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EN
The non-protein amino acid homocysteine (Hcy) enters the first step of protein synthesis and forms an aminoacyl-tRNA synthetase-bound homocysteinyl adenylate (Hcy-AMP). Direct incorporation of Hcy into tRNA and protein is prevented by editing activities of aminoacyl-tRNA synthetases that convert Hcy-AMP into Hcy thiolactone. Editing of Hcy occurs in all cell types investigated, including human. S-Nitrosylation of Hcy prevents its editing by MetRS and allows formation of S-nitroso-Hcy-tRNAMet, as well as incorporation of Hcy into proteins at positions specified by methionine codons. This provides an example of how the genetic code can be expanded by invasion of the metionine coding pathway by Hcy. Hcy can also be incorporated into protein post-translationally by a facile reaction of Hcy thiolactone with ?-amino groups of protein lysine residues. Hcy is present in human blood proteins, such as hemoglobin, serum albumin, and ?-globulins. Hcy thiolactonase, a component of high-density lipoprotein, minimizes protein N-homocysteinylation. Incorporation of Hcy into protein provides plausible chemical mechanism by which elevated levels of Hcy contribute to human cardiovascular disease.
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Molecular basis of dendritic arborization

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EN
The pattern of dendritic branching along with the receptor and channel composition and density of synapses regulate the electrical properties of neurons. Abnormalities in dendritic tree development lead to serious dysfunction of neuronal circuits and, consequently, the whole nervous system. Not surprisingly, the complicated and multi-step process of dendritic arbor development is highly regulated and controlled at every stage by both extrinsic signals and intrinsic molecular mechanisms. In this review, we analyze the molecular mechanisms that contribute to cellular processes that are crucial for the proper formation and stability of dendritic arbors, in such distant organisms as insects (e.g. Drosophila melanogaster), amphibians (Xenopus laevis), and mammals.
EN
The ribosomal peptidyl transferase ribozyme resides in the large ribosomal subunit and catalyzes the two principal chemical reactions of protein synthesis, peptide bond formation and peptidyl-tRNA hydrolysis. With the presentations of atomic structures of the large ribosomal subunit, the questions how an RNA active site can catalyze these chemical reactions gained a new level of molecular significance. The peptidyl transferase center represents the most intense accumulation of universally conserved ribosomal RNA nucleotides in the entire ribosome. Thus, it came as a surprise that recent findings revealed that the nucleobase identities of active site residues are actually not critical for catalysis. Instead RNA backbone groups have been identified as key player in transpeptidation and peptide release. While the ribose 2' -OH of the 23S rRNA residue A2451 plays an important role in peptidyl transfer, its contribution to peptidyl-tRNA hydrolysis is only minor. On the other hand, the ribose 2'-OH of the terminal adenosine of P-site bound tRNA seems to play equally crucial roles in peptide bond formation and tRNA hydrolysis. While it seems that details of ribosome-catalyzed peptidyl-tRNA hydrolysis are just emerging, our molecular insights into transpeptidation are already very advanced. It has been realized that an intricate interaction between the ribose 2'-OH groups of 23S rRNA residue A2451 and tRNA nucleotide A76 are crucial for proton shuttling that is required for efficient amide bond synthesis.
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EN
The article summarizes the most meaningful studies which have provided evidence that protein synthesis in neurons can occur not only in cell perikarya but also locally in dendrites. The presence of the complete machinery required to synthesize cytoplasmic and integral membrane proteins in dendrites, identification of binding proteins known to mediate mRNA trafficking in dendrites and the ability to trigger 'on-site' translation make it possible for the synthesis of particular proteins to be regulated by synaptic signals. Until now over 100 different mRNAs coding the proteins involved in neurotransmission and modulation of synaptic activity have been identified in dendrites. Local protein synthesis is postulated to provide the basic mechanism of fast changes in the strength of neuronal connections and to be an important factor in the molecular background of synaptic plasticity, giving rise to enduring changes in synaptic function, which in turn play a role in local homeostatic responses. Local protein synthesis points to some autonomy of dendrites which makes them 'the brains of the neurons' (Jim Eberwine; from the interview with J. Eberwine ? Barinaga 2000).
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