title: Calcium interacts with antifreeze proteins and chitinase from cold-acclimated winter rye.
abstract: During cold acclimation, winter rye (Secale cereale) plants accumulate pathogenesis-related proteins that are also antifreeze proteins (AFPs) because they adsorb onto ice and inhibit its growth. Although they promote winter survival in planta, these dual-function AFPs proteins lose activity when stored at subzero temperatures in vitro, so we examined their stability in solutions containing CaCl2, MgCl2, or NaCl. Antifreeze activity was unaffected by salts before freezing, but decreased after freezing and thawing in CaCl2 and was recovered by adding a chelator. Ca2+ enhanced chitinase activity 3- to 5-fold in unfrozen samples, although hydrolytic activity also decreased after freezing and thawing in CaCl2. Native PAGE, circular dichroism, and Trp fluorescence experiments showed that the AFPs partially unfold after freezing and thawing, but they fold more compactly or aggregate in CaCl2. Ruthenium red, which binds to Ca(2+)-binding sites, readily stained AFPs in the absence of Ca2+, but less stain was visible after freezing and thawing AFPs in CaCl2. We conclude that the structure of AFPs changes during freezing and thawing, creating new Ca(2+)-binding sites. Once Ca2+ binds to those sites, antifreeze activity, chitinase activity and ruthenium red binding are all inhibited. Because free Ca2+ concentrations are typically low in the apoplast, antifreeze activity is probably stable to freezing and thawing in planta. Ca2+ may regulate chitinase activity if concentrations are increased locally by release from pectin or interaction with Ca(2+)-binding proteins. Furthermore, antifreeze activity can be easily maintained in vitro by including a chelator during frozen storage.
authors.affiliation: Departments of Biology, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada.
authors.forename: Maja
authors.initials: M
authors.lastname: Stressmann
authors.forename: Satoshi
authors.initials: S
authors.lastname: Kitao
authors.forename: Marilyn
authors.initials: M
authors.lastname: Griffith
authors.forename: Christine
authors.initials: C
authors.lastname: Moresoli
authors.forename: León A
authors.initials: LA
authors.lastname: Bravo
authors.forename: Alejandro G
authors.initials: AG
authors.lastname: Marangoni
mesh_terms: D000064:Acclimatization / Q000187:drug effects / Q000502:physiology*; D021301:Antifreeze Proteins / Q000378:metabolism*; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism / Q000494:pharmacology*; D002122:Calcium Chloride / Q000494:pharmacology; D002614:Chelating Agents / Q000494:pharmacology; D002688:Chitinases / Q000378:metabolism*; D005615:Freezing; D015636:Magnesium Chloride / Q000494:pharmacology; D010940:Plant Proteins / Q000737:chemistry / Q000378:metabolism; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D012434:Secale / Q000187:drug effects / Q000502:physiology*; D012965:Sodium Chloride / Q000494:pharmacology
chemical_list: D021301:Antifreeze Proteins; D002614:Chelating Agents; D010940:Plant Proteins; C053376:pathogenesis-related proteins, plant; D015636:Magnesium Chloride; D012965:Sodium Chloride; D002688:Chitinases; D002122:Calcium Chloride; D002118:Calcium