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Meeting Abstract
Four domain voltage-gated Na+ and Ca2+ channels mediate rapid physiological responses in metazoans. Fast Na+ and Ca2+-based action potentials likely evolved in single celled organisms before the evolution of multicellular neuromuscular systems, although their functional role is not well understood. We recently discovered that fast action potentials in some marine phytoplankton may be underpinned by homologs of single domain bacterial Na+ channels (NavBac), suggesting a unique role of these proteins in unicellular eukaryotes. We therefore heterologously expressed two NavBac homologs, one from the diatom Odontella sinsensis and one from the coccolithophore Scyphosphaera apsteinii in the mammalian cell line HEK293. Whole cell patch clamp analysis revealed that the expressed proteins from both species mediate fast activating, rapidly inactivating inward currents in response to depolarization. Both channels exhibited steady state inactivation with recovery from inactivation. The coccolithophore protein exhibited much faster activation and inactivation kinetics (τact=0.62 ms ± 0.40, τinact=1.16ms ± 0.34) compared to the diatom protein (τact=1.18 ms ± 0.53, τinact=46.90ms ± 8.50). These kinetic features are similar to the properties of four-domain voltage-gated Na+ channels in animals, implying the single-domain proteins tetramerize to form a functional voltage gated channel. Differentiation in the amino acids of the selectivity filter suggests that the coccolithophore channel selects for Na+ over Ca2+, while the diatom channel is permeant to Ca2+. Our data suggest unique functional roles for single-domain voltage-gated Na+ and Ca2+ channels among the diverse lineages of marine protists.