Antifreeze proteins of teleost fishes

Marine teleosts at high latitudes can encounter ice-laden seawater that is approximately 1 degreesC colder than the colligative freezing point of thei r body fluids. They avoid freezing by producing small antifreeze proteins ( AFPs) that adsorb to ice and halt its growth, thereby producing an addition al non-colligative lowering of the freezing point. AFPs are typically secre ted by the liver into the blood. Recently, however. it has become clear tha t AFP isoforms are produced in the epidermis (skin, scales, fin. and gills) and may serve as a first line of defense against ice propagation into the fish. The basis for the adsorption of AFPs to ice is something of a mystery and is complicated by the extreme structural diversity of the five antifre eze types. Despite the recent acquisition of several AFP three-dimensional structures and the definition of their ice-binding sites by mutagenesis, no common ice-binding motif or even theme is apparent except that surface-sur face complementarity is important for binding. The remarkable diversity of antifreeze types and their seemingly haphazard phylogenetic distribution su ggest that these proteins might have evolved recently in response to sea le vel glaciation occurring just 1-2 million years ago in the northern hemisph ere and 10-30 million years ago around Antarctica. Not surprisingly, the ex pression of AEP genes from different origins can also be quite dissimilar. The most intensively studied system is that of the winter flounder, which h as a built-in annual cycle of antifreeze expression controlled by growth ho rmone (GH) release from the pituitary in tune with seasonal cues. The signa l transduction pathway, transcription factors, and promoter elements involv ed in this process are just beginning to be characterized.