A bimodular mechanism of calcium control in eukaryotes

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  • Henning Tidow
  • ,
  • Lisbeth Rosager Poulsen, Institut for Plantebiologi og Bioteknologi, Københavns Universitet, Danmark
  • Antonina Andreeva, MRC Laboratory of Molecular Biology, Storbritannien
  • Michael Knudsen
  • Kim L Hein, Danmark
  • Carsten Wiuf, Department of Mathematical Sciences, University of Copenhagen, Danmark
  • Michael G. Palmgren, Institut for Plantebiologi og Bioteknologi, Københavns Universitet, Danmark
  • Poul Nissen
Calcium ions (Ca(2+)) have an important role as secondary messengers in numerous signal transduction processes, and cells invest much energy in controlling and maintaining a steep gradient between intracellular (∼0.1-micromolar) and extracellular (∼2-millimolar) Ca(2+) concentrations. Calmodulin-stimulated calcium pumps, which include the plasma-membrane Ca(2+)-ATPases (PMCAs), are key regulators of intracellular Ca(2+) in eukaryotes. They contain a unique amino- or carboxy-terminal regulatory domain responsible for autoinhibition, and binding of calcium-loaded calmodulin to this domain releases autoinhibition and activates the pump. However, the structural basis for the activation mechanism is unknown and a key remaining question is how calmodulin-mediated PMCA regulation can cover both basal Ca(2+) levels in the nanomolar range as well as micromolar-range Ca(2+) transients generated by cell stimulation. Here we present an integrated study combining the determination of the high-resolution crystal structure of a PMCA regulatory-domain/calmodulin complex with in vivo characterization and biochemical, biophysical and bioinformatics data that provide mechanistic insights into a two-step PMCA activation mechanism mediated by calcium-loaded calmodulin. The structure shows the entire PMCA regulatory domain and reveals an unexpected 2:1 stoichiometry with two calcium-loaded calmodulin molecules binding to different sites on a long helix. A multifaceted characterization of the role of both sites leads to a general structural model for calmodulin-mediated regulation of PMCAs that allows stringent, highly responsive control of intracellular calcium in eukaryotes, making it possible to maintain a stable, basal level at a threshold Ca(2+) concentration, where steep activation occurs.
OriginalsprogEngelsk
TidsskriftNature
Vol/bind491
Sider (fra-til)468-472
Antal sider5
ISSN0028-0836
DOI
StatusUdgivet - 15 nov. 2012

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