An Antarctic toothfish eye lens protein resists thermal stress even when extensively deamidated

Crystallins are highly stable, soluble proteins that refract light and maintain transparency in the vertebrate eye lens. They are not replaced after early development, making them an excellent system for studying protein stability and solubility in crowded environments. To better understand the effects of deamidation on these ubiquitous vertebrate crystallins, we investigated a particularly extreme example, a lens protein from the long-lived Antarctic toothfish (Dissostichus mawsoni), γS1 crystallin (DmγS1). This protein remains soluble in the crowded fish lens, maintaining its transparency even at -2°C and at concentrations more than twofold that of humans (nearly 1000 mg/mL) and over a comparable timescale. As the organism ages, crystallins accumulate oxidative damage such as deamidation of Asn and Gln side chains, leading to aggregation and cataract. Previous studies of human γS crystallin (HγS) have shown that extensive deamidation reduces stability and increases aggregation propensity. Here, we present the biophysical characterization of wild-type DmγS1 and variants with three, five, and seven deamidation sites. In sharp contrast to results for human γS-crystallin, increasing the number of deamidations does not significantly change the thermal stability of DmγS1. These proteins are startlingly resistant to thermal denaturation; despite their psychrophilic origin, they have midpoint unfolding temperatures between 56°C and 63°C. Extensive deamidation does make the protein more vulnerable to chemical denaturation as well as aggregation below the unfolding temperature; however, all the variants resist aggregation well above the fish's physiological temperature. These proteins present a useful model system for aggregation resistance in extreme environments; most studies of protein solubility focus on unusually aggregation-prone proteins, but understanding the underlying biophysics also requires studying extremely soluble proteins.