis, and inhibited epithelial-mesenchymal transition (EMT). BTF3 increases EMT and metastatic potential by increasing the JAK2 or STAT3 signaling pathways. Having FOXM1, eliminates several of these cancer-related issues. This shows that BTF3 is a critical upstream regulator of this cancer-causing transcription factor. It supports normal cellular functions as well as contributes to pathogenic processes.^[7]

This demonstrates its importance in coordinating gene expression with cell development and survival. BTF3's significance in both essential biological processes and disease pathways explains why it exists in eukaryotic cells. Overall, it demonstrates how alterations in its expression can cause important developmental or pathological outcomes.

Interactions

BTF3 has been shown to interact with CSNK2B.^[14]

BTF3 had been shown to interact is BMI1. Most recently it shows that it sustains prostate cancer stemness.^[15]

BTF3 directly interacts with FOXM1, this promotes proliferation and glycolysis in hepatocellular carcinoma (HCC) cells.^[16]

BTF3 binds to the PDCD2L promoter and stimulates transcription in hepatocellular cancer. BTF3 binds to the PDCD2L proximal promoter and increases transcription, connecting BTF3 to a significant proliferation and apoptosis regulating mechanism in liver cancer.^[17]

BTF3 functionally interacts with p53-associated signaling via PDCD2L. By raising PDCD2L, BTF3 indirectly suppresses p53 pathway activity, lowering p53, p21, and Bax levels while increasing Bcl-2. This puts BTF3 in an important position in balancing survival and death.^[17]

BTF3 controls the JAK2/STAT3 signaling pathway, which controls cell migration, invasion, and EMT. Loss of BTF3 lowers JAK2 and STAT3 phosphorylation, but IL-6 stimulation restores pathway activity and rescues EMT markers, showing that BTF3 maintains STAT3-driven metastatic behaviors.^[18]

Clinical significance

BTF3 is clinically significant because it is found in different types of cancers and promotes tumor growth.^[9][19] BTF3 has been identified as an oncogene linked to a poor prognosis in a multiple of cancers, such as gastric, pancreatic, cervical, breast, prostate, osteosarcoma, hypopharyngeal squamous cell carcinoma, and colorectal cancer.^[9] BTF3 functions in two key ways. First, it functions as a transcription factor, binding promoter elements like TATA and CAAT box sequences. Second, it is a component of the nascent-polypeptide-associated complex (NAC), which interacts with nascent polypeptides to prevent mistargeting and is involved in protein ubiquitination and proteolysis.^[9]

BTF3 promotes CRC growth by increasing CHD1L transcription and stabilizing p53 by suppressing HERC2-mediated degradation. Reduced miR-497-5p expression boosts BTF3 levels.

Colorectal cancer (CRC) is one of the most common cancers and the main cause of cancer-related death globally. Although BTF3 has been related to many of tumor growth mechanisms, its exact role and targets in CRC remain unknown. A study looked at BTF3 and subsequent targets in CRC.^[9] A combination of RNA-Seq and ChIP-Seq analysis in BTF3-knockdown and control HT29 colon cancer cells showed 292 differentially expressed genes and 149 genomic locations with changed BTF3 binding.^[9] Combining these datasets showed two possible direct transcriptional targets, of which CHD1L was chosen for further evidence due to its known oncogenic function in CRC. ChIP-Seq indicated decreased BTF3 binding at the CHD1L promoter after BTF3 knockdown, and qRT-PCR revealed similar decreases in CHD1L expression.^[9] Functional experiments found that BTF3 overexpression increases CRC cell proliferation, migration, and invasion.^[9] However,  CHD1L knockdown reverses these effects, meaning that CHD1L is a critical transcriptional target of BTF3. ^[9] TCGA dataset analysis found that both BTF3 and CHD1L are significantly increased in CRC tissues and have a positive correlation with each other as well as clinicopathological parameters such as lymphatic invasion and tumor stage.[5] Their expression was also significantly associated with CD8+ T-cell infiltration, showing a potential role in tumor immune control. To look into BTF3's NAC-related function, immunoprecipitation followed by mass spectrometry was used, resulting in the identification of 542 BTF3-interacting proteins, and its NAC partner NACA.^[9] GO analyses suggested an increase in protein targeting to the endoplasmic reticulum, cytosolic ribosome components, RNA binding, and DNA binding. This is in line with both its NAC and transcriptional functions. Using several ubiquitin ligase prediction databases, HERC2 was identified as a possible E3 ligase related with BTF3.^[9] Since p53 was the most predicted HERC2 substrate, BTF3 could improve tumor growth by facilitating HERC2-mediated p53 ubiquitination and proteolysis. The post-transcriptional control of BTF3 by microRNAs was looked into. Cross-referencing miRNA prediction databases with CRC expression datasets shows that miR-497-5p is significantly downregulated in CRC and has a strong negative connection with BTF3 expression. Showing miR-497-5p mimics into HT29 cells decreased BTF3 levels. Then, a dual luciferase test found that miR-497-5p binds directly to the BTF3 3′UTR.^[9] This proved that it suppressed its expression. So, this data implies that low miR-497-5p levels lead to abnormal BTF3 overexpression in CRC. The study provided a thorough molecular characterization of BTF3 in CRC. It showed that BTF3 acts as an oncogene through both transcriptional activation of CHD1L and possible NAC-associated regulation of p53 stability, while its own overexpression may be caused by decreased miR-497-5p levels.^[9]

