Regulation of Cellular Functions by Phosphorylation and Dephosphorylation of Proteins: An Introduction

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During the last decade it has become apparent that activation of specific protein kinases by intracellular generated signals — such as calcium, cAMP, cGMP and other less well defined small molecules — and;concomitant phosphorylation of key regulatory proteins or enzymes is one important mechanism by which many hormones, neurotransmitters and autacoids (locally generated and acting hormones) regulate cellular functions. From the results obtained so far it is quite clear that protein phosphorylation is not the only regulatory mechanism triggered by the binding of hormones to plasma membrane receptors. This mechanism is only operative in such situations in which the cellular response follows hormone binding within seconds to minutes. This mechanism phosphorylation/dephosphorylation has not been observed in very fast responses (milliseconds) — presumably since the protein kinase catalyzed phophotransfer is too slow to modify a significant number of protein molecules before the onset of such fast cellular responses. In addition protein phosphorylation is not primarily involved in slow cellular responses occurring within hours — presumably since the covalent modification carried out by the protein kinase is not stable for hours but easily reversed by the action of one or several protein phosphatases. However, although the very fast and very slow cellular responses are not mediated by phosphorylation of some regulatory proteins, these responses may be modulated by the phosphorylation or dephosphorylation of some proteins involved in these responses.