Remodelling Ca2+ Responsiveness of Cav2.3 by Cavb Subunits: Role of an N-Terminal Polyacidic Motif

Abstract

Ca2+-dependent inactivation of Cav2 channels is highly sensitive to intracellular Ca2+ buffers. Therefore, it seems likely that the cytoplasmic Ca2+ buffering scenario will have a large impact on the activity of Cav2.3 channels, which mediate Ca2+ influx associated with medium to slow neurotransmitter release. Using the whole-cell patch-clamp technique, here we show that the kinetics of the fast and slow components of macroscopic inactivation, τf and τs, of Cav2.3 are significantly slower when the cell is dyalized with 0.5 mM EGTA than when is dyalized with a solution containing no intracellular chelators. Rat Cavβ3 and a Cavβ subunit from the human parasite Schistosoma mansoni (CaβSm) eliminate the sensitivity of τf, but not of τs, to 0.5 mM EGTA. Interestingly, CavβSmalso eliminates the sensitivity of τf to 5 mM BAPTA, whereas Cavβ3 does not. Differently from mammalian Cavβ's, CavβSm contains a long N-terminal poly-acidic motif (NPAM). Does this motif interfere with responsiveness of τf to BAPTA? Coexpression with a CavβSm subunit without NPAM increased the sensitivity of τf to 5 mM BAPTA and enhanced the sensitivity of τs to EGTA and BAPTA. Coexpression with a chimaeric Cavβ3 subunit that contains an NPAM suppressed the sensitivity of both τf and τs to intracellular buffering. Thus, we conclude that presence of NPAM in Cavβ subunits reduces or suppresses the sensitivity of Cav2.3 inactivation to intracellular chelators. Perhaps NPAMs compete for Ca2+ with cellular buffers in the microdomains associated with Cav channels. We propose that the NPAM is a built-in buffer within the architecture of the CavβSm subunit with a function in modulating inactivation of schistosome Cav channels. Recombinant mammalian Cavβ subunits containing NPAMs could potentially offer a novel therapeutic strategy for diseases associated with enhanced Ca2+ entry.