The observations from Dunlap and Fischbach that transmittermediated shortening of the duration of action potentials could be caused by a decrease in calcium conductance led to numerous studies of the mechanisms of modulation of voltagedependent calcium channels. Calcium channels are well known targets for inhibition by receptor-G protein pathways, and multiple forms of inhibition have been described. Inhibition of Ca 2ϩ channels can be mediated by G protein ␤␥-subunits or by kinases, such as protein kinase C and tyrosine kinases. In the last few years, it has been shown that integration of G protein signaling can take place at the level of the calcium channel by regulation of the interaction of the channel pore-forming subunit with different cellular proteins.Communication between neurons and their targets depends upon the precise timing of electrical and chemical signals. In the nervous system, short, local signals are necessary to convey appropriate timing information. Defects in such timing events produce problematic errors in the final physiological response. Voltage-dependent calcium channels are the primary triggers for electrically evoked release of chemical transmitters; therefore, understanding the molecular components and events underlying their regulation is central to the development of a mechanistic picture of key events in neuronal signaling. Inhibition of Ca 2ϩ channels can be voltage-dependent and is mediated by direct interaction of G protein ␤␥-subunits with the pore-forming ␣ 1 -subunit of the channel (Hille, 1994;Ikeda and Dunlap, 1999). In addition, phosphorylation by kinases such as protein kinase C (Rane and Dunlap, 1986) and tyrosine kinases (Diversé-Pierluissi et al., 1997) has been shown to inhibit Ca 2ϩ channels. Classification and Structure of High-VoltageActivated Calcium ChannelsHigh voltage-activated calcium channels are classified into Ca v 1.1-3 and Ca v 2.1-3 types based on the gene encoding the pore subunit and their electrophysiological and pharmacological properties. Ca v 1-type channels are ubiquitous (Table 1). They play an important role in excitation-contraction in cardiac and skeletal muscle (Catterall, 2000). Ca v 2.1 and Ca v 2.2 channels are involved in neurotransmission and can coexist in the same nerve terminals (Dunlap et al., 1995). Until recently, the Ca v 2.3 channel was less characterized because of its lack of sensitivity to any known pharmacological blocker. The use of SNX482 as a blocker for this type of channel should facilitate the studies of the function and regulation of Ca v 2.3 channels (Bourinet et al., 2001).Voltage-dependent calcium channels are multimeric proteins composed of ␣ 1 -, ␤-, ␣ 2 ␦-, and ␥-subunits (Fig. 1). The ␣ 1 -subunit is the pore-forming subunit and accounts for the voltage-dependence of the channel. Channel blockers and naturally occurring toxins exert their actions by binding to this subunit. So far, four ␤-subunits, four ␦-subunits (Qin et al., 2002), and eight ␥-subunits (Tomita et al., 2003)

Activation of GABA B receptors in chick dorsal root ganglion (DRG) neurons inhibits the Ca v 2.2 calcium channel in both a voltage-dependent and voltage-independent manner. The voltage-independent inhibition requires activation of a tyrosine kinase that phosphorylates the ␣ 1 subunit of the channel and thereby recruits RGS12, a member of the "regulator of G protein signaling" (RGS) proteins. Here we report that RGS12 binds to the SNARE-binding or "synprint" region (amino acids 726 -985) in loop II-III of the calcium channel ␣1 subunit. A recombinant protein encompassing the Nterminal PTB domain of RGS12 binds to the synprint region in protein overlay and surface plasmon resonance binding assays; this interaction is dependent on tyrosine phosphorylation and yet is within a sequence that differs from the canonical NPXY motif targeted by other PTB domains. In electrophysiological experiments, microinjection of DRG neurons with synprint-derived peptides containing the tyrosine residue Tyr-804 altered the rate of desensitization of neurotransmitter-mediated inhibition of the Ca v 2.2 calcium channel, whereas peptides centered about a second tyrosine residue, Tyr-815, were without effect. RGS12 from a DRG neuron lysate was precipitated using synprint peptides containing phosphorylated Tyr-804. The high degree of conservation of Tyr-804 in the SNAREbinding region of Ca v 2.1 and Ca v 2.2 calcium channels suggests that this region, in addition to the binding of SNARE proteins, is also important for determining the time course of the modulation of calcium current via tyrosine phosphorylation.Multiple G protein-mediated signaling pathways are known to modulate Ca v 2.2 (N-type) calcium channels (1, 2) via direct G protein-ion channel interactions, activation of second messenger cascades, and activation of tyrosine kinases (3, 4). This modulation of voltage-dependent calcium channels is a transient phenomenon. Upon prolonged exposure to a neurotransmitter, neurons become unresponsive or desensitized. Despite the common requirement for the activation of a G proteincoupled receptor kinase (GRK3) for desensitization of the neurotransmitter-mediated inhibition of calcium current (5), G i -and G o -mediated pathways exhibit different rates of desensitization (6) that may result from selective effects of the G␣-directed GTPase-accelerating activity borne by "regulator of G protein signaling" (RGS) 1 proteins (7,8).In dorsal root ganglion (DRG) neurons, the activation of ␥-aminobutyric acid type B (GABA B ) receptors induces both voltage-dependent and voltage-independent inhibition of Ca v 2.2 channels (9). Voltage-independent inhibition requires the activation of a tyrosine kinase that phosphorylates the pore-forming ␣-subunit of the calcium channel (10). The tyrosine-phosphorylated form of the ␣-subunit becomes a target for the phosphotyrosine binding (PTB) domain of RGS12, a member of the RGS protein superfamily that specifically accelerates the rate of desensitization of this response (10).To better understand the molecular b...

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