Identification of heteromolecular binding sites in transcription factors Sp1 and TAF4 using high-resolution nuclear magnetic resonance spectroscopy

Q-rich activation domains: flexible 'rulers' for transcription start site selection?

Recent findings broadened the function of RNA polymerase II (Pol II) proximal promoter motifs from quantitative regulators of transcription to important determinants of transcription start site (TSS) position. These motifs are recognized by transcription factors (TFs) that we propose to term 'ruler' TFs (rTFs), such as NRF1, NF-Y, YY1, ZNF143, BANP, and members of the SP, ETS, and CRE families, sharing as a common feature a glutamine-rich (Q-rich) effector domain also enriched in valine, isoleucine, and threonine (QVIT-rich). We propose that rTFs guide TSS location by constraining the position of the pre-initiation complex (PIC) during its promoter recognition phase through a specialized, and still enigmatic, class of activation domains.

Viral Appropriation of Specificity Protein 1 (Sp1): The Role of Sp1 in Human Retro- and DNA Viruses in Promoter Activation and Beyond

Specificity protein 1 (Sp1) is a highly ubiquitous transcription factor and one employed by numerous viruses to complete their life cycles. In this review, we start by summarizing the relationships between Sp1 function, DNA binding, and structural motifs. We then describe the role Sp1 plays in transcriptional activation of seven viral families, composed of human retro- and DNA viruses, with a focus on key promoter regions. Additionally, we discuss pathways in common across multiple viruses, highlighting the importance of the cell regulatory role of Sp1. We also describe Sp1-related epigenetic and protein post-translational modifications during viral infection and how they relate to Sp1 binding. Finally, with these insights in mind, we comment on the potential for Sp1-targeting therapies, such as repurposing drugs currently in use in the anti-cancer realm, and what limitations such agents would have as antivirals.

Glutamine-rich regions of the disordered CREB transactivation domain mediate dynamic intra- and intermolecular interactions

The cyclic AMP response element (CRE) binding protein (CREB) is a transcription factor that contains a 280-residue N-terminal transactivation domain and a basic leucine zipper that mediates interaction with DNA. The transactivation domain comprises three subdomains, the glutamine-rich domains Q1 and Q2 and the kinase inducible activation domain (KID). NMR chemical shifts show that the isolated subdomains are intrinsically disordered but have a propensity to populate local elements of secondary structure. The Q1 and Q2 domains exhibit a propensity for formation of short β-hairpin motifs that function as binding sites for glutamine-rich sequences. These motifs mediate intramolecular interactions between the CREB Q1 and Q2 domains as well as intermolecular interactions with the glutamine-rich Q1 domain of the TATA-box binding protein associated factor 4 (TAF4) subunit of transcription factor IID (TFIID). Using small-angle X-ray scattering, NMR, and single-molecule Förster resonance energy transfer, we show that the Q1, Q2, and KID regions remain dynamically disordered in a full-length CREB transactivation domain (CREBTAD) construct. The CREBTAD polypeptide chain is largely extended although some compaction is evident in the KID and Q2 domains. Paramagnetic relaxation enhancement reveals transient long-range contacts both within and between the Q1 and Q2 domains while the intervening KID domain is largely devoid of intramolecular interactions. Phosphorylation results in expansion of the KID domain, presumably making it more accessible for binding the CBP/p300 transcriptional coactivators. Our study reveals the complex nature of the interactions within the intrinsically disordered transactivation domain of CREB and provides molecular-level insights into dynamic and transient interactions mediated by the glutamine-rich domains.

Bex1 is essential for ciliogenesis and harbours biomolecular condensate-forming capacity

BACKGROUND

Primary cilia are sensory organelles crucial for organ development. The pivotal structure of the primary cilia is a microtubule that is generated via tubulin polymerization reaction that occurs in the basal body. It remains to be elucidated how molecules with distinct physicochemical properties contribute to the formation of the primary cilia.

RESULTS

Here we show that brain expressed X-linked 1 (Bex1) plays an essential role in tubulin polymerization and primary cilia formation. The Bex1 protein shows the physicochemical property of being an intrinsically disordered protein (IDP). Bex1 shows cell density-dependent accumulation as a condensate either in nucleoli at a low cell density or at the apical cell surface at a high cell density. The apical Bex1 localizes to the basal body. Bex1 knockout mice present ciliopathy phenotypes and exhibit ciliary defects in the retina and striatum. Bex1 recombinant protein shows binding capacity to guanosine triphosphate (GTP) and forms the condensate that facilitates tubulin polymerization in the reconstituted system.

CONCLUSIONS

Our data reveals that Bex1 plays an essential role for the primary cilia formation through providing the reaction field for the tubulin polymerization.

