Protein kinase Subtypes

The ANP leceptoi exists in two forms, ANP and ANPg, both of which have been cloned. These membrane-bound guanylate cyclases have a single transmembrane domain, an intracellular protein kinase-like domain, and a catalytic cyclase domain, activation of which results in the accumulation of cychc guanosine monophosphate (cGMP). A third receptor subtype (ANP ) has been identified that does not have intrinsic guanylate cyclase activity and may play a role in the clearance of ANP. 

Activation of Mi, M3, and M5 mAChRs does not only lead to the generation of IP3 followed by the mobilization of intracellular Ca2+, but also results in the stimulation of phospholipase A2, phospholipase D, and various tyrosine kinases. Similarly, M2 and M4 receptor activation does not only mediate the inhibition of adenylyl cyclase, but also induces other biochemical responses including augmentation of phospholipase A2 activity. Moreover, the stimulation of different mAChR subtypes is also linked to the activation of different classes of mitogen-activated protein kinases (MAP kinases), resulting in specific effects on gene expression and cell growth or differentiation. 

Protein kinase B, or Akt, was discovered as the product of an oncogene of the acutely transforming retrovirus AKT8, causing T-cell lymphomas in mice. It encodes a fusion product of a cellular serine/threonine protein kinase and the viral structural protein Gag. This kinase is similar to both protein kinase Ce (PKCe 73% identity to the catalytic domain) and protein kinase A (PKA 68%). It differs from other protein kinases in that it contains a pleckstrin homology (PH) domain, which allows it to bind to polyphosphoinositide head groups (and also to G-protein fly subunits). To date, three subtypes have been identified a, (3, and y, all of which show a broad tissue distribution. It... 

FIGURE 21-6 Schematic illustration of the overall structure and regulatory sites of eleven different phosphodiesterase subtypes. The catalytic domain of the phosphodiesterases are relatively conserved, and the preferred substrate(s) for each type is shown. The regulatory domains are more variable and contain the sites for binding of Ca2+/calmodulin (CaM) and cGMP, as well as GAF and PAS domains. The regulatory domains also contain sites of phosphorylation by cAMP-dependent protein kinase (PKA). 

Regulation of neurotransmitter receptors the p-adren-ergic receptor. This receptor, of which three subtypes have been cloned, mediates many of the effects of norepinephrine and epinephrine in the brain and peripheral tissues. One of the dramatic features of P-adrenergic receptor function is its rapid desensitization in response to agonist stimulation. It is now known that one important mechanism for this desensitization is phosphorylation of the receptor both by PKA and by a receptor-associated protein kinase, PARK (also called GRK2 Fig. 23-6). 

Nakajima, Y., Yamamoto, T Nakayama, T and Nakanishi, S. (1999) A relationship between protein kinase C phosphorylation and calmodulin binding to the metabotropic glutamate receptor subtype 7. J. Biol. Chem. 274,27573-27577. 

Protein kinase B (PKB), which is also known as Akt, is a serine/threonine kinase that also belongs to the AGC kinase subtype. Three mammalian isoforms—PKBo , /3, and y have been identified. These proteins are broadly expressed and, although isoform-specific patterns of expression exists in some tissues, the three kinases have a similar organizational structure an amino-terminal PH domain, a central serine/threonine catalytic domain, and a short regulatory region at the carboxy terminal end containing the so-called hydrophobic motif [118-120]. 

All muscarinic receptors are members of the seven transmembrane domain, G protein-coupled receptors, and they are structurally and functionally unrelated to nicotinic ACh receptors. Activation of muscarinic receptors by an agonist triggers the release of an intracellular G-protein complex that can specifically activate one or more signal transduction pathways. Fortunately, the cellular responses elicited by odd- versus even-numbered receptor subtypes can be conveniently distinguished. Activation of Ml, M3, and M5 receptors produces an inosine triphosphate (IP3) mediated release of intracellular calcium, the release of diacylglyc-erol (which can activate protein kinase C), and stimulation of adenylyl cyclase. These receptors are primarily responsible for activating calcium-dependent responses, such as secretion by glands and the contraction of smooth muscle. 

