G protein modulation of N-type calcium channels is facilitated by physical interactions between syntaxin 1A and Gβγ

The direct modulation of N-type calcium channels by G protein beta gamma subunits is considered a key factor in the regulation of neurotransmission. Some of the molecular determinants that govern the binding interaction of N-type channels and G beta gamma have recently been identified (see, Le., Zamponi, G. W., Bourinet, E., Nelson, D., Nargeot, J., and Snutch, T. P. (1997) Nature 385, 442-446); however, little is known about cellular mechanisms that modulate this interaction. Here we report that a protein of the presynaptic vesicle release complex, syntaxin 1A, mediates a crucial role in the tonic inhibition of N-type channels by G beta gamma. When syntaxin 1A was coexpressed with (N-type) alpha(1B) + alpha(2)-delta + beta(1b) channels in tsA-201 cells, the channels underwent a 18 mV negative shift in half-inactivation potential, as well as a pronounced tonic G protein inhibition as assessed by its reversal by strong membrane depolarizations. This tonic inhibition was dramatically attenuated following incubation with botulinum toxin C, indicating that syntaxin 1A expression was indeed responsible for the enhanced G; protein modulation. However, when G protein beta gamma subunits were concomitantly coexpressed, the toxin became ineffective in removing G protein inhibition, suggesting that syntaxin LA optimizes, rather than being required for G protein modulation of N-type channels. We also demonstrate that G beta gamma physically binds to syntaxin 1A, and that syntaxin 1A can simultaneously interact with G beta gamma and the synprint motif of the N-type channel II-III linker. Taken together, our experiments suggest a mechanism by which syntaxin IA mediates a colocalization of G protein beta gamma subunits and N-type calcium channels, thus resulting in more effective G protein coupling to, and regulation of, the channel. Thus, the interactions between syntaxin, G proteins, and N-type calcium channels are part of the structural specialization of the presynaptic terminal.