Ectoprotein Kinase in the Regulation of Cellular Responsiveness to Extracellular ATPa

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Abstract
The reversible phosphorylation of intracellular proteins has been established as a key regulatory mechanism in numerous cellular functions. In this process the enzyme protein kinase transfers the gamma-phosphate of ATP to form a covalent bond with specific proteins. Another line of investigation has demonstrated that extracellular ATP is a potent physiological regulator in various cellular systems. Although many of the physiological effects of extracellular ATP were shown to be mediated by the action of purinergic receptors, it is possible that extracellular protein phosphorylation systems are also implicated in the mechanisms underlying the responsiveness of cells to extracellular ATP. The identification of ectoprotein kinase at the surface of various cells has provided evidence for the existence of such mechanisms, and revealed how the regulatory powers of protein phosphorylation systems can extend to the extracellular environment. The versatile roles that extracellular protein phosphorylation activity may play in the regulation of cellular functions is underscored by the presence of multiple protein substrates for this activity at the cell surface. Each such surface phosphoprotein may have a unique function. FIGURE 5 depicts the hypothetical relationships between the extracellular ATP secreted by exocytosis and the specific physiological function of these secreting neurons. Based on findings described in this article, we propose that extracellular ATP can be utilized by two types of extracellular protein kinase: a membrane-bound ectoprotein kinase, and a soluble exoprotein kinase. The exoprotein kinase can originate by detachment of an ectokinase from the cell surface, or be an intravesicular protein that is coreleased with ATP by exocytosis from stimulated cells. Phosphorylation of specific proteins at the surface of a secreting cell may have an important feedback control over its own presynaptic activity. The ectoprotein kinase could exert this feedback regulation by phosphorylating ion channels involved in secretion, and/or by phosphorylating transporters that carry out the reuptake of released transmitter molecules. Phosphorylation of receptors can regulate intercellular communication, and phosphorylation of integrins could regulate the interaction of the cell surface with components of the extracellular matrix. Although most of the relationships suggested in FIGURE 5 are still hypothetical, it should be possible to test them experimentally in a direct manner by raising antibodies against the phosphorylated sites of specific surface phosphoproteins. The ability of such antibodies to inhibit protein phosphorylation without penetrating the cells provides an experimental paradigm for the direct testing of potential physiological function of ecto- and exoprotein kinase activities in a variety of cells.