Histidyl-tRNA synthetase (HisRS) is responsible for the synthesis of histidyl-transfer RNA, which is essential for the incorporation of histidine into proteins. This amino acid has uniquely moderate basic properties and is an important group in many catalytic functions of enzymes.A compilation of currently known primary structures of HisRS shows that the subunits of these homodimeric enzymes consist of 420 -550 amino acid residues. This represents a relatively short chain length among aminoacyl-tRNA synthetases (aaRS), whose peptide chain sizes range from about 300 to 1100 amino acid residues.The crystal structures of HisRS from two organisms and their complexes with histidine, histidyl-adenylate and histidinol with ATP have been solved. HisRS from Escherichia coli and Thermus thermophilus are very similar dimeric enzymes consisting of three domains: the N-terminal catalytic domain containing the sixstranded antiparallel -sheet and the three motifs characteristic of class II aaRS, a HisRS-specific helical domain inserted between motifs 2 and 3 that may contact the acceptor stem of the tRNA, and a C-terminal ␣/ domain that may be involved in the recognition of the anticodon stem and loop of tRNA His .The aminoacylation reaction follows the standard two-step mechanism. HisRS also belongs to the group of aaRS that can rapidly synthesize diadenosine tetraphosphate, a compound that is suspected to be involved in several regulatory mechanisms of cell metabolism. Many analogs of histidine have been tested for their properties as substrates or inhibitors of HisRS, leading to the elucidation of structure-activity relationships concerning configuration, importance of the carboxy and amino group, and the nature of the side chain.HisRS has been found to act as a particularly important antigen in autoimmune diseases such as rheumatic arthritis or myositis. Successful attempts have been made to identify epitopes responsible for the complexation with such auto-antibodies.
The Fli-1 protein is a member of the ets proto-oncogene family, whose overexpression is a consequence of Friend murine leukemia virus (F-MuLV) integration in Friend erythroleukemic cells. We present evidence that Fli-1 and the retinoic acid receptor (RARa) can reciprocally repress one another's transcriptional activation. Overexpression of Fli-1 inhibits the retinoic acid-induced activation of genes carrying a functional retinoic acid response element (RARE). Conversely, RARa is able to repress Fli-1-mediated transcriptional activation. Transfection analysis of RARa and Fli-1 mutants in cultured cells demonstrate that the DNA binding domain of RARa and the N-terminal region of Fli-1 are required for repression. Gel retardation analysis demonstrates that RARa cannot bind to the Fli-1 binding site in the E74 promoter and the expression of Fli-1 does not aect RARa binding to DNA. Furthermore, the data suggest an indirect interaction between Fli-1 and RARa mediated by a`bridging' factor(s) present in nuclear extracts from RM10 erythroleukemia cells. Fli-1 also interferes with the action of receptors for thyroid or glucocorticoid hormone in several hematopoietic cell lines. The RAinduced dierentiation and decrease of cell proliferation was blocked in myeloblastic leukemia HL-60 cells overexpressing the N-terminal region of Fli-1 at physiological concentrations of RA. These data suggest that accumulation of Fli-1 can oppose the transcriptional activity of hormone receptors in hematopoietic cells.
In a search for nucleotide binding proteins associated with the T‐cell receptor (TCR)‐CD3 complex, a novel labeling technique involving introduction of [alpha‐32P]GTP or [alpha‐32P]ATP into permeabilized cells followed by in situ periodate oxidation was developed. To test the method we first demonstrated that p21ras and other classical GTP binding proteins could be labeled in a GTP‐specific manner. In human T lymphocytes the TCR zeta chain was found to be specifically labeled by GTPoxi but not by ATPoxi or CTPoxi. Labeling kinetics and competition experiments demonstrated that zeta had a capacity to bind GTP and GDP but not GMP or ATP. Proteolytic cleavage experiments identified lysine 128 as the GTP crosslinking site. This result was confirmed by studies using oligonucleotide‐directed mutagenesis. Lysine residues 128, 135 and 149 were each replaced by arginine and glycine 134 by valine and mutated proteins were expressed in CHO cells. Labeling of mutants K128R and G134V was abrogated whereas mutant proteins K135R and K148R could still be specifically crosslinked to GTP. We conclude that Lys128 and Gly134 are part of a GTP/GDP binding site suggesting that zeta is a unique GTP/GDP binding structure.
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