Amino acids cellular regulation

Both the overall rate of protein synthesis and the translation of certain specific mRNAs are controlled by agents such as hormones, growth factors, and other extracellular stimuli. As precursors for protein assembly, amino acids also regulate the translational machinery. Because protein synthesis consumes a high proportion of cellular metabolic energy, the energy status of the cell also modulates translation factors. 

The ankyrin repeat motif is one of the most common protein-protein interaction domains. Ankyrin repeats are modules of about 33 amino acids repeated in tandem. They are found in a large number of proteins with diverse cellular functions such as transcriptional regulators, signal transducers, cell-cycle regulators, and cytoskeletal proteins. 

Hormonal actions on target neurons are classified in terms of cellular mechanisms of action. Hormones act either via cell-surface or intracellular receptors. Peptide hormones and amino-acid derivatives, such as epinephrine, act on cell-surface receptors that do such things as open ion-channels, cause rapid electrical responses and facilitate exocytosis of hormones or neurotransmitters. Alternatively, they activate second-messenger systems at the cell membrane, such as those involving cAMP, Ca2+/ calmodulin or phosphoinositides (see Chs 20 and 24), which leads to phosphorylation of proteins inside various parts of the target cell (Fig. 52-2A). Steroid hormones and thyroid hormone, on the other hand, act on intracellular receptors in cell nuclei to regulate gene expression and protein synthesis (Fig. 52-2B). Steroid hormones can also affect cell-surface events via receptors at or near the cell surface. 

The members of the protein kinase C family are central signal proteins and as such, are involved in the regulation of a multitude of cellular processes. A problem in the identification of substrates of protein kinase C is its low substrate specificity which often cannot be differentiated from that of protein kinase A, particularly in in vitro experiments. The consensus sequence of the phosphorylation sites in substrate proteins are similar to those of protein kinase A, in that basic amino acids are required in the neighborhood of the Ser/Thr residue to be phosphorylated. The following consensus sequences may be formulated for phosphorylation by protein kinase C ( = phosphorylation site) S /T XK/R K/RXXS /T K/RXXS /T XK/R K/RXS /T K/RXS /T XK/R (Pearson and Kemp, 1991). 

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