Adaptation of proteins to the cold in Antarctic fish: A role for Methionine?

The evolution of antifreeze glycoproteins has enabled notothenioid fish to flourish in the freezing waters of the Southern Ocean. Whilst successful at the biodiversity level to life in the cold, paradoxically at the cellular level these stenothermal animals have problems producing, folding and degrading proteins at their ambient temperatures of down to −1.86°C. In this first multi-species transcriptome comparison of the amino acid composition of notothenioid proteins with temperate teleost proteins, we show that, unlike psychrophilic bacteria, Antarctic fish provide little evidence for the mass alteration of protein amino acid composition to enhance protein folding and reduce protein denaturation in the cold. The exception was the significant over-representation of positions where leucine in temperate fish proteins was replaced by methionine in the notothenioid orthologues. Although methionine may increase stability in critical proteins, we hypothesise that a more likely explanation for the extra methionines is that they have been preferentially assimilated into the genome because they act as redox sensors. This redox hypothesis is supported by the enrichment of duplicated genes within the notothenioid transcriptomes which centre around Mapk signalling, a major pathway in the cellular cascades associated with responses to environmental stress. Whilst notothenioid fish show cold-associated problems with protein homeostasis, they may have modified only a selected number of biochemical pathways to work efficiently below 0°C. Even a slight warming of the Southern Ocean might disrupt the critical functions of this handful of key pathways with considerable impacts for the functioning of this ecosystem in the future.