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TAF1-TAF7 module of TFIID in Cell Proliferation and Human Cancer

The expression of protein encoding genes is essential for the proper execution of all cellular processes.  Abnormal gene expression patterns are a feature of cancer and commonly are the result of mutations in transcription factors. TAF1 is the largest subunit of the general transcription factor complex TFIID.  TAF1 possesses promoter DNA binding, histone acetylation, and protein phosphorylation activities. The TAF1 histone acetyltransferase (HAT) activity maps to a conserved central domain.  We, and others, have shown that TAF1 HAT activity is critical for transcription of a subset of cell cycle regulatory genes.  Interestingly, direct binding of TAF7, another TFIID subunit, can block the HAT activity of TAF1. Our studies suggest that phosphorylation of TAF7 at S264 causes its release from TFIID, alleviating its inhibitory effect on TAF1. Therefore, we study the dynamics of the TAF1-TAF7 module within TFIID and its emerging role in the control of cell cycle progression.

Molecular Interactions and Structural Features of Human TAF1 Protein

 

Dash lines connects proteins and DNA that interact with the indicated domain in TAF1. Interacting proteins are TBP: TATA binding protein; TAF7: TBP associated factor 7; RAP74: RNA pol II associated protein 74; AcH4: acetylated histone H4.  Structure features are WH: winged-helix; ZnK: zinc knuckle; 2xBRM: double bromodomain

Genetic and biochemical evidence implicate misregulation of TAF1 in tumorigenesis.  A survey of uterine serous carcinoma (USC) by whole-exome sequencing revealed that approximately 13% of USC tumors analyzed carried TAF1 mutations at positions strictly conserved during evolution. A number of these mutations map to the HAT or TAF7 binding domain of TAF1. TAF1 is required for the expression of cyclin D1, a proto-oncogene frequently overexpressed in various cancers.  TAF1 interacts with human papilloma virus (HPV) E2, a protein important for HPV induced cervical cancer.  TAF1 also enhances cell cycle progression by promoting degradation of the tumor suppressor protein p53. Therefore, we aim to elucidate the TAF1 dependent molecular mechanisms and signaling pathways controlling cell proliferation and to use this information to uncover new targets for anti-cancer drugs.

Core promoter DNA binding activity of TAF1

TFIID is the first general transcription factor to bind to the core promoter of protein encoding genes.  This binding event initiates the assembly of a functional pre-initiation complex that transcribes DNA into RNA. A number of TFIID subunits are well-documented to directly contact core promoter DNA elements, which include the TATA box, Initiator (Inr), motif 10 element (MTE) and downstream promoter element (DPE). Specifically, TAF1 binds to the Inr and less well-defined downstream core element (DCE). Our research aims to define the TAF1 domain(s) that interfaces with core promoter DNA and the mechanism that conveys DNA binding sequence specificity.

In collaboration with Dr. Ning Zheng, we solved the structure of a TAF1-TAF7 complex, by X-ray crystallography, and discovered a winged helix (WH) fold embedded in the TAF1 HAT domain. The TAF1 WH, which displays structural similarity to the DNA binding transcription factor E2F4, contains amino acids essential for TAF1 to contact promoter DNA.  In contrast to E2F4, the WH domain of TAF1 displayed little sequence specificity suggesting the involvement of other determining factors. Sequence analysis of the TAF1 protein revealed an evolutionarily conserved Zn knuckle motif absent from the TAF1 fragment used in our DNA binding and structure studies. TAF1 also interacts with TAF2, another TFIID subunit that directly binds to the Inr sequence.  We have begun to investigate whether the Zn knuckle or presence of TAF2 provides additional points of contact with DNA that dictate sequence binding specificity.

Ribbon diagram of TAF1-TAF7 structure

Genetics of Human Disorders: Role of TAF1 and TAF2 mutations

Intellectual disability (ID) is classified as a developmental disability characterized by limited cognitive and adaptive function, commonly defined by an IQ of less than 70. An estimated 1-2% of the general population is affected by ID, with the age of onset before 18.  Whole genome and exome sequencing of individuals diagnosed with ID and their unaffected family members has uncovered de novo mutations in the TAF1 and TAF2 subunits of TFIID that segregate with the disorder.  X-linked dystonia-parkinsonism (XDP) is a neurodegenerative disorder indigenous to Panay Island in the Phillipines.  Nearly all affected individuals harbor the same collection of seven disease-specific genetic changes, three of which map to the TAF1 locus. The significance of these TAF1 mutations to disease progression is further substantiated by the ranking of TAF1 as the 53rd most constrained human gene among the top 1000.  We plan to introduce disease associated mutations into the TAF1 and TAF2 proteins and to investigate the functional consequences using molecular and cellular biology, biochemistry and structural approaches.

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