Regulation of adrenergic receptor function by phosphorylation.
Mounting evidence suggests that the physiological function of the various subtypes of adrenergic receptors is controlled by phosphorylation/dephosphorylation reactions. It seems intuitively unlikely that this phenomenon will be limited simply to the adrenergic receptors, since these receptors share transmembrane signaling pathways with a host of other plasma membrane receptors. Different types of kinases appear to be involved. On the one hand, phosphorylation reactions may operate in a classical feedback regulatory sense. Thus, the cAMP-dependent protein kinase, once activated by a beta-agonist, can feedback-regulate the function of the receptors by phosphorylating and desensitizing them. Similarly, protein kinase C appears to be able to feedback-regulate the function of alpha 1-adrenergic receptors by phosphorylation. There may also be "cross talk" between the systems. Thus, protein kinase C, when stimulated by phorbols, is able to phosphorylate and desensitize the beta-adrenergic receptors. Moreover, very recently we have found that the cAMP-dependent protein kinase can phosphorylate the alpha 1-adrenergic receptors in vitro. These are examples of one transmembrane signaling system regulating the function of another. Perhaps most interestingly, it appears that there may be a previously unappreciated class of receptor kinases in the cytosol of cells. The first of these, which we have recently found and named beta-ARK, serves to phosphorylate only the agonist-occupied form of the beta-adrenergic receptor. As noted, it is somewhat analogous to the rhodopsin kinase. Such highly specific receptor kinases, which can phosphorylate only the agonist-occupied form of a receptor, represent a potentially elegant mechanism for controlling the function of receptors in a fashion which is linked to their physiological stimulation. How widespread such kinases are, and the actual roles which they play in regulating receptor function, remain to be determined. Finally, it should be stressed that although this review has focused on the regulatory role of receptor phosphorylation, it is by no means our intent to suggest that receptors are the only locus for physiological control of sensitivity to hormone and drug reaction. There is already evidence that guanine nucleotide regulatory proteins can be regulated, and it seems likely that each of the components of the system, including the adenylate cyclase, are likely to be involved in various forms of complex regulation. To date, however, the receptors represent that component of the system whose regulation we understand in the greatest detail.
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