Resistance of presynaptic CaV2.2 channels to voltage-dependent inactivation: dynamic palmitoylation and voltage sensitivity.

Presynaptic CaV2.2 (N type) calcium channels gate the influx of calcium ions to trigger transmitter release. We have previously demonstrated at the chick ciliary ganglion presynaptic calyx terminal that the bulk of these channels are highly resistant to voltage dependent inactivation [E.F. Stanley, G. Goping, Characterization of a calcium current in a vertebrate cholinergic presynaptic nerve terminal, J. Neurosci. 11 (1991) 985-993; E.F. Stanley, Syntaxin I modulation of presynaptic calcium channel inactivation revealed by botulinum toxin C1, Eur. J. Neurosci. 17 (2003) 1303-1305; E.F. Stanley, R.R. Mirotznik, Cleavage of syntaxin prevents G-protein regulation of presynaptic calcium channels, Nature (Lond.) 385 (1997) 340-343]. Recent studies have suggested that CaV2.2 can be rendered inactivation resistant when expressed with the palmitoylated beta2A subunit and that this effect can be eliminated by tunicamycin, a general inhibitor of dynamic palmitoylation [J.H. Hurley, A.L. Cahill, K.P. Currie, A.P. Fox, The role of dynamic palmitoylation in Ca(2+) channel inactivation, Proc. Natl. Acad. Sci. U.S.A. 97 (2000) 9293-9298]. We find that while tunicamycin treatment had no effect on CaV2.2 current in the inactivation-sensitive isolated chick dorsal root ganglion (DRG) neuron, it caused a 10mV hyperpolarized shift in the profile of the inactivation-resistant presynaptic CaV2.2 population. This shift occurred without any effect on the voltage sensitivity of the inactivation process, as measured by a Boltzmann slope factor. Our findings suggest that dynamic palmitoylation contributes to the hyperpolarized steady inactivation profile of presynaptic CaV2.2. However, some other factor must also contribute since its inhibition does is not restore the inactivation profile to that of channels in the cell soma.