Ribosomal 56 protein phosphorylation

Criteria for Establishment of the Biological Significance of Ribosomal Protein Phosphorylation J. Gordon, P. J. Nielsen, K. L. Manchester, H. Towbin, L. Jimenez De Asua, and G. Thomas... 

Roles of Cytokinin and ABA in Modulation of Ribosomal Protein Phosphorylation... 

A sugar-dependent in vitro antiproliferative activity was observed along with inhibition of S6 ribosomal protein phosphorylation. The 6-alkylated perillyl glu-coside 61 was converted into the corresponding 6 -triazole perillyl glucoside 62 via treating with 3-azido propanol using CuAAC in 82% yield (Scheme 6.16) [38]. 

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. 

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... 

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). 

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. 

Takagi M, Absalon MJ, McLure KG, Kastan MB (2005) Regulation of p53 translation and induction after DNA damage by ribosomal protein L26 and nucleolin. Cell 123 49-63 Tawfic S, Goueli SA, Olson MO, Ahmed K (1994) Androgenic regulation of phosphorylation and stability of nucleolar protein nucleolin in rat ventral prostate. Prostate 24 101-106. 

A further susceptible point for insulin-regulated signaling pathways is the ribosomal protein S6. Under the influence of insulin, S6 is phosphorylated by a specific protein kinase, the p70 kinase, resulting in increased levels of translation of certain mRNAs. Several pathways including the MAPK/ERK pathway (see chapter 10) and the Akt kinase pathway can contribute to the activation of the p70 kinase. 

The membrane-associated Akt kinase is now a substrate for protein kinase PDKl that phosphorylates a specific Thr and Ser residue of Akt kinase. The double phosphorylation converts Akt kinase to the active form. It is assumed that the Akt kinase now dissociates from the membrane and phosphorylates cytosolic substrates such as glycogen synthase kinase, 6-phosphofructo-2-kinase and ribosomal protein S6 kinase, p70 . According to this mechanism, Akt kinase regulates central metabolic pathways of the cell. Furthermore, it has a promoting influence on cell division and an inhibitory influence on programmed cell death, apoptosis. A role in apoptosis is suggested by the observation that a component of the apoptotic program. Bad protein (see Chapter 15) has been identified as a substrate of Akt kinase. 

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... 

Nielsen, P. J., Manchester, K. L., Towbin, H., Gordon, J., and Thomas, G. (1982) The phosphorylation of ribosomal protein S6 in rat tissues following cycloheximide injection, in diabetes, and after denervation of diaphragm. J. Biol. Chem. 257, 12316-12321. 

There is recent data to suggest that there may in fact be a biological role for the internalized insulin and EGF receptors (both of which are themselves tyrosine kinases). Thus, microinjection of insulin-occupied insulin receptors into Xenopus oocytes causes the increased phosphorylation of ribosomal protein S6 (a known substrate for the insulin receptor/kinase) [62] and the EGF receptor in endocytic vesicles has been shown to retain its kinase activity [63]. Whether the internalized insulin receptor/kinase or EGF receptor/kinase has a physiological role or not is as yet unknown. Clearly, though, these data suggest that there is much more to be learned about the role of internalized hormone-receptor complexes, especially those where the receptor possesses intrinsic enzymatic activity. 

Regulation of eukaryotic translation provides a rapid way to control gene expression and is used frequently. In some cases this is achieved by altering components of the translational complex. This is expected to affect the translation of all mRNAs. Such a mechanism is the phosphorylation of 40 S ribosomal proteins, which results in higher polysome levels in cells stimulated by growth factors. 

Rapid phosphorylation of the other detected phosphoproteins does occur but no definite roles have yet been ascribed to them. The 33 kDa protein may be the S6 ribosomal protein involved in the control of protein synthesis. The 57 kDa protein has been identified as the regulatory suhunit of the cyclic AMP-dependent protein kinase [44]. Of the other proteins the 76, 43 and 20 kDa may be connected with the microfilaments (43 kDa actin, 76 kDa myosin light chain kinase and 20 kDa myosin light chain) but this must be further investigated. These proteins may only play a permissive role in. steroidogenesis. The fact that the pattern of protein phosphorylation is very similar after stimulation of protein kinase C with phorbol esters supports this because the latter only marginally increase steroidogenesis [18]. 

The molecular basis for regulation of enzymatic activity through phosphorylation and dephosphorylation has been established in many enzyme systems (29). The significance of these reactions in histones, ribosomal proteins and KNA polymerase is not known. In an attempt to establish the specificity of the cyclic AMP-dependent protein kinases, the structure of several substrates have been determined (30). The data indicate that the sequence around the phosphorylated serine residue all contain two basic amino acids separated by no more than two residues from the N-terminal of the susceptible serine (e.g. -Arg-Arg-X-Y-Ser-). 

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