The eye lens of the Antarctic toothfish avoids cold-cataracts at temperatures cold enough to freeze sea water by maintaining the right concentration of three isoforms of crytallin proteins.

Proteins are the workhorses of life, and they are very fashion conscious. That is, proteins are large molecules with elaborate carbon-chain frames decorated with a variety of chemical accessories. But nothing will work right if every fold, every dangling accessory, and every chemical undergarment is not in its proper position. They're fussy about temperature, too. Taken out of their comfort zone, they're a disorganized, dysfunctional mess. Egg white proteins are a common example. In their comfort zone, egg albumin, dressed to the nines, is clear and fluid. Turn up the heat and she loses her cool, turning opaque and solid.

Eye lenses are also made up largely of proteins, particularly three forms of crystallin (alpha, beta, and gamma crystallin). When all is well, lenses are clear, but too hot or too cold, these proteins lose their finesse and turn opaque. But not the eye of the giant nototheniid fish, Dissostichus mawsoni, an Antarctic toothfish living in the coldest marine environment--the Antarctic region of the Southern Ocean--where water temperatures are perennially at or near the freezing point of seawater (-2°C, 28.4°F). Its eye lens remains clear at this freezing temperature and even as cold as -12°C (10.4°F). Although science does not know for sure how toothfish lenses remain clear, the relative concentration of the gamma isoform of crystallin protein in the toothfish lens appears to be key in its ability to maintain optical clarity at temperatures cold enough to freeze sea water solid.


Comparison of the behavior of two isoforms of crystallin protein, found in the cow lens (A) and the toothfish lens (B). Copyright: All rights reserved. See gallery for details.

References

"Their ability to survive in the frigid Antarctic marine environments is due to the evolution of blood-borne antifreeze glycoproteins, which allowed them to avoid freezing...Since cold temperatures depress the rates of biochemical reactions as well as affect the stability of protein structure, these fishes also exhibit a suite of biochemical and physiological adaptations to low temperature, including cold-efficient catalytic enzymes, cold-effective protein translocation, membrane phospholipid unsaturation, and tubulins that polymerise at subzero temperatures." (Kiss 2004:4633)

"In mammals, the most abundant protein is α crystallin, a large oligomeric structure composed of two polypeptides, [alpha]A and [alpha]B which belong to the small Heat Shock Protein (sHSP) family with chaperone-like ability to protect unrelated proteins from heat or chemical induced protein denaturation." (Kiss 2004:4634)

"[T]oothfish lens remained clear even when cooled to temperatures as low as –12°C. This low temperature stability is in contrast to the endothermic mammalian lens, which undergoes rapid cold-induced lens opacity (cold-cataract) below 20°C...The extraordinary cold stability of Antarctic toothfish lens exemplifies the preservation of normal protein function at the coldest known extreme of marine ectothermic vertebrate life." (Kiss 2004:4643)

"The percentage abundance of γ crystallins in toothfish (43%) and bigeye tuna (41%) lens is twofold higher than in the bovine (19%) lens...Examination of the thermal response of individual crystallin[alpha] and γ) from the three species showed that γ crystallin is a more heat-labile component relative to [alpha]." (Kiss 2004:4645)

Journal article
Cold-stable eye lens crystallins of the Antarctic nototheniid toothfish Dissostichus mawsoni NormanJournal of Experimental BiologyMarch 12, 2004
A. J. Kiss

Organism
Giant Antarctic CodDissostichus mawsoniSpecies