title: Back to the future with the AGP-Ca2+ flux capacitor.
abstract: BACKGROUND
  Arabinogalactan proteins (AGPs) are ubiquitous in green plants. AGPs comprise a widely varied group of hydroxyproline (Hyp)-rich cell surface glycoproteins (HRGPs). However, the more narrowly defined classical AGPs massively predominate and cover the plasma membrane. Extensive glycosylation by pendant polysaccharides O-linked to numerous Hyp residues like beads of a necklace creates a unique ionic compartment essential to a wide range of physiological processes including germination, cell extension and fertilization. The vital clue to a precise molecular function remained elusive until the recent isolation of small Hyp-arabinogalactan polysaccharide subunits; their structural elucidation by nuclear magentic resonance imaging, molecular simulations and direct experiment identified a 15-residue consensus subunit as a β-1,3-linked galactose trisaccharide with two short branched sidechains each with a single glucuronic acid residue that binds Ca(2+) when paired with its adjacent sidechain.
  
  
  METHODS
  AGPs bind Ca(2+) (Kd ∼ 6 μm) at the plasma membrane (PM) at pH ∼5·5 but release it when auxin-dependent PM H(+)-ATPase generates a low periplasmic pH that dissociates AGP-Ca(2+) carboxylates (pka ∼3); the consequential large increase in free Ca(2+) drives entry into the cytosol via Ca(2+) channels that may be voltage gated. AGPs are thus arguably the primary source of cytosolic oscillatory Ca(2+) waves. This differs markedly from animals, in which cytosolic Ca(2+) originates mostly from internal stores such as the sarcoplasmic reticulum. In contrast, we propose that external dynamic Ca(2+) storage by a periplasmic AGP capacitor co-ordinates plant growth, typically involving exocytosis of AGPs and recycled Ca(2+), hence an AGP-Ca(2+) oscillator.
  
  
  CONCLUSIONS
  The novel concept of dynamic Ca(2+) recycling by an AGP-Ca(2+) oscillator solves the long-standing problem of a molecular-level function for classical AGPs and thus integrates three fields: AGPs, Ca(2+) signalling and auxin. This accounts for the involvement of AGPs in plant morphogenesis, including tropic and nastic movements.
authors.affiliation: School of Life Sciences, University of Sussex, Falmer, Brighton BN1 9QG, UK Derekt.t.a.lamport@googlemail.com.
authors.forename: Derek T A
authors.initials: DT
authors.lastname: Lamport
authors.affiliation: School of Life Sciences, University of Sussex, Falmer, Brighton BN1 9QG, UK.
authors.forename: Peter
authors.initials: P
authors.lastname: Varnai
authors.affiliation: Seed Conservation Department, Royal Botanic Gardens, Kew, Wakehurst Place, Ardingly, West Sussex RH17 6TN, UK.
authors.forename: Charlotte E
authors.initials: CE
authors.lastname: Seal
mesh_terms: D002118:Calcium / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism; D002473:Cell Wall / Q000378:metabolism; D005685:Galactans / Q000378:metabolism; D018506:Gene Expression Regulation, Plant; D006031:Glycosylation; D006863:Hydrogen-Ion Concentration; D007210:Indoleacetic Acids / Q000378:metabolism*; D008954:Models, Biological; D009088:Mucoproteins / Q000378:metabolism*; D063245:Plant Development; D010937:Plant Growth Regulators / Q000378:metabolism*; D010940:Plant Proteins / Q000378:metabolism; D010944:Plants / Q000378:metabolism*; D015398:Signal Transduction
chemical_list: D005685:Galactans; D007210:Indoleacetic Acids; D009088:Mucoproteins; D010937:Plant Growth Regulators; D010940:Plant Proteins; C006619:arabinogalactan proteins; C005653:arabinogalactan; D002118:Calcium
keywords: AGP–Ca2+ flux capacitor; Arabinogalactan proteins; calcium signalling; hydroxyproline-rich glycoproteins; ion currents; plant cell wall protein