The rapid and reversible phosphorylation of proteins catalyzed by protein kinases and protein phosphatases is a well recognized mechanism of regulation in cells. This bidirectional process is a highly flexible method of influencing cellular activity in response to a variety of incoming stimuli. A physiological role for protein phosphorylation was first identified about 50 years ago while investigating the regulation of glycogen metabolism (Fischer and Krebs 1955; Sutherland 1972). In fact, many aspects of gene regulation; cell cycle control, transport, and secretion; actin remodeling; and cell adhesion are controlled by this mechanism (Krebs 1985; Hunter 1995; Pawson and Scott 1997; Cohen 2000; Goodman and Smolik 2000; Pawson and Nash 2000). The utility of protein phosphorylation as the predominant form of covalent modification of proteins in vivo is exemplified by the finding that∼ 30% of intracellular proteins are phosphoproteins (Hunter 1987). Not surprisingly, the breakdown in signal transduction may be the cause or consequence of many diseases, including cancer, diabetes, arthritis, and Alzheimer’s (Cohen 1999). Most signaling pathways are composed of common elements. The initial signal is transduced through a receptor at the plasma membrane (such as a G-protein coupled receptor, or a receptor tyrosine kinase or phosphatase), which results in activation of the receptor or the mobilization of receptor-associated proteins to generate some form of intracellular message. This signal is then directed throughout the cell either by the diffusion of a small soluble second messenger or the translocation of an activated enzyme. At a molecular level, phosphorylation mediates the regulation of enzymatic activities by causing allosteric conformational changes, or by directly enhancing or blocking access to enzyme catalytic sites (Johnson and Barford 1990; Barford et al. 1991; Johnson and O’Reilly 1996). More recently it has been realized that an essential feature of signaling by protein phosphorylation is the modulation of protein–protein interactions. These are mediated by a growing number of protein interaction modules including WW, SH2, PTB, 14-3-3, WD-40, FHA, and FF domains that may associate with their binding partners in a phosphorylation-dependent manner (Cantley et al. 1991; Pawson and Gish 1992; Pawson 1995). These protein–protein interactions generate molecular networks that drive intracellular signaling events. This fascinating topic was recently the subject of several excellent reviews (Burack and Shaw 2000; Hunter 2000; Jordan et al. 2000; Pawson and Nash 2000; Sudol and Hunter 2000). In this article we emphasize how the subcellular location of protein kinases and phosphatases provides a means to restrict where and when phosphorylation events occur. In particular, we discuss the compartmentalization of the cAMP-dependent protein kinase (protein kinase A; PKA) through its association with A-kinase anchoring proteins (AKAPs). cAMP-dependent signaling mechanisms

The second messenger cAMP was identified in 1957 as a factor that stimulates glycogen phosphorylase (Rall et al. 1957; Rall and Sutherland 1958). However, this discovery as well as the development of the second messenger concept was not fully appreciated until a cAMP-dependent protein kinase was characterized over a decade later (Walsh et al. 1968). Ever since this initial discovery, an intense global research effort has elucidated many molecular and cellular actions of cAMP. Engagement of transmembrane receptors and the recruitment of intermediary G proteins activate adenylyl cyclases on the inner face of the plasma membrane (Tang and …