Deletion of the synaptic protein interaction site of the N-type (CaV2.2) calcium channel inhibits secretion in mouse pheochromocytoma cells

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National Academy of Sciences

RESUMO

Presynaptic N-type Ca2+ channels (Cav2.2, α1B) are thought to bind to SNARE (SNAP-25 receptor) complex proteins through a synaptic protein interaction (synprint) site on the intracellular loop between domains II and III of the α1B subunit. Whether binding of syntaxin to the N-type Ca2+ channels is required for coupling Ca2+ ion influx to rapid exocytosis has been the subject of considerable investigation. In this study, we deleted the synprint site from a recombinant α1B Ca2+ channel subunit and transiently transfected either the wild-type α1B or the synprint deletion mutant into mouse pheochromocytoma (MPC) cell line 9/3L, a cell line that has the machinery required for rapid stimulated exocytosis but lacks endogenous voltage-dependent Ca2+ channels. Secretion was elicited by activation of exogenously transfected Ca2+ channel subunits. The current-voltage relationship was similar for the wild-type and mutant α1B-containing Ca2+ channels. Although total Ca2+ entry was slightly larger for the synprint deletion channel, compared with the wild-type channel, when Ca2+ entry was normalized to cell size and limited to cells with similar Ca2+ entry (≈150 × 106 Ca2+ ions/pF cell size), total secretion and the rate of secretion, determined by capacitance measurements, were significantly reduced in cells expressing the synprint deletion mutant channels, compared with wild-type channels. Furthermore, the amount of endocytosis was significantly reduced in cells with the α1B synprint deletion mutant, compared with the wild-type subunit. These results suggest that the synprint site is necessary for efficient coupling of Ca2+ influx through α1B-containing Ca2+ channels to exocytosis.

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