A Ca 2+ dependent bacterial antifreeze protein domain has a novel beta-helical ice-binding fold
AFPs (antifreeze proteins) are produced by many organismsthat inhabit ice-laden environments. They facilitate survivalat sub-zero temperatures by binding to, and inhibiting, thegrowth of ice crystals in solution. The Antarctic bacterium Marinomonas primoryensis produces an exceptionally large(>1...
| Published in: | Biochemical Journal |
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| Main Authors: | , , , , , |
| Format: | Article in Journal/Newspaper |
| Language: | English |
| Published: |
Oxford University Press
2008
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| Subjects: | |
| Online Access: | http://www.biochemj.org/bj/default.htm https://doi.org/10.1042/BJ20071372 http://www.ncbi.nlm.nih.gov/pubmed/18095937 http://ecite.utas.edu.au/53340 |
| Summary: | AFPs (antifreeze proteins) are produced by many organismsthat inhabit ice-laden environments. They facilitate survivalat sub-zero temperatures by binding to, and inhibiting, thegrowth of ice crystals in solution. The Antarctic bacterium Marinomonas primoryensis produces an exceptionally large(>1 MDa) hyperactive Ca 2+ -dependent AFP. We have cloned,expressed and characterized a 322-amino-acid region of theprotein where the antifreeze activity is localized that showssimilarity to the RTX (repeats-in-toxin) family of proteins. Therecombinant protein requires Ca 2+ for structure and activity, and itis capable of depressing the freezing point of a solution in excessof 2C at a concentration of 0.5 mg/ml, therefore classifying itas a hyperactive AFP. We have developed a homology-guidedmodel of the antifreeze region based partly on the Ca 2+ -bound β-roll from alkaline protease. The model has identified both a novelβ-helical fold and an ice-binding site. The interior of the !-helixcontains a single row of bound Ca 2+ ions down one side of thestructure and a hydrophobic core down the opposite side. The icebindingsurface consists of parallel repetitive arrays of threonineand aspartic acid/asparagine residues located down the Ca 2+ -bound side of the structure. The model was tested and validatedby site-directed mutagenesis. It explains the Ca 2+ -dependency ofthe region, as well its hyperactive antifreeze activity. This is thefirst bacterial AFP to be structurally characterized and is one ofonly five hyperactive AFPs identified to date. |
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