Protein kinase from mammalian cells

Everolimus is an oral rapamycin analogue which selectively inhibits the mammalian target of rapamycin, mTOR, an intracellular protein kinase implicated in cell cycle control, specifically in the progression of cells from the Gi to S phase, which acts as a sensor that integrates extracellular and intracellular events to coordinate growth and proliferation [33 ]. 

FIGURE 15.7 Cyclic AMP-dependent protein kinase (also known as PKA) is a 150- to l70-kD R9C9 tetramer in mammalian cells. The two R (regulatory) subunits bind cAMP ( = 3 X 10 M) cAMP binding releases the R subunits from the C (catalytic) subunits. C subunits are enzymatically active as monomers. 

Figure 10.6 Schematic representation of the mitogen-activated protein kinase (MAP) cascades in mammalian cells. (From Voet and Voet, 2004. Reproduced with permission from John Wiley Sons., Inc.)...

A calpain inhibitor, the DPK of N-dimethyltyrosine, was isolated from Streptomyces griseus and this compound showed activity in the calpain assay as described by Alvarez etal Calpain is a cytosolic protease regulated by calcium and is distributed in mammalian and avian cells. Calpain catalyzes proteolysis of target protein in cells, causing changes in metabolic processes such as the activation of protein kinase C, neuropeptide metabolism, and the activation of platelets. It is proposed that these inhibitors can be used in the treatment of neurodegen-erative diseases. 

Histone kinases responsible for N-phosphorylation have been isolated from regenerating rat liver [109] and Walker-256 carcinosarcoma cells [110]. One kinase with a pH optimum of 9.5 phosphorylated His-18 and His-75 of H4, while the other with a pH optimum of 6.5 phosphorylated lysine of HI. The enzyme from regenerating rat liver phosphorylated H4 at 1-phosphoryl histidine, while the carcinosarcoma enzyme phosphorylated H4 His at the position 3 [111]. Both kinases were cAMP independent [110]. Matthews and colleagues purified a 32-kDa histidine H4 kinase from yeast, Saccharomyces cerevisiae [112,113]. The enzyme phosphorylated His-75 (1-phosphoryl histidine) in H4. His-18 of H4 and other histidines in other core histones were not phosphorylated by this kinase [112]. Protein phosphatases 1, 2A, and 2C could dephosphorylate His-75 of H4 [114]. Applying a gel kinase approach to detect mammalian H4 histidine kinases, Besant and Attwood detected four activities in the 34-41 kDa range with extracts from porcine thymus [115]. 

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