Insect Antifreeze Proteins

Abstract

Insects are the most abundant and diverse group of animals on earth. Therefore, it is not surprising that they have evolved numerous, generally non-mutually exclusive, adaptations to permit survival at subzero temperatures. In general, these adaptations can be divided into those that prevent freezing of freeze susceptible species (freeze avoidance) and those that allow the insect to freeze and survive (freeze tolerance). Three types of ice-binding proteins (IBPs) are often involved in either freeze tolerance or avoidance, but they are not required for either, as numerous other adaptations are often employed either in concert with IBPs or in their absence: (1) Antifreeze proteins (AFPs) in freeze avoiding insects lower the freezing point of the hemolymph below the melting point, generating a difference between the two (thermal hysteresis) of generally 2–9 °C. AFPs (A) inhibit inoculative freezing initiated by surface ice across the cuticle and (B) promote supercooling by inhibiting ice nucleators. (2) Recrystallization inhibition proteins (RIPs), found in many freeze-tolerant species, produce a much smaller hemolymph thermal hysteresis, generally less than 1 °C, and function to prevent damaging recrystallization of ice. (3) Hemolymph ice-nucleating proteins (INPs), present in many freeze-tolerant insects, function to initiate ice nucleation in the extracellular fluid at high subzero temperatures, thereby inhibiting lethal intracellular freezing that can follow freezing after significant supercooling. Details of the physiology and biochemistry of these IBPs will be discussed, along with the roles of the other subzero adaptations. Also, potential AFP functions other than prevention of freezing will be mentioned.