The cardiac L-type Ca2+ channel is a textbook example of an ion channel regulated by protein phosphorylation; however, the molecular events that underlie its regulation remain unknown. Here, we report that in transiently transfected HEK293 cells expressing L-type channels, elevations in cAMP resulted in phosphorylation of the alpha1C and beta2a channel subunits and increases in channel activity. Channel phosphorylation and regulation were facilitated by submembrane targeting of protein kinase A (PKA), through association with an A-kinase anchoring protein called AKAP79. In transfected cells expressing a mutant AKAP79 that is unable to bind PKA, phosphorylation of the alpha1C subunit and regulation of channel activity were not observed. Furthermore, we have demonstrated that the association of an AKAP with PKA was required for beta-adrenergic receptor-mediated regulation of L-type channels in native cardiac myocytes, illustrating that the events observed in the heterologous expression system reflect those occurring in the native system. Mutation of Ser1928 to alanine in the C-terminus of the alpha1C subunit resulted in a complete loss of cAMP-mediated phosphorylation and a loss of channel regulation. Thus, the PKA-mediated regulation of L-type Ca2+ channels is critically dependent on a functional AKAP and phosphorylation of the alpha1C subunit at Ser1928.

Dihydropyridine-sensitive voltage-dependent L-type calcium channels are critical to excitation-secretion and excitation-contraction coupling. The channel molecule is a complex of the main, pore-forming subunit α 1 and four additional subunits: α 2 , δ, β, and γ (α 2 and δ are encoded by a single messenger RNA). The α 1 subunit messenger RNA alone directs expression of functional calcium channels in Xenopus oocytes, and coexpression of the α 2 /δ and β subunits enhances the amplitude of the current. The α 2 , δ, and γ subunits also have pronounced effects on its macroscopic characteristics, such as kinetics, voltage dependence of activation and inactivation, and enhancement by a dihydropyridine agonist. In some cases, specific modulatory functions can be assigned to individual subunits, whereas in other cases the different subunits appear to act in concert to modulate the properties of the channel.

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GIRK (Kir3) channels are activated by neurotransmitters coupled to G proteins, via a direct binding of G(beta)(gamma). The role of G(alpha) subunits in GIRK gating is elusive. Here we demonstrate that G(alpha)(i) is not only a donor of G(beta)(gamma) but also regulates GIRK gating. When overexpressed in Xenopus oocytes, GIRK channels show excessive basal activity and poor activation by agonist or G(beta)(gamma). Coexpression of G(alpha)(i3) or G(alpha)(i1) restores the correct gating parameters. G(alpha)(i) acts neither as a pure G(beta)(gamma) scavenger nor as an allosteric cofactor for G(beta)(gamma). It inhibits only the basal activity without interfering with G(beta)(gamma)-induced response. Thus, GIRK is regulated, in distinct ways, by both arms of the G protein. G(alpha)(i) probably acts in its GDP bound form, alone or as a part of G(alpha)(beta)(gamma) heterotrimer.

Activation by agonist binding of G-protein-coupled receptors (GPCRs) controls most signal transduction processes. Although these receptors span the cell membrane, they are not considered to be voltage sensitive. Recently it was shown that both the activity of GPCRs and their affinity towards agonists are regulated by membrane potential. However, it remains unclear whether GPCRs intrinsically respond to changes in membrane potential. Here we show that two prototypical GPCRs, the m2 and m1 muscarinic receptors (m2R and m1R), display charge-movement-associated currents analogous to 'gating currents' of voltage-gated channels. The gating charge-voltage relationship of m2R correlates well with the voltage dependence of the affinity of the receptor for acetylcholine. The loop that couples m2R and m1R to their G protein has a crucial function in coupling voltage sensing to agonist-binding affinity. Our data strongly indicate that GPCRs serve as sensors for both transmembrane potential and external chemical signals.

Complementary DNAs encoding three novel and distinct beta subunits (CaB2a, CaB2b and CaB3) of the high voltage activated (L‐type) calcium channel have been isolated from rabbit heart. Their deduced amino acid sequence is homologous to the beta subunit originally cloned from skeletal muscle (CaB1). CaB2a and CaB2b are splicing products of a common primary transcript (CaB2). Northern analysis and specific amplification of CaB2 and CaB3 specific cDNAs by polymerase chain reactions showed that CaB2 is predominantly expressed in heart, aorta and brain, whereas CaB3 is most abundant in brain but also present in aorta, trachea, lung, heart and skeletal muscle. A partial DNA sequence complementary to a third variant of the CaB2 gene, subtype CaB2c, has also been cloned from rabbit brain. Coexpression of CaB2a, CaB2b and CaB3 with alpha 1heart enhances not only the expression in the oocyte of the channel directed by the cardiac alpha 1 subunit alone, but also effects its macroscopic characteristics such as drug sensitivity and kinetics. These results together with the known alpha 1 subunit heterogeneity, suggest that different types of calcium currents may depend on channel subunit composition.

Activity of several ion channels is controlled by heterotrimeric GTP-binding proteins (G proteins) via a membrane-delimited pathway that does not involve cytoplasmic intermediates. The best studied example is the K+ channel activated by muscarinic agonists in the atrium, which plays a crucial role in regulating the heartbeat. To enable studies of the molecular mechanisms of activation, this channel, denoted KGA, was cloned from a rat atrium cDNA library by functional coupling to coexpressed serotonin type 1A receptors in Xenopus oocytes. KGA displays regions of sequence homology to other inwardiy rectifying channels as well as unique regions that may govern G-protein interaction. The expressed KGA channel is activated by serotonin 1A, muscarinic m2, and b-opioid receptors via G proteins. KGA is activated by guanosine 5'-[ythio]triphosphate in excised patches, confirming activation by a membrane-delimited pathway, and displays a conductance equal to that of the endogenous channel in atrial cells. The hypothesis that similar channels play a role in neuronal inhibition is supported by the cloning of a nearly identical channel (KGB1) from a rat brain cDNA library.A major signal transduction mechanism in cardiac physiology and neurobiology is the direct coupling of neurotransmitter receptors to ion channels by a membrane-delimited pathway that does not involve cytoplasmic intermediates (for reviews, see refs. 1 and 2). The best studied member of this group is the G protein-activated K+ channel found in atria of all vertebrates. This channel, which we denote KGA, figured in the original discovery of chemical synaptic transmission (3,4) and plays a crucial role in regulating the heartbeat. The KGA channel rectifies at the single-channel level, allowing much larger inward than outward currents (5, 6). It is activated by acetylcholine acting on muscarinic m2 receptors via a pathway that includes a pertussis toxin (PTX)-sensitive G protein, probably of the G, family (1,(7)(8)(9)(10)(11)(12)