Investigating the dual role of a marine antifreeze protein, winter flounder AFP6

Abstract

Antifreeze proteins (AFPs) can protect organisms from freezing by binding to ice crystals, thereby preventing further crystal growth. The most thoroughly studied of these proteins is AFP6, a 37-residue α-helical protein found in the blood plasma of winter flounder, Pseudopleuronectes americanus. The role of the AFPs in flounder is to lower the freezing point to that of the surrounding seawater. However, recent results using recombinant AFP6 have shown that, at lower temperatures, this protein can trigger ice formation. The transition of AFP6 from an ice growth inhibitor to an ice nucleator is expected to require a change in the solution conditions or in the assembly state of the protein. Therefore, AFP6 was hypothesized to oligomerize, resulting in an extended ice crystal template that would allow ice nucleation to occur. Results from chemical crosslinking and size exclusion chromatography suggested oligomerization of AFP6. The ice nucleation of crosslinked AFP6 was then investigated, suggesting no effect on this activity. Ice nucleation by AFP6 was then examined in the presence of a set of relevant additives. These additives did not induce significant changes to nucleation temperatures; however, some differences observed would be worthy of further study.