Alternative splicing of the β4 subunit has α1 subunit subtype-specific effects on Ca2+ channel gating

Abstract

Ca2+ channel β subunits are important molecular determinants of the kinetics and voltage dependence of Ca2+ channel gating. Through direct interactions with channel-forming α1 subunits, β subunits enhance expression levels, accelerate activation, and have variable effects on inactivation. Four distinct β subunit genes each encode five homologous sequence domains (D1-5), three of which (D1, D3, and D5) undergo alternative splicing. We have isolated from human spinal cord a novel alternatively spliced β4 subunit containing a short form of domain D1 (β4a) that is highly homologous to N termini of Xenopus and rat β3 subunits. The purpose of this study was to compare the gating properties of various α1 subunit complexes containing β4 with those of complexes containing a β4 subunit with a longer form of domain D1, β4b. Expression in Xenopus oocytes revealed that, relative to α1A and α1B complexes containing β4b, the voltage dependence of activation and inactivation of complexes containing β4a were shifted to more depolarized potentials. Moreover, α1A and α1B complexes containing β4a inactivated at a faster rate. Interestingly, β4 subunit alternative splicing did not influence the gating properties of α1C and α1E subunits. Experiments with β4 deletion mutants revealed that both the N and C termini of the β4 subunit play critical roles in setting voltage-dependent gating parameters and that their effects are α1 subunit specific. Our data are best explained by a model in which distinct modes of activation and inactivation result from β-subunit splice variant-specific interactions with an α1 subunit gating structure.