Transcription Factor II H (TFIIH) Pleckstrin homology (PH) domain
The transcription factor II H (TFIIH) is one of the general transcription factors (GTFs) known to be a target of the transactivation domain (TAD) of p53. Human TFIIH and its homologous yeast counterpart (factor b) are composed of ten subunits that can be divided into two groups, the core TFIIH (XPB/Ssl2, p62/Tfb1, p52/Tfb2, p44/Ssl1, p34/Tfb4, and TTDA/Tfb5 in human/yeast) and the CAK complex (cdk7/Kin28, cyclin H/Ccl1, and MAT1/Tfb3). These two complexes are linked by the XPD/Rad3 subunit. The helicase activities of XPB and XPD are essential to the formation of the open complex during transcription initiation and the kinase activity of cdk7 phosphorylates the C-terminal domain (CTD) of the RNA Pol II largest subunit, enabling RNA Pol II to progress from the initiation phase to the elongation phase of transcription. The PH domain of p62/Tfb1 has been shown to interact with herpes simplex virus protein 16 (VP16) TAD and the binding of p53 TAD is mediated by the TAD2 subdomain. TFIIE recruits TFIIH to complete the preinitiation complex (PIC) formation and regulates enzymatic activities of TFIIH. The PH domain of the human TFIIH p62 subunit binds to the C-terminal acidic (AC) domain of the human TFIIEalpha subunit. This interaction could be a switch to replace p53 with TFIIE on TFIIH in transcription. PH domains have diverse functions, but in general are involved in targeting proteins to the appropriate cellular location or in the interaction with a binding partner. They share little sequence conservation, but all have a common fold, which is electrostatically polarized. Less than 10% of PH domains bind phosphoinositide phosphates (PIPs) with high affinity and specificity. PH domains are distinguished from other PIP-binding domains by their specific high-affinity binding to PIPs with two vicinal phosphate groups: PtdIns(3,4)P2, PtdIns(4,5)P2 or PtdIns(3,4,5)P3 which results in targeting some PH domain proteins to the plasma membrane. A few display strong specificity in lipid binding. Any specificity is usually determined by loop regions or insertions in the N-terminus of the domain, which are not conserved across all PH domains. PH domains are found in cellular signaling proteins such as serine/threonine kinase, tyrosine kinases, regulators of G-proteins, endocytotic GTPases, adaptors, as well as cytoskeletal associated molecules and in lipid associated enzymes.
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