Molecular moieties masking Ca²⁺-dependent facilitation of voltage-gated Ca_v2.2 Ca²⁺ channels

Voltage-gated Ca_v2.1 (P/Q-type) Ca²⁺ channels undergo Ca²⁺-dependent inactivation (CDI) and facilitation (CDF), both of which contribute to short-term synaptic plasticity. Both CDI and CDF are mediated by calmodulin (CaM) binding to sites in the C-terminal domain of the Ca_v2.1 α₁ subunit, most notably to a consensus CaM-binding IQ-like (IQ) domain. Closely related Ca_v2.2 (N-type) channels display CDI but not CDF, despite overall conservation of the IQ and additional sites (pre-IQ, EF-hand-like [EF] domain, and CaM-binding domain) that regulate CDF of Ca_v2.1. Here we investigate the molecular determinants that prevent Ca_v2.2 channels from undergoing CDF. Although alternative splicing of C-terminal exons regulates CDF of Ca_v2.1, the splicing of analogous exons in Ca_v2.2 does not reveal CDF. Transfer of sequences encoding the Ca_v2.1 EF, pre-IQ, and IQ together (EF-pre-IQ-IQ), but not individually, are sufficient to support CDF in chimeric Ca_v2.2 channels; Ca_v2.1 chimeras containing the corresponding domains of Ca_v2.2, either alone or together, fail to undergo CDF. In contrast to the weak binding of CaM to just the pre-IQ and IQ of Ca_v2.2, CaM binds to the EF-pre-IQ-IQ of Ca_v2.2 as well as to the corresponding domains of Ca_v2.1. Therefore, the lack of CDF in Ca_v2.2 likely arises from an inability of its EF-pre-IQ-IQ to transduce the effects of CaM rather than weak binding to CaM per se. Our results reveal a functional divergence in the CDF regulatory domains of Ca_v2 channels, which may help to diversify the modes by which Ca_v2.1 and Ca_v2.2 can modify synaptic transmission.