Ribosomes sub-units

Exposure of cells to IFN normally results in the induction of a protein kinase termed eIF-2a protein kinase. The enzyme, which is synthesized in a catalytically inactive form, is activated by exposure to dsRNA. The activated kinase then phosplorylates its substrate, eIF-2a, which is the smallest subunit of initiation factor 2 (eIF2). This in turn blocks construction of the smaller ribosomal sub-unit, thereby preventing translation of all viral (and cellular) mRNA (Figure 4.6). 

The binding, entry, and action of ricin, abrin, and modeccin on eukaryotic cellular systems have been reviewed.Modeccin binds to surface receptors containing terminal D-galactosyl residues, and like abrin and ricin it inhibits protein synthesis by inactivating the 60 S ribosomal sub-units. Competition experiments with various glycoproteins have indicated that the modeccin receptors are different from abrin receptors. Mutant cell lines resistant to abrin and ricin were not resistant to modeccin and vice-versa. 

Ribosomal RNA (rRNA) is associated with about 50 different proteins in the ribosomal sub-units (see Section 5.4). A precursor of the rRNA is synthesized in the nucleoli and is then partly methylated there. 

Fig. 5.7 Diagrammatic representation of protein synthesis taking place on the surface of a pair of ribosome sub-units. The peptide bond is being formed between two groups marked with an asterisk.

Note When proteins are Insoluble in a concentrated aqueous solution of urea, electrophoresis may be possible in solvent systems of a different type. Phenol, acetic acid and water (2 1 1) has been used. This solvent system has been used to separate proteins of ribosome sub-units. 

The transcribed mRNA is bound by a particle called a ribosome. Ribosomes are huge complexes consisting of fifty-two separate proteins (of which several are present in multiple copies) and three pieces of RNA with lengths of 120,1,542, and 2,904 nucleotides. The ribosome can be readily broken down into two large pieces, called the 30S sub-unit and the 50S subunit.6 Incredibly, the ribosome is self-assembling. Experiments have shown that when ribosomes are separated into their components and then remixed, under the right conditions the components will spontaneously reform ribosomes. 

A ribosome can be dissociated into a large subunit (5OS) and a small sub-unit (30S). These subunits can be further split into their constituent proteins and RNAs. The 30S subunit contains 21 different proteins (referred to as Si through S21) and a 16S RNA molecule. The SOS subunit contains 34 different proteins (LI through L34) and two RNA molecules, a 23S and a. 55... 

Ribosomes are subcellular particles generally consisting of two sub units each of which contains several r-RNA molecules and numerous specific ribosomal proteins arranged in a complex structure. Many thousand are present in cells, with diameters of 25-30 nm. They are sites for protein synthesis and in particular for translating the genetic code on m-RNA into the amino add sequence of the new protein [16-19]. 

Under the transmission electron microscope, the interior of the bacterial cell appears granular. This is due to the ribosomes, which are essential players in protein synthesis. They ensure, along with the t-RNA, the translation of the genetic code. The ribosomes consist of two parts characterized by their sedimentation speed, expressed in Svedberg values (S). These two sub-units are different in size 30 S and 50 S in procaryotes. The assembled ribosome has a sedimentation constant of 70 S. The 30 S sub-unit contains a 16 S ribosomal RNA molecule (1542 nucleotides) and 21 different protein molecules its molecular mass is 900 KDa. The larger 50 S sub-unit contains two ribosomal... 

Lapinaite and colleagues have published a particularly impressive study of the box C/D ribonucleoprotein (RNP) from the archaeon Pyrococcus furiosus, a complex of 390 kDa responsible for methylating ribosomal RNA. ° By I, L, V-methyl labelling the three protein components of the complex, methyl TROSY spectra were used to monitor chemical shifts and PREs. Models for the entire complex could thus be calculated both with and without substrate RNA present using previously-determined structures of protein and RNA sub-units. Domain boundaries were allowed to be flexible during docking using NMR and SANS data, with... 

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