Transcription activator Dimerization

Rel homology domain (RHD) that encompasses a sequence-specific DNA-binding domain, a dimerization domain and a nuclear translocation signal (NLS) (Fig. la). RelA, cRel, and RelB contain a transcription activation domain (TAD). NF-kB 1 and NF-kB2 are synthesized as large precursors, pi 05 and pi 00, that are posttranslationnally processed to generate the mature forms, p50 and p52, which lack a TAD. 

The core unit of the chromatin, the nucleosome, consists of histones arranged as an octamer consisting of a (H3/ H4)2-tetramer complexed with two histone H2A/H2B dimers. Accessibility to DNA-binding proteins (for replication, repair, or transcription) is achieved by posttranslational modifications of the amino-termini of the histones, the histone tails phosphorylation, acetylation, methylation, ubiquitination, and sumoyla-tion. Especially acetylation of histone tails has been linked to transcriptional activation, leading to weakened interaction of the core complexes with DNA and subsequently to decondensation of chromatin. In contrast, deacetylation leads to transcriptional repression. As mentioned above, transcriptional coactivators either possess HAT activity or recruit HATs. HDACs in turn act as corepressors. 

A bacterial two-hybrid system has been developed that, similar to the yeast system, functions via activation of transcription (Dove and Hochschild, 1998 Joung et al., 2000). RNA polymerase (RNAP) in E. coli consists of an enzymatic core composed of the a, (3, and (3 subunits in the stoichiometry a2(3(3, and one of several c factors that enable the enzyme to recognize specific promoters (Heilman and Chamberlin, 1988). Many bacterial transcriptional activator proteins bind the promoters they regulate and interact directly with subunits of RNAP. The most commonly observed contact is between activator proteins and the a subunit of RNAP (Ebright and Busby, 1995). The function of the a subunit is to initiate the assembly of RNAP by forming a dimer (Igarashi et al., 1991). 

ERs have domains responsible for nuclear location, hormone binding, dimerization, DNA binding, and transcription activation (Figs. 1.2 and 1.3) (Beato et al. 1996 Beato 1989 Fawell et al. 1990 Hall et al. 1999 Kumar et al. 1987). 

The formation of a transcriptionally active complex requires the interaction of all transcription cofactors with their respective specific DNA sequences. Once they have bound to their specific sequences, it is on these that the remaining elements of the complex that do not interact directly with the DNA are assembled (Chin 1995 Filardo 2002). The elements of the complex that do not come into direct contact with DNA have their own specificity of interaction with the remaining proteins of the complex. Therefore, they include important restrictions so that a fully active transcriptional complex can be assembled with difficulty on a receptor dimer that has incorrectly recognized a HRE. Indeed, an incorrect interaction can imply a noticeable degree of transcription inhibition. 

Figure I. The hetero dimeric Core Binding Factor (CBF) transcription factor is comprised of one of three RUNX family proteins (a snbnnit) and a b-subimit, which is encoded by a single gene. The RUNX proteins contain two conserved and functional domains the runt homology domain (RHD) and the transcription activation domain (TAD). Interactions between the RHD and the hetero dimerization domain (HD) of CBFb are essential for most of the known activities of CBF. Synerigistic activity with a number of different transcription factors is well established.

Jorumycin (211) Dimeric isoquinoline alkaloid Zalypsis (PM00104/50) (212) Oncology DNA binding and transcriptional activity Phase I (solid tumors or lymphoma) PharmaMar 950, 951... 

Each stage described above involves at least one critical RNA-RNA and/or RNA-protein interaction. Initiation of reverse transcription involves the binding of a specific tRNA, which is packaged in the viral particle, to a short sequence of the viral RNA. Transcription and accumulation of viral RNA is dependent upon the sequence-specific interaction between two essential viral regulatory proteins. Tat and Rev, with their respective RNA sites, TAR and RRE (Figure 10.1). Tat is a transcriptional activator, whereas Rev acts post-transcriptionally to increase the cytoplasmic accumulation of the viral gag-pol and env messenger RNAs. Viral assembly is initiated by formation of an RNA-RNA dimer where... 

Fig. 1.7. Basic leudne zipper and heltx-loop-heltx motif in complex with DNA. A) The basic leucine zipper of the transcription activator GCN4 of yeast consists of two slightly curved a-hehces, which dimerize with the help of the leucine zipper motif. The sequence specific binding of DNA occurs via the basic ends of the two helices. They insert themselves into the major groove of the DNA. B) The helix-loop-helix motif of the eucaryotic transcription factor Max complexed with DNA. Molscript drawing (Kraulis 1991).

Most transcription activators bind to DNA as a dimer or higher multimer (see 2.4). The dimerization relies on structural motifs which commonly occur in many different proteins. Examples for dimerization motifs are the helix-loop-hdix motif and the leucine zipper. The dimerization motifs permit the formation of DNA-boimd homodimers or heterodimers, depending upon whether the same or different proteins interact with each other (Fig. 1.38). The different dimers have different requirements for the... 

The heterotypic dimerization significantly expands the repertoire for tissue-specific regulation of transcription activity. The tissue-specific expression of a particular pattern of transcriptional activators can be used to select only certain DNA-binding elements out of a series of similar elements, and thus to specifically induce certain genes. This strategy is extensively used by the receptors for retinoic acid (see chapter 4). 

The Stat proteins have SH2 and SH3 domains, a DNA binding domain and a C-ter-minal domain required for transcription activation. The activating phosphorylation takes place for Statl on Tyr701 in the vicinity of the C-terminus. In the imphosphoryla-ted form, the Stat proteins exist as monomers, whereas in the phosphorylated form, they are dimers. 

Each of the monomeric proteins c-jun and c-fos, as well as other members of the leucine zipper family, has an N-terminal DNA-binding domain rich in positively charged basic amino acid side chains, an activation domain that can interact with other proteins in the initiation complex, and the leucine-rich dimerization domain.363 The parallel coiled-coil structure (Fig. 2-21) allows for formation of either homodimers or heterodimers. However, cFos alone does not bind to DNA significantly and the cjun/cFos heterodimer binds much more tightly than does cjun alone.364 The yeast transcriptional activator protein GCN4 binds to the same 5 -TGACTCA sequence as does the mammalian AP-1 and also has a leucine zipper structure.360 364 365... 

In yeast model systems it has been observed that myc is a transcription activator but only when present in a heterodimer with max. Max appears to be essential for DNA binding. Max dimer can bind to DNA on its own but it does not activate transcription on its own. Mammalian genes that are normally activated by myc are still not known. 

The DNA binding ability of peptide nanostructures has also been reported by the artificial dimerization of peptide sequences corresponding to the contact region of the transcriptional activator protein GCN4. Cuenoud and Schepartz... 

Hope, I.A. Struhl, K. (1987). GCN4, a eukaryotic transcriptional activator protein, binds as a dimer to target DNA. The EMBO Journal 6, 2781-4. 

---