MRNA translocation

Meshorer, E., Erb, C., Gazit, R., Pavlovsky, L., Kaufer, D., Friedman, A., Glick, D., Ben-Arie, N., Soreq, H. (2002). Alternative splicing and neuritic mRNA translocation under long-term neuronal h q)ersensitivity. Science 295 508-12. 

Schroder, H. C., Diehl-Seifert, B., Rottmann, M., Messer, R., Bryson, B. A., Agutter, P. S., and Muller, W. E., Functional dissection of nuclear envelope mRNA translocation system Effects of phorbol ester and a monoclonal antibody recognizing cytoskeletal structures, Arch. Biochem. Biophys., 261 [2], 394,1988. 

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]. 

Elongation is a cycUc process on the ribosome in which one amino acid at a time is added to the nascent peptide chain. The peptide sequence is determined by the order of the codons in the mRNA. Elongation involves several steps catalyzed by proteins called elongation factors (EFs). These steps are (1) binding of aminoacyl-tRNA to the A site, (2) peptide bond formation, and (3) translocation. 

These macromolecules include histones, ribosomal proteins and ribosomal subunits, ttansctiption factors, and mRNA molecules. The transport is bidirectional and occurs through the nucleat pote complexes (NPCs). These are complex stmctures with a mass approximately 30 times that of a ribosome and are composed of about 100 diffetent proteins. The diameter of an NPC is approximately 9 run but can increase up to ap-ptoximately 28 nm. Molecules smaller than about 40 kDa can pass through the channel of the NPC by diffusion, but special translocation mechanisms exist fot latget molecules. These mechanisms are under intensive investigation, but some important features have already emerged. 

Fig. 8.4 Outline of the main events in protein synthesis initiation, elongation, translocation and termination. AUG is an initiation codon on the mRNA it codes for Af-fomiylmelhionine and initiates the formation of the 70S rihosome. UAG is a termination codon it does not code for any amino acid and brings about termination of protein synthesis.

At these concentrations, CHX inhibits the 60S subunit translocation and retains ribosomal association with mRNA. 

After formation of the initiation dipeptide, the first EF-G-dependent translocation allows binding of the third aminoacyl-tRNA in the A-site so that a tripeptide is formed. The apparent rate of this event may depend upon the nature of the initiation complex initially formed, being slower, for instance, with those containing mRNAs with an extended SD sequence than with those having either very short or no SD complementarity (C. O. G. and M. Rodnina, unpublished results). Furthermore, very powerful translocation inhibitors may block tripeptide formation to such an extent that they mimic translation initiation inhibitors. 

For processive peptide polymerization, the rihosome has to move along the mRNA. Following peptide bond formation, the rihosomal A site is occupied hy a peptidyl-tRNA whereas the P site contains a deacylated tRNA. During translocation, the complex of the two tRNAs with the mRNA has to move relative to the ribosome to... 

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