A novel mode of interaction between intrinsically disordered proteins

An increasing number of proteins, which have neither regular secondary nor well-defined tertiary structures, have been found to be present in cells. The structure of these proteins is highly flexible and disordered under physiological (native) conditions, and they are called “intrinsically disordered” proteins (IDPs). Many of the IDPs are involved in interactions with other biomolecules such as DNA, RNA, carbohydrates, and proteins. While these IDPs are largely unstructured by themselves, marked conformational changes often occur upon binding to an interacting partner, which is known as the “coupled folding and binding mechanism”, which enable them to change the conformation to become compatible with the shape of the multiple target biomolecules. We have studied the structure and interaction of eukaryotic transcription factors Sp1 and TAF4, and found that both of them have long intrinsically disordered regions (IDRs). One of the IDRs in Sp1 exhibited homo-oligomer formation. In addition, the same region was used for the interaction with another IDR found in the TAF4 molecule. In both cases, we have not detected any significant conformational change in that region, suggesting a prominent and novel binding mode for IDPs/IDRs, which are not categorized by the well-accepted concept of the coupled folding and binding mechanism.

Overexpression of TWA1 predicts poor prognosis in patients with gastric cancer

TWA1 is associated with microtubule dynamics, cell migration, nucleokinesis and chromosome segregation. However, the role of TWA1 in gastric cancer (GC) remains unclear. In this study, Cosmic database revealed that the expression level of TWA1 ranks in the top 20 of overexpressed genes in GC. Further bioinformatic analysis revealed that the expression level of TWA1 was not in connection with the infection status of HP or EB. IHC and IF showed that TWA1 protein was present in both the cytoplasm and nucleus, but mainly in the cytoplasm. The high expression level of TWA1 was also related to tumor size, depth of invasion, lymph node metastasis, TNM stage, cancerous node and vascular invasion. Furthermore, higher TWA1 expression was also associated with shorter PFS and OS in GC. The univariate and multivariate analysis suggested the expression of TWA1 was an independent poor prognostic factor in GC. DNA copy number gain contributes to TWA1 overexpression and promoter methylation of TWA1 predicts profitable prognosis. Co-expression showed that TAF4 may function as a transcription factor (TF) regulates TWA1 expression, which further to mediate tumor invasion and metastasis. These findings revealed that TWA1 plays an important role in the development of GC and is expected to become an important biomarker and therapeutic target of tumors.

Transcriptional activation is weakened when Taf1p N-terminal domain 1 is substituted with its Drosophila counterpart in yeast TFIID

Transcription factor II D (TFIID), a multiprotein complex consisting of TATA-binding protein (TBP) and 13-14 TBP-associated factors (Tafs), plays a central role in transcription and regulates nearly all class II genes. The N-terminal domain of Taf1p (TAND) can be divided into two subdomains, TAND1 and TAND2, which bind to the concave and convex surfaces of TBP, respectively. The interaction between TAND and TBP is thought to be regulated by TFIIA, activators and/or DNA during transcriptional activation, as the TAND1-bound form of TBP cannot bind to the TATA box. We previously demonstrated that Drosophila TAND1 binds to TBP with a much stronger affinity than yeast TAND1 and that the expression levels of full-length chimeric Taf1p, whose TAND1 is replaced with the Drosophila counterpart, can be varied in vivo by substituting several methionine residues downstream of TAND2 with alanine residues in various combinations. In this study, we examined the transcriptional activation of the GAL1-lacZ reporter or endogenous genes such as RNR3 or GAL1 in yeast cells expressing various levels of full-length chimeric Taf1p. The results showed that the substitution of TAND1 with the Drosophila counterpart in yeast TFIID weakened the transcriptional activation of GAL1-lacZ and RNR3 but not that of GAL1. These findings strongly support a model in which TBP must be released efficiently from TAND1 within TFIID upon transcriptional activation.

Structure of human TFIID and mechanism of TBP loading onto promoter DNA

The general transcription factor IID (TFIID) is a critical component of the eukaryotic transcription preinitiation complex (PIC) and is responsible for recognizing the core promoter DNA and initiating PIC assembly. We used cryo-electron microscopy, chemical cross-linking mass spectrometry, and biochemical reconstitution to determine the complete molecular architecture of TFIID and define the conformational landscape of TFIID in the process of TATA box-binding protein (TBP) loading onto promoter DNA. Our structural analysis revealed five structural states of TFIID in the presence of TFIIA and promoter DNA, showing that the initial binding of TFIID to the downstream promoter positions the upstream DNA and facilitates scanning of TBP for a TATA box and the subsequent engagement of the promoter. Our findings provide a mechanistic model for the specific loading of TBP by TFIID onto the promoter.