Specific sites in the Beta Interaction Domain of a schistosome Ca2+ channel beta subunit are key to its role in sensitivity to the anti-schistosomal drug praziquantel

Abstract

Praziquantel, the drug of choice against schistosomiasis, disrupts calcium (Ca2+) homeostasis in schistosomes via an unknown mechanism. Voltage-gated Ca2+ channels are heteromultimeric transmembrane protein complexes that contribute to impulse propagation and also regulate intracellular Ca2+ levels. Beta subunits modulate the properties of the pore-forming alpha1 subunit of high voltage-activated Ca2+ channels. Unlike other Ca2+ channel beta subunits, which have current stimulatory effects, a beta subunit subtype found in S. mansoni (SmbetaA) and S. japonicum (Sjbeta) dramatically reduces current levels when co-expressed with Ca2+ channel alpha1 subunits in Xenopus oocytes. It also confers praziquantel sensitivity to the mammalian Cav2.3 alpha1 subunit. The Beta Interaction Domains (BIDs) of SmbetaA and Sjbeta lack 2 conserved serines that each constitute a consensus site for protein kinase C (PKC) phosphorylation. Here, we use site-directed mutagenesis of schistosome beta subunits to show that these unique functional properties are correlated with the absence of these consensus PKC sites in the BID. Furthermore, a second schistosome beta subunit subtype contains both serines in the BID, enhances currents through alpha1 subunits, and does not confer praziquantel sensitivity. Thus, phosphorylation sites in the BID may play important roles in defining the modulatory properties and pharmacological sensitivities of schistosome Ca2+ channel beta subunits.