Alternative Mechanisms for Fast Na+/Ca2+ Signaling in Eukaryotes via a Novel Class of Single-Domain Voltage-Gated Channels

Abstract

Rapid Na+/Ca2+-based action potentials govern essential cellular functions in eukaryotes, from the motile responses of unicellular protists, such as Paramecium [1Fujiu K. Nakayama Y. Yanagisawa A. Sokabe M. Yoshimura K. Chlamydomonas CAV2 encodes a voltage- dependent calcium channel required for the flagellar waveform conversion.Curr. Biol. 2009; 19: 133-139Abstract Full Text Full Text PDF PubMed Scopus (69) Google Scholar, 2Lodh S. Yano J. Valentine M.S. Van Houten J.L. Voltage-gated calcium channels of Paramecium cilia.J. Exp. Biol. 2016; 219: 3028-3038Crossref PubMed Scopus (26) Google Scholar], to complex animal neuromuscular activity [3Hille B. Ion Channels of Excitable Membranes.Third Edition. Sinauer, 2001Google Scholar]. A key innovation underpinning this fundamental signaling process has been the evolution of four-domain voltage-gated Na+/Ca2+ channels (4D-Cavs/Navs). These channels are widely distributed across eukaryote diversity [4Verret F. Wheeler G. Taylor A.R. Farnham G. Brownlee C. Calcium channels in photosynthetic eukaryotes: implications for evolution of calcium-based signalling.New Phytol. 2010; 187: 23-43Crossref PubMed Scopus (109) Google Scholar], albeit several eukaryotes, including land plants and fungi, have lost voltage-sensitive 4D-Cav/Navs [5Liebeskind B.J. Hillis D.M. Zakon H.H. Phylogeny unites animal sodium leak channels with fungal calcium channels in an ancient, voltage-insensitive clade.Mol. Biol. Evol. 2012; 29: 3613-3616Crossref PubMed Scopus (38) Google Scholar, 6Pozdnyakov I. Matantseva O. Skarlato S. Diversity and evolution of four-domain voltage-gated cation channels of eukaryotes and their ancestral functional determinants.Sci. Rep. 2018; 8: 3539Crossref PubMed Scopus (18) Google Scholar, 7Edel K.H. Marchadier E. Brownlee C. Kudla J. Hetherington A.M. The evolution of calcium-based signalling in plants.Curr. Biol. 2017; 27: R667-R679Abstract Full Text Full Text PDF PubMed Scopus (157) Google Scholar]. Because these lineages appear to lack rapid Na+/Ca2+-based action potentials, 4D-Cav/Navs are generally considered necessary for fast Na+/Ca2+-based signaling [7Edel K.H. Marchadier E. Brownlee C. Kudla J. Hetherington A.M. The evolution of calcium-based signalling in plants.Curr. Biol. 2017; 27: R667-R679Abstract Full Text Full Text PDF PubMed Scopus (157) Google Scholar]. However, the cellular mechanisms underpinning the membrane physiology of many eukaryotes remain unexamined. Eukaryotic phytoplankton critically influence our climate as major primary producers. Several taxa, including the globally abundant diatoms, exhibit membrane excitability [8Oami K. Naitoh Y. Sibaoka T. Modification of voltage-sensitive inactivation of Na+ current by external Ca2+ in the marine dinoflagellate Noctiluca miliaris.J. Comp. Physiol. A. 1995; 176: 635-640Crossref Google Scholar, 9Taylor A.R. Brownlee C. A novel Cl- inward-rectifying current in the plasma membrane of the calcifying marine phytoplankton Coccolithus pelagicus.Plant Physiol. 2003; 131: 1391-1400Crossref PubMed Scopus (37) Google Scholar, 10Taylor A.R. A fast Na+/Ca2+-based action potential in a marine diatom.PLoS ONE. 2009; 4: e4966Crossref PubMed Scopus (41) Google Scholar]. We previously demonstrated that certain diatom genomes encode 4D-Cav/Navs [4Verret F. Wheeler G. Taylor A.R. Farnham G. Brownlee C. Calcium channels in photosynthetic eukaryotes: implications for evolution of calcium-based signalling.New Phytol. 2010; 187: 23-43Crossref PubMed Scopus (109) Google Scholar] but also proteins of unknown function, resembling prokaryote single-domain, voltage-gated Na+ channels (BacNavs) [4Verret F. Wheeler G. Taylor A.R. Farnham G. Brownlee C. Calcium channels in photosynthetic eukaryotes: implications for evolution of calcium-based signalling.New Phytol. 2010; 187: 23-43Crossref PubMed Scopus (109) Google Scholar]. Here, we show that single-domain channels are actually broadly distributed across major eukaryote phytoplankton lineages and represent three novel classes of single-domain channels, which we refer collectively to as EukCats. Functional characterization of diatom EukCatAs indicates that they are voltage-gated Na+- and Ca2+-permeable channels, with rapid kinetics resembling metazoan 4D-Cavs/Navs. In Phaeodactylum tricornutum, which lacks 4D-Cav/Navs, EukCatAs underpin voltage-activated Ca2+ signaling important for membrane excitability, and mutants exhibit impaired motility. EukCatAs therefore provide alternative mechanisms for rapid Na+/Ca2+ signaling in eukaryotes and may functionally replace 4D-Cavs/Navs in pennate diatoms. Marine phytoplankton thus possess unique signaling mechanisms that may be key to environmental sensing in the oceans.