Ribosome associated protein-kinase

The increase in nuclear cyclin B/CDKl activity promotes phosphorylation of nuclear substrates that are necessary for mitosis, such as nuclear envelope breakdown, spindle formation, chromatin condensation, and restmcturing of the Golgi and endoplasmic reticulum (85, 86). Numerous cyclin B/CDKl substrates have been dehned, which include nuclear lamins, nucleolar proteins, centrosomal proteins, components of the nuclear pore complex, and microtubule-associated proteins (87-89). Cyclin B/CDKl complexes also phosphorylate MCM4 to block replication of DNA, the TFIIH subunit of RNA polymerase II to inhibit transcription, and the ribosomal S6 protein kinase to prevent translation during mitosis (90-92). 

It is, therefore, likely, that phosphorylation of eIF-2 impairs one of its function required for met-tRNA binding to 4OS ribosomes, as shown recently in reticulocyte lysate for the haemin-regulated eIF-2 phosphorylation (14, I5). A role for the other ribosome-associated proteins phosphorylated by the dsRNA-dependent protein kinase, e.g. protein 67 and I7 (4)> "the translational inhibition is, however, not excluded. Factor Ml may also be involved (5) ... 

There are at least three major effector pathways that are activated by neurotrophic factor-Trk receptors. The best-characterized pathway is the extracellular-regulated kinase (ERK) cascade, which is regifiated by activation of Ras, a small membrane-bound G protein. Activation of Ras occurs when activated Trk receptor associates with adaptor proteins and a GTP exchange factor (see Russell and Duman 2002 for details). Ras in turn recruits and activates a serine threonine kinase, Raf, to the membrane resulting in the activation of ERK kinase (also referred to as MEK) and ERK (also known as mitogen activated protein kinase or MAPK). Activation of the Ras-Raf-MEK-ERK cascade can lead to regifiation of many celMar proteins, including ribosomal S6-kinase (RSK). 

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. 

Signal transduction events associated with IL-2 receptor interaction have been extensively studied. Surprisingly, since the IL-2 receptor does not contain a tyrosine kinase domain, one of the earliest events observed is tyrosine phosphorylation of the IL-2 receptor p chain (FI 1). Later events include the protein kinase C-depen-dent phosphorylation of a 68-kDa protein (Ell) with apparent identity to the cy-toskeletal protein /-plastin (Z6) and a signal transduction event shared by several hematopoietic growth factor systems (E13). Phosphorylation of the ribosomal S6 protein and increased protein synthesis also follow IL-2 receptor interaction (E12). Negative signals appear to involve cAMP-mediated events (F9, FIO). 

Roberts WK, Hovanessian A, Brown RE, Clemens MJ, Kerr IM (1976) Interferon-mediated protein kinase and low-molecular-weight inhibitor of protein synthesis. Nature 264 477 80 Roller RJ, Roizman B (1990) The herpes simplex virus Us 11 open reading frame encodes a sequence-specific RNA-binding protein. J Virol 64 3463-3470 Roller RJ, Roizman B (1992) The herpes simplex virus 1 RNA binding protein US 11 is a virion component and associates with ribosomal 60S subunits. J Virol 66 3624-3632... 

Low concentrations of dsRNA also activate a protein kinase which phosphorylates the small subimit of eIF-2 (23). This kinase differs from the haem-controlled enzyme in its subcellular location (it is mainly associated with the ribosomes), in its somewhat broader substrate specificity, and in the nature of the agents which control its activation. Thus, whilst haemin and high GTP levels (36) each antagonise the activation of the haem-controlled kinase, high levels of dsRNA have no effect on this process, yet do specifically prevent the activation of the dsRNA-controlled kinase... 

The presence of dsRNA in lysates leads to phosphorylation of eIF-2 (Farrell et al., 1977) at the same site in the a-subunit that is phosphorylated by HCR (Ernst et al., 1980). The protein kinase responsible for this phosphorylation is activated by dsRNA and is an-tigenically distinct from HCR (Petryshyn et al., 1979). This dsRNA-activated inhibitor of translation is ribosome-associated, while HCR is found in the postribosomal supernatant (Farrell et al., 1977). The purified dsRNA-activated kinase undergoes autophosphorylation upon activation, yielding a labeled 67,000 component (Levin et al, 1981). 

Samuel, C. E., 1979, Mechanism of interferon action Phosphorylation of protein synthesis initiation factor eIF-2 in interferon treated human cells by a ribosome associated kinase processing site specificity similar to hemin-regulated rabbit reticulocyte kinase, Proc. Natl Acad. Sci. USA 76 600. 

In contrast, several lines of evidence suggest that the activation of the Ras-Raf-ERKl/2 pathway may oppose JNK-mediated effects with respect to mitochondria-dependent apoptosis (Fig. 4). ERKl/2 can phosphorylate and activate mitogen-and stress-activated kinase 1 (MSKl) and pp90 ribosomal S6 kinase (RSK) [83,84]. RSK can phosphorylate BAD, thereby inhibiting its proapoptotic effects [83]. Furthermore, both RSK and MSKl are potent activators of cyclic adenosine monophosphate (cAMP) element binding protein (CREB), a transcription factor for Blc-2, and therefore important for cell survival [84]. The upregulation or overexpression of Bcl-2 has been reported to be associated with the inactivation of JNK [75,85]. Thus, ERKl/2 activation could ultimately lead to JNK inactivation. Finally, the activation of c-Ras has been shown to result in a suppression of Bax expression, via a mechanism that involved activation of both the ERKl/2 signaling cascade and the phosphatidylinositol-3 -kinase (PI-3)/Akt pathway [80]. 

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