Nasopharyngeal carcinoma (NPC) is a malignant tumor that grows in the nasopharynx. It is behind the nose and above the back of the throat.^[19] BTF3 was shown to be overexpressed in NPC tissues in comparison to surrounding normal tissues, both at the mRNA and protein levels.^[19] This means that there is a possible role in tumor growth and progression. When Immunohistochemistry was done, it was found that BTF3 staining was much stronger in cancerous tissue. Meaning higher expression related to more advanced tumor stages, lymph node metastasis, and distant metastasis.^[19] Having BTF3-silencing in NPC cell lines showed that there were less cell proliferation and colony formation, proving that BTF3 increases tumor cell growth.^[19] This could be linked to BTF3's significance for apoptosis and cell cycle regulation, since overexpression can reduce programmed cell death while increasing tumor growth.^[19] Through survival analyses, BTF3's was revealed to have a significant importance. Patients with high BTF3 expression exhibited a significantly elevated risk of overall mortality.^[19] Kaplan-Meier and multivariate Cox regression models identified BTF3 positivity as an important risk factor for shorter overall survival.^[19] Together, these findings argue that BTF3 not only contributes to NPC progression, but may also be a useful molecular target for prognostic evaluation and future therapeutic intervention.

Regulations

• BTF3 regulates important cell-cycle regulators through the FOXM1 pathway.^[18] In gastric cancer, BTF3 regulates FOXM1 gene expression, which then regulates Ki-67, PCNA, and p27. BTF3 loss causes stunted growth and death, but FOXM1 production rescues these deficiencies.^[17]
• Regulation of PDCD2L and the p53 pathway^[17] BTF3 increases PDCD2L, which inhibits the p53 axis. When BTF3 or PDCD2L are suppressed, p53, p21, and Bax rise, while Bcl-2 falls. This restores pro-apoptotic signaling. This identifies BTF3 as a key regulator of p53-dependent stress and survival pathways.^[17]
• BTF3 regulates the JAK2/STAT3 signaling axis, which controls EMT and metastatic behavior.^[17] When BTF3 is silenced, JAK2 and STAT3s break down. This results in decreased EMT and impaired migration. Yet, IL-6 stimulation can restore STAT3 activation, proving that BTF3 modulates EMT states through cytokine-responsive signaling pathways.
• BTF3 regulates EMT-related transcriptional states.^[17]
• In hepatocellular carcinoma, BTF3 transcriptionally promotes PDCD2L, which influences p53 signaling.^[17]
• BTF3 increases transcriptional initiation by participating in the general TFII machinery
• Coordination of transcription and translation: BTF3 regulates both promoter-level transcription and ribosome-associated protein maturation, helping it to integrate upstream gene expression with downstream proteostasis.^[6]
• Regulation of apoptosis and cell-cycle transitions: BTF3 regulates transitions through S and G2/M phases in cancer cells by acting on PDCD2L, FOXM1, and downstream p53 signaling. It also affects proliferation marker expression and defines apoptotic thresholds.^[17][18]
• Regulation of invasion and migration^[17] BTF3 knockdown reduces invasive and migratory threat by modifying EMT-related transcriptional outputs, such as enhanced E-cadherin and decreased N-cadherin and ZEB2.