Eukaryotes signal transduction requirements

The posttranslational modification of proteins is required for the specific function of the proteins, as well as their stability, degradation, and control of various biological processes. For example, certain proteins must be phosphorylated, i.e., one or more PO4 groups are added to the protein chain. After phosphorylation, proteins become active in the signal transduction pathway, cell division, and other systems in an organism. In higher eukaryotes, one third of all proteins are phosphorylated. 

Figure 17-1 Universal regulation by protein phosphorylation. Protein phosphorylation requires the coordinated actions of protein kinases, which transfer a phosphoryl group to a target protein, and protein phosphatases, which remove it via hydrolysis. Phosphorylation of a target protein can change its biological activity in many ways including enzymatic activity, intracellular localization, and its ability to interact with other macromolecules such as DNA, RNA, and proteins. The most common amino acids which are phosphorylated in eukaryotic organisms are serine, threonine, and tyrosine. Phosphorylation on histidine with subsequent phosphoryl transfer to aspartic acid represents a coitunon modification in prokaryotic two-component signal transduction s)rstems (see Figure 17-15A).

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