Control of cell function by reversible protein phosphorylation.

In conclusion, multiple protein kinases and phosphatases are involved in the mediation of the physiological response of a specific cell to extracellular stimuli. This can be achieved by changes in the intracellular levels of "mediators" or "second messengers", which alter the activity of selective protein kinases and/or phosphatases. The involvement of reversible protein phosphorylation in the modulation of the function of cell surface receptors, in the synthesis and degradation of the second messengers, altered cellular metabolism, protein synthesis and gene expression suggests a vital role for this regulatory mechanism in the control of cellular function. Protein phosphorylation may be implicated in the interplay between various second messenger systems resulting in a complex regulation of target proteins by multi-site phosphorylation. The presence of unique as well as common targets for the multiple protein kinases and phosphatases suggests a coordinated mechanism for the regulation of cellular function, which can also account for the diversity of cellular response. Future studies will establish the temporal and spatial organization of the regulatory pathways utilized by specific hormones to alter the function of subcellular compartments featured in the overall physiological response.