Abstract
BackgroundThe function of voltage-gated calcium (Cav) channels greatly depends on coupling to cytoplasmic accessory β subunits, which not only promote surface expression, but also modulate gating and kinetic properties of the α1 subunit. Schistosomes, parasitic platyhelminths that cause schistosomiasis, express two β subunit subtypes: a structurally conventional β subunit and a variant β subunit with unusual functional properties. We have previously characterized the functional properties of the variant Cavβ subunit. Here, we focus on the modulatory phenotype of the conventional Cavβ subunit (SmCavβ) using the human Cav2.3 channel as the substrate for SmCavβ and the whole-cell patch-clamp technique.ResultsThe conventional Schistosoma mansoni Cavβ subunit markedly increases Cav2.3 currents, slows macroscopic inactivation and shifts steady state inactivation in the hyperpolarizing direction. However, currents produced by Cav2.3 in the presence of SmCavβ run-down to approximately 75% of their initial amplitudes within two minutes of establishing the whole-cell configuration. This suppressive effect was independent of Ca2+, but dependent on intracellular Mg2+-ATP. Additional experiments revealed that SmCavβ lends the Cav2.3/SmCavβ complex sensitivity to Na+ ions. A mutant version of the Cavβ subunit lacking the first forty-six amino acids, including a string of twenty-two acidic residues, no longer conferred sensitivity to intracellular Mg2+-ATP and Na+ ions, while continuing to show wild type modulation of current amplitude and inactivation of Cav2.3.ConclusionThe data presented in this article provide insights into novel mechanisms employed by platyhelminth Cavβ subunits to modulate voltage-gated Ca2+ currents that indicate interactions between the Ca2+ channel complex and chelated forms of ATP as well as Na+ ions. These results have potentially important implications for understanding previously unknown mechanisms by which platyhelminths and perhaps other organisms modulate Ca2+ currents in excitable cells.
Highlights
The function of voltage-gated calcium (Cav) channels greatly depends on coupling to cytoplasmic accessory β subunits, which promote surface expression, and modulate gating and kinetic properties of the α1 subunit
high-voltage activated (HVA) Cav channels are heteromeric protein complexes composed of a pore-forming α1 subunit and auxiliary β and α2δ subunits [7]
We have identified two Cav channel β subunits from schistosomes and other platyhelminths: a conventional β subunit (SmCavβ), and a variant β subunit (SmCavβvar), which appears to be unique to platyhelminths and has unusual structural and functional features [14,15]
Summary
The function of voltage-gated calcium (Cav) channels greatly depends on coupling to cytoplasmic accessory β subunits, which promote surface expression, and modulate gating and kinetic properties of the α1 subunit. With the ultimate goal of understanding the molecular basis for neuromuscular transmission in these parasitic flatworms, we have previously cloned three transcripts from Schistosoma mansoni that code for one L-type-like and two non L-type high voltage-activated Cav channel α1 subunits [13]. Heterologous expression of these α1 subunits in Xenopus oocytes and mammalian cell lines has proved problematic, perhaps because of the relatively high A-T content of these coding regions, or the lack of a specific chaperone in these systems. We have identified two Cav channel β subunits from schistosomes and other platyhelminths: a conventional β subunit (SmCavβ), and a variant β subunit (SmCavβvar), which appears to be unique to platyhelminths and has unusual structural and functional features [14,15]
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