Ribosomes Phosphorylation

Rapamycin is an immunosuppressive diug and an inhibitor of S6K1 (also known as p70S6-kinase) which phosphorylates ribosomal S6 protein. S6K1 is activated in response to insulin via activation of Akt. Rapamycin binds to a specific target protein (mTOR, mammalian target of rapamycin) which is functionally located downstream of Akt, but upstream... 

Kinases that specifically phosphorylate ribosomal protein S6... 

Phosphorylated IRS-1 activates a second signaling pathway by interacting with an 85-kDa SH2-containing protein that is a subunit of phophatidylinositol 3-kinase.384 386 This activates the 110-kDa catalytic subunit of the 3-kinase, which catalyzes formation of phosphatidylinositol 3-phosphate as well as Ptdlns (3,4)P2 and Ptdlns (3,4,5)P3.387/387a These compounds, which remain within membranes, activate other branches of the signaling cascade, some of which may converge with those of the MAP kinase cascade. However, there appears to be specific activation of a ribosomal Ser/Thr kinase that, among other activities, phosphorylates ribosomal protein S6, a component of the small ribosomal subunit.388 It also phosphorylates some isoforms of protein kinase C and other enzymes. Ptdlns 3-kinase may also activate 6-phosphofructo-2-kinase (Fig. 11-2, step ti).384/388... 

Two protein kinases can phosphorylate ribosomal proteins in vitro one of the enzymes is attached to the ribosome and the other is found in the cytosol. Both enzymes require cyclic AMP for activity. The y-phosphoryl group of ATP or GTP is transferred to primarily serine and, to a lesser degree, threonine residues. Four proteins of the 40 S unit and 10 proteins of the 60 S units are phosphorylated in vitro. 

Since hormonal regulation of Sg protein phosphorylation was observed only in vivo and was absent in vitro, it is probable that some cofactors or second messengers are necessary for transduction of the hormonal signal to the protein kinase phosphorylating ribosomal proteins. 

Although the interior of a prokaryotic cell is not subdivided into compartments by internal membranes, the cell still shows some segregation of metabolism. For example, certain metabolic pathways, such as phospholipid synthesis and oxidative phosphorylation, are localized in the plasma membrane. Also, protein biosynthesis is carried out on ribosomes. 

S6K1 (also known as p70S6 kinase) is a serine/ threonine protein kinase which is involved in the regulation of translation by phosphorylating the 40S ribosomal protein S6. Insulin and several growth factors activate the kinase by phosphorylation in a PI 3-kinase dependent and rapamycin-sensitive manner. Phosphorylation of S6 protein leads to the translation of mRNA with a characteristic 5 polypyrimidine sequence motif. 

DNA sequencing reveals the order in which amino acids are added to the nascent polypeptide chain as it is synthesized on the ribosomes. However, it provides no information about posttranslational modifications such as proteolytic processing, methylation, glycosylation, phosphorylation, hydroxylation of prohne and lysine, and disulfide bond formation that accompany mamra-tion. While Edman sequencing can detect the presence of most posttranslational events, technical hmitations often prevent identification of a specific modification. 

The activity of 4E is regulated in a second way, and this also involves phosphorylation. A recently discovered set of proteins bind to and inactivate 4E. These proteins include 4E-BP1 (BPl, also known as PHAS-1) and the closely related proteins 4E-BP2 and 4E-BP3. BPl binds with high affinity to 4E. The [4E] [BP1] association prevents 4E from binding to 4G (to form 4F). Since this interaction is essential for the binding of 4F to the ribosomal 40S subunit and for correctly positioning this on the capped mRNA, BP-1 effectively inhibits translation initiation. 

Sturgill TW, Ray LB, Erikson E, Mailer JL 1988 Insulin-stimulated MAP-2 kinase phosphorylates and activates ribosomal protein S6 kinase II. Nature 334 715-718... 

Not all the cellular DNA is in the nucleus some is found in the mitochondria. In addition, mitochondria contain RNA as well as several enzymes used for protein synthesis. Interestingly, mitochond-rial RNA and DNA bear a closer resemblance to the nucleic acid of bacterial cells than they do to animal cells. For example, the rather small DNA molecule of the mitochondrion is circular and does not form nucleosomes. Its information is contained in approximately 16,500 nucleotides that func-tion in the synthesis of two ribosomal and 22 transfer RNAs (tRNAs). In addition, mitochondrial DNA codes for the synthesis of 13 proteins, all components of the respiratory chain and the oxidative phosphorylation system. Still, mitochondrial DNA does not contain sufficient information for the synthesis of all mitochondrial proteins most are coded by nuclear genes. Most mitochondrial proteins are synthesized in the cytosol from nuclear-derived messenger RNAs (mRNAs) and then transported into the mito-chondria, where they contribute to both the structural and the functional elements of this organelle. Because mitochondria are inherited cytoplasmically, an individual does not necessarily receive mitochondrial nucleic acid equally from each parent. In fact, mito-chondria are inherited maternally. 

Mazroui, R., Sukarieh, R., Bordeleau, M. E., Kaufman, R. J., Northcote, P., Tanaka, J., Gallouzi, I., and Pelletier, J. (2006). Inhibition of ribosome recruitment induces stress granule formation independendy of eukaryotic initiation factor 2alpha phosphorylation. Mol. Biol. Cell 17, 4212-4219. 

The main method through which these agents regulate the translational apparatus is via changes in the states of phosphorylation of translation factors and related proteins phosphorylation may, for example, alter the intrinsic activity of translation factors or affect their ability to bind other components (other factors, the ribosome, or RNA). 

Figure 13.1 Schematic diagram of eukaryotic translation initiation. The sites of action of small molecule inhibitors are shown with dashed lines. Kinases that affect the phosphorylation of 4E-BP and eIF2a, and exert effects on ribosome recruitment and ternary complex formation, respectively, are shown in a black box. See text for details.

Gribskov, M. (1992). Translational initiation factors IF-1 and eIF-2 alpha share an RNA-binding motif with prokaryotic ribosomal protein SI and polynucleotide phosphoryl-ase. Gene 119, 107-111. 

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