Transcriptional activator amino acids

Hepatic protein synthesis proceeds via the subcellular stages of gene transcription (in the nucleus) and gene translation (in the cytoplasm). The DNA is transcribed into various types of RNA by the action of the different DNA-dependent RNA polymerases I (A), II (B) and III (C). RNA polymerase I is responsible for the transcription of ribosomal RNA, RNA polymerase II mediates the transcription of messenger RNA, and RNA polymerase III forms transcriptal RNA. These three different RNA types move out of the nucleus into the cytoplasm. Here the ribosomes acquire the genetic information needed for protein synthesis via mRNA, and tRNA transports the activated amino acids to the ribosomes, which are themselves activated (and if necessary replicated) by rRNA. (s. figs. 2.9, 2.17 3.5)... 

A theory proposing that the effect of steroids is to stimulate amino acid activation and, as a result, protein synthesis is not without objections. If the primary effect of steroid hormones is to activate amino acids, the rate of synthesis of all proteins should be increased in the target organ. In contrast, steroids stimulate the biosynthesis of highly specific proteins, and therefore it seems more logical to assume that the steroids act at a step of the biosynthetic pathway that determines the specificity of the proteins synthesized. Such a mechanism could involve transcription of messenger RNA on DNA template. The rate of incorporation of labeled nucleotide triphosphate into the rapidly labeled nuclear RNA by the methods described by Weiss is markedly decreased in castrated animals. 

Two domains, t1 and t2, exist which affect the GR post-DNA binding transcription activity (37). The major (t1) transactivation domain is 185 amino acid residues ia length with a 58-tesidue a-heUcal functional cote (38). The t1 domain is located at the N terminus of the proteia the minor (t2) trans activation domain residues on the carboxy-terminal side of the DNA binding domain. 

Cellular protein biosynthesis involves the following steps. One strand of double-stranded DNA serves as a template strand for the synthesis of a complementary single-stranded messenger ribonucleic acid (mRNA) in a process called transcription. This mRNA in turn serves as a template to direct the synthesis of the protein in a process called translation. The codons of the mRNA are read sequentially by transfer RNA (tRNA) molecules, which bind specifically to the mRNA via triplets of nucleotides that are complementary to the particular codon, called an anticodon. Protein synthesis occurs on a ribosome, a complex consisting of more than 50 different proteins and several stmctural RNA molecules, which moves along the mRNA and mediates the binding of the tRNA molecules and the formation of the nascent peptide chain. The tRNA molecule carries an activated form of the specific amino acid to the ribosome where it is added to the end of the growing peptide chain. There is at least one tRNA for each amino acid. 

One of the most important molecular functions of p53 is therefore to act as an activator of p21 transcription. The wild-type protein binds to specific DNA sequences, whereas tumor-derived p53 mutants are defective in sequence-specific DNA binding and consequently cannot activate the transcription of p5 3-con trolled genes. As we will see more than half of the over one thousand different mutations found in p53 involve amino acids which are directly or indirectly associated with DNA binding. 

Cyclosporine A (CsA) is a water-insoluble cyclic peptide from a fungus composed of 11 amino acids. CsA binds to its cytosolic receptor cyclophilin. The CsA/cyclophilin complex reduces the activity of the protein phosphatase calcineurin. Inhibition of this enzyme activity interrupts antigen receptor-induced activation and translocation of the transcription factor NEAT to the nucleus which is essential for the induction of cytokine synthesis in T-lymphocytes. 

Small tfbiquitin-like modifier represents a family of evolutionary conserved proteins that are distantly related in amino-acid sequence to ubiquitin, but share the same structural folding with ubiquitin proteins. SUMO proteins are covalently conjugated to protein substrates by an isopeptide bond through their carboxyl termini. SUMO addition to lysine residues of target proteins, termed SUMOylation, mediates post-transla-tional modification and requires a set of enzymes that are distinct from those that act on ubiquitin. SUMOylation regulates the activity of a variety of tar get proteins including transcription factors. 

Alternatively, one interesting drug delivery technique exploits the active transport of certain naturally-occurring and relatively small biomacromolecules across the cellular membrane. For instance, the nuclear transcription activator protein (Tat) from HIV type 1 (HlV-1) is a 101-amino acid protein that must interact with a 59-base RNA stem-loop structure, called the traus-activation region (Tar) at the 5 end of all nascent HlV-1 mRNA molecules, in order for the vims to replicate. HIV-Tat is actively transported across the cell membrane, and localizes to the nucleus [28]. It has been found that the arginine-rich Tar-binding region of the Tat protein, residues 49-57 (Tat+9 57), is primarily responsible for this translocation activity [29]. 

Recently, Tse et al. [73] and Orlowski et al. [74] have cloned a third isoform of Na /H exchanger (named NHE-3). The inferred 832-amino acid sequence of rabbit NHE-3 is 41% identical with NHE-1, 44% identical with NHE-2, and has a similar secondary structure. In contrast to NHE-1 and NHE-2, NHE-3 is only expressed in epithelia in intestine and kidney. Moreover, administration of glucocorticoids, which stimulates transport activity of the apical Na /H" exchanger in rabbit intestine, increased levels of NHE-3 transcripts but did not affect NHE-1 or NHE-2 [75]. Taken together, these results suggest that NHE-3 may encode a resistant-type Na /H exchanger of epithelia. A fourth Na /H exchanger isoform (NHE-4) is preferentially expressed in stomach [74]. 

Our studies of the ERT cell cycles show that they are regulated by nutrition (Britton Edgar 1998). If the newly hatched larva is starved for dietary amino acids, DNA replication in most ERTs is not initiated. Under starvation conditions these tissues express low levels of cyclin E and E2F, the transcription factor which is probably responsible for cyclin E expression. If either E2F or cyclin E is induced in starved larvae, DNA replication in the ERTs is activated, and thus expression of these genes appears to limit the ERT cell cycle. When nutrient-deprived larvae are fed, expression of E2F and cyclin E mRNAs increases approximately sixfold, and DNA replication is initiated in most ERT cells. If the animal is first fed and then starved, the ERT cell cycle is activated and then inactivated quite rapidly. These experiments all indicate that the ERT cell cycle is nutrition-responsive, rather than controlled by a rigid developmental program. 

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