As temperatures drop, proteins tend to become more rigid. Not so for the muscles of Atlantic salmon (Salmo salar).
New research shows that the tail muscles of these powerful fish maintain flexibility at temperatures below 10 °C, thanks to a tiny tweak in tropomyosin, a protein involved with muscle contraction. The salmon, who regularly experience temperatures below this threshold in the North Atlantic Ocean, owe this trick to a single amino acid difference compared with the tropomyosin found in mammals (Biochemistry 2020, DOI: 10.1021/acs.biochem.0c00416).
For muscles to contract properly, tropomyosin must maintain conformational flexibility and the ability to interact with other muscle proteins, says David H. Heeley, a biochemist at Memorial University who helmed the study. Mammalian forms of tropomyosin become rigid and less able to bind their target as the temperature drops.
The salmon protein, Tpm1 (α-fast), turns the tide. Replacing positively charged lysine with neutral threonine at position 77 disrupts some electrostatic interactions within tropomyosin, destabilizing the protein. At a mammal’s body temperature, this variant would lose stability and function. But in salmon, the destabilization grants the protein just enough molecular flexibility to prevent cold-induced rigidity, allowing the fish to surge through the frigid Atlantic waters.