80S ribosome

Puromycin. Puromycin (19), elaborated by S. alboniger (1—4), inhibits protein synthesis by replacing aminoacyl-tRNA at the A-site of peptidyltransferase (48,49). Photosensitive analogues of (19) have been used to label the A-site proteins of peptidyltransferase and tRNA (30). Compound (19), and its carbocycHc analogue have been used to study the accumulation of glycoprotein-derived free sialooligosaccharides, accumulation of mRNA, methylase activity, enzyme transport, rat embryo development, the acceptor site of human placental 80S ribosomes, and gene expression in mammalian cells (51—60). 

The binding of the 60S ribosomal subunit to the 48S initiation complex involves hydtolysis of the GTP bound to elF-2 by elF-5. This teaction tesults in telease of the initiation factots bound to the 48S initiation complex (these factots then ate tecycled) and the tapid association of the 40S and 60S subunits to fotm the 80S ribosome. At this point, the met-tRNA is on the P site of the ribosome, ready for the elongation cycle to commence. 

Figure 7.5 Model of ferritin (and erythroid a-aminolaevulinate synthase) translation/ribosome binding regulation by IRP. In (a), with IRP not bound to the IRE (1) binding of the 43S preinitiation complex (consisting of the small ribosomal 40S subunit, GTP and Met-tRNAMet) to the mRNA is assisted by initiation factors associated with this complex, as well as additional eukaryotic initiation factors (elFs) that interact with the mRNA to facilitate 43S association. Subsequently (2), the 43S preinitiation complex moves along the 5 -UTR towards the AUG initiator codon, (3) GTP is hydrolysed, initiation factors are released and assembly of the 80S ribosome occurs. Protein synthesis from the open reading frame (ORF) can now proceed. In (b) With IRP bound to the IRE, access of the 43S preinitiation complex to the mRNA is sterically blocked. From Gray and Hentze, 1994, by permission of Oxford University Press.

Of particular interest, the data depicted in Fig. 4.1 demonstrate that ribosomes remain membrane-bound after termination and, surprisingly, are recovered predominantly as 80S rather than the predicted posttermination 40S + 60S subunits. It would be of high interest to determine if 80S post-termination, ER-bound ribosomes are competent for initiation, to identify the cohort of initiation factors necessary for initiation with 80S ribosomes, and also to determine whether initiation and scanning require dissociation of the 80S couplet. Methods to address these questions are under development but unavailable at the time of preparation of this contribution. 

Ribosomes (79-87) are small organelles 17-23 nm in diameter. They can exist in clusters known as polysomes or be attached to the er where they bind to pores in the er membrane. A major constituent of the er pore is translocon, the heterotrimetric Sec 61 protein complex. Sec 61 binds to the 80s ribosomes (86). Ribosomes consist of subunits, a 30s subunit (16srRNA and 21 proteins), and a 50s subunit (23s and 5s RNAs, > proteins and the catalytic site of peptidyl transferase). Ribosomes are the sites of protein synthesis. 

Spahn CM, Beckmann R, Eswar N, Penczek PA, Sali A, Blobel G, Frank J, Helmers J (2001) Structure of the 80S ribosome from Saccharomyces cerevisiae—tRNA-ribosome and subunit-subunit interactions architecture of the protein-conducting channel associated with the translating 80S ribosome. Cell 107 373-386... 

Ribosomes are large complexes of protein and rRNA (Figure 31.8). They consist of two subunits—one large and one small—whose relative sizes are generally given in terms of their sedimentation coefficients, or S (Svedberg) values. [Note Because the S values are determined both by shape as well as molecular mass, their numeric values are not strictly additive. For example, the prokaryotic 50S and 30S ribosomal subunits together form a ribosome with an S value of 70. The eukaryotic 60S and 40S subunits form an 80S ribosome.] Prokaryotic and eukaryotic ribosomes are similar in structure, and serve the same function, namely, as the "factories" in which the synthesis of proteins occurs. 

S ribosomes into their 40S and 60S subunits. This depends upon the 700-kDa eIF3, a complex of 5-11 peptides of mass 30 to 170 kDa each, which binds to the 40S subunit (Fig. 29-11, step a). 306,311,318-320 jn a sepa ... 

The elongation cycle for E. coli is shown in Fig. 29-12. That for eukaryotic ribosomes is similar except that 40S and 60S subunits are involved in formation of the complete 80S ribosome. 

A prokaryotic 70S ribosome comprises two subunits (50S and 30S). The 50S subunit has 23S and 5S rRNAs complexed with 34 polypeptides whereas the 30S subunit contains 16S rRNA and 21 polypeptides. A eukaryotic 80S ribosome comprises two subunits (60S and 40S). The 50S subunit has 28S, 5.8S and 5S rRNAs complexed with approx. 49 polypeptides whereas the 40S subunit contains 18S rRNA and about 33 polypeptides. 

Tetracyclines inhibit protein biosynthesis by acting on the 70S and 80S ribosomes. 

Pestova, T.Y, I.B. Lomakin, and C.U. Hellen. 2004. Position of the CrPV IRES on the 40S subunit and factor dependence of IRES/80S ribosome assembly. EMBO Rep 5 906-913. 

PS - 80S ribosomal 60S subunit PT at or near A site (binds at Anisomycin site) [antileukaemic, antitumour]... 

PS — non-competitive 80S ribosomal 60S subunit PT, competes with Trichothecin (triggers ribotoxic stress response activating JNK1)... 

PS - 80S ribosomal 60S subunit PT [apoptotic, cytotoxic, toxic] PS - 80S ribosomal 60S subunit PT [apoptotic, cytotoxic, toxic] PS - 80S ribosomal 60S subunit PT (at 10) [caspase-3 JNK1 activation (at 10)]... 

SERPR, serine protease SialylT, sialyltransferase SLOX, soya bean 15-lipoxygenase, soya bean lipoxygenase SNF1K, SNF1 protein kinase kinase 80S PT, 80S ribosome (eukaryote) peptidyl transferase... 

Beckmann, R., Spahn, C. M. T., Eswar, N., Helmers, J., Penczek, P. A., Sali, A., Frank, J., and Blobel, G. (2001). Architecture of the protein-conducting channel associated with the translating 80S ribosome. Cell 107, 361-372. 

Hummel, H., Bock, A., 23S ribosomal RNA mutations in halobacteria conferring resistance to the anti-80S ribosome targeted antibiotic anisomydn. Nuddc Acids Res. 1987, 15, 2431-2443. 

The ribosomal subunits migrate through the nuclear pores into the cytoplasm where they complex with mRNA, forming 80S ribosomes. Because sedimentation coefficients reflect both shape and particle weight, they are not additive. 

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