Fish Antifreeze Provided by Pseudogene
Freezing water forms crystals that can rip and tear at cells. Yet there are fish in arctic waters that can survive even below the freezing point of sea water. They accomplish this by means of special “antifreeze proteins” that interfere with the damaging effects of water crystals.
Scientists knew about AFP (anti-freeze protein) Type I in winter flounder, and knew its properties. They became puzzled how the fish survived temperatures lower than the protection AFP type I could provide. They suspected another antifreeze protein was at work, and found the gene that codes for it. They explain why this gene, named 5a, had escaped detection for 30 years: “The two proteins differ slightly in their amino-terminal sequence and amino-acid composition. At the time of its discovery, the 5a gene was dismissed as an antifreeze-protein pseudogene, largely because the protein it encodes would have been grossly different from type I AFP and had never been detected in the flounder.”
The protein is normally present in low concentrations and degrades at room temperature. At low temperatures, however, it roars into action. It becomes extraordinarily hyperactive, providing more protection against freezing than the previously-known AFP by an order of magnitude.
1Marshall, Fletcher, and Davies, “Hyperactive antifreeze protein in a fish,” Nature 429, 153 (13 May 2004); doi:10.1038/429153a.
Another wonderful discovery, all the more interesting for the last line: “The evolutionary relationship between our 5a-like antifreeze protein and type I AFP, which also contains short tracts of alanine, remains to be solved.”
Here is an example of “junk DNA” proving to be functionally important. Based on Darwinian assumptions, scientists had dismissed the gene as a degenerating relic of a gene duplication event sometime in the fish’s prehistory. Such a mindset is proving to be a hindrance to the advance of science (see 05/10/2004 headline).