Norepinephrine is released into the synapse from vesicles [(1) in Fig. 2.7] amphetamine facilitates this release. Norepinephrine acts in the CNS at two different types of noradrenergic receptors, the a and the P [see (2a), (2b) and (3) in Fig. 2.7]. a-Adrenergic receptors can be subdivided into receptors (coupled to phospholipase and located postsynaptically) and tt2 receptors (coupled to Gj and located primarily presynapti-cally) (Insel, 1996). P-Adrenergic receptors in the CNS are predominantly of the P subtype (3 in Fig. 2.7). P receptors are coupled to and lead to an increase in cAMP. Cyclic AMP triggers a variety of events mediated by protein kinases, including phosphorylation of the P receptor itself and regulation of gene expression via phosphorylation of transcription factors. 

The nature of the second messenger response to a given neurotransmitter depends on the subtype of receptor to which it binds and the G protein to which the receptor is coupled. Three of the most commonly utilized G proteins include G, which stimulates adenylyl cyclase to produce cyclic AMP (cAMP) Gj, which inhibits adenylyl cyclase, resulting in lower intracellullar levels of cAMP and Gq, which activates phospholipase C to produce the second messengers IP3 and DAG. In general, these activities refer to the function of the a subunit however, it should be pointed out that the py complex has its own set of activities (on adenylyl cyclase, phospholipase C, channels, mitogen-activated protein kinase [MAPK]) that are just now becoming better clarified. 

Fig. 5.23. Diversity of regulation of adenylyl cyclase. The figure summarizes schematically the regulation of adenylyl cyclases of type I, II, V and VI (after Taussig and Gilman, 1995). The individual subtypes are negatively (-) or positively (+) regulated by heterotrimeric G-proteins by various pathways. Regulation takes place both via G -subunits and via Py-complexes. For nomenclature of the G-proteins, see 5.5.1 PKC protein kinase C Ca VCaM Ca Vcalmodulin (see Chapters 6 and 7).

Other members of class I of the PI3-kinases, such as PI3-kinase of the y subtype, are stimulated by interaction with Pycomplexes (see Chapter 5.5.7) and have their own regulatory subunit. It is interesting that both a lipid kinase activity and a protein kinase activity have been identified in the catalytic domain of the P13-kinase y subtype in brain (Bondeva et al., 1998). Activation of the MAPK pathway (see Chapter 10) may take place via the protein kinase activity, so that this enzyme can produce a bifurcated signal the lipid kinase activity stimulates the Akt kinase (see below), the protein kinase the MAPK pathway. Proliferation promoting signals are transmitted via both pathways. 

Like most of the Ser/Thr-specific protein kinase family, the protein kinase C family also shows significant heterogeneity. At the present time, at least 12 different subtypes of protein kinase C have been discovered in mammals, based on different criteria such... 

The existence of the many subtypes of protein kinase C in mammals suggests that the individual subtypes perform specific functions in the organism (Hug and Sarre, 1993 Dekker and Parker, 1994). The different subtypes are distinguished by different cellular localization, different activation by cofactors, and a different pattern of substrate proteins. For example, the a, 6 and subtypes are widespread in almost all tissues whereas the other subtypes only occur in specialized tissues. 

The mitogenic activated protein kinase (MAPK) downstream from the Ras protein is organized in modules containing three types of protein kinases, which are successively activated by sequential phosphorylation events. The cell contains different MAPK modules, which differ in the nature of the triggering stimuli and the nature and specificity of the protein kinase components. The signal transducing function of a MAPK pathway is thus determined by the nature of the MAPK module involved this, in turn, depends on the properties of the protein kinases it contains, which differ in regulation and substrate specificity. The exact composition of the MAPK module is not fixed rather, different subtypes of protein kinase may be recruited to a module in a variable... 

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