Cross talk between β subunits, intracellular Ca²⁺ signaling, and SNAREs in the modulation of Ca_V 2.1 channel steady-state inactivation

Modulation of Ca_V 2.1 channel activity plays a key role in interneuronal communication and synaptic plasticity. SNAREs interact with a specific synprint site at the second intracellular loop (LII-III) of the Ca_V 2.1 pore-forming α_1A subunit to optimize neurotransmitter release from presynaptic terminals by allowing secretory vesicles docking near the Ca²⁺ entry pathway, and by modulating the voltage dependence of channel steady-state inactivation. Ca²⁺ influx through Ca_V 2.1 also promotes channel inactivation. This process seems to involve Ca²⁺ -calmodulin interaction with two adjacent sites in the α_1A carboxyl tail (C-tail) (the IQ-like motif and the Calmodulin-Binding Domain (CBD) site), and contributes to long-term potentiation and spatial learning and memory. Besides, binding of regulatory β subunits to the α interaction domain (AID) at the first intracellular loop (LI-II) of α_1A determines the degree of channel inactivation by both voltage and Ca²⁺ . Here, we explore the cross talk between β subunits, Ca²⁺ , and syntaxin-1A-modulated Ca_V 2.1 inactivation, highlighting the α_1A domains involved in such process. β₃ -containing Ca_V 2.1 channels show syntaxin-1A-modulated but no Ca²⁺ -dependent steady-state inactivation. Conversely, β_2a -containing Ca_V 2.1 channels show Ca²⁺ -dependent but not syntaxin-1A-modulated steady-state inactivation. A LI-II deletion confers Ca²⁺ -dependent inactivation and prevents modulation by syntaxin-1A in β₃ -containing Ca_V 2.1 channels. Mutation of the IQ-like motif, unlike CBD deletion, abolishes Ca²⁺ -dependent inactivation and confers modulation by syntaxin-1A in β_2a -containing Ca_V 2.1 channels. Altogether, these results suggest that LI-II structural modifications determine the regulation of Ca_V 2.1 steady-state inactivation either by Ca²⁺ or by SNAREs but not by both.