"Ever since the discovery of these antifreeze proteins, it was assumed they had to be produced in the liver, since the vertebrate liver is well known as a source of secreted plasma protein, so there was no reason to think otherwise," said Chi-Hing "Christina" C. Cheng, a professor of animal biology. "It turns out that the liver has no role in the freezing avoidance in these fishes at all."
Instead, antifreeze glycoproteins (AFGP) originate primarily from the exocrine pancreas and the stomach, say Cheng, Paul A. Cziko and Clive W. Evans in a paper appearing online this week ahead of regular publication in the Proceedings of the National Academy of Sciences.
Cziko is a research specialist at Illinois. Evans is a professor of molecular genetics and development at the University of Auckland in New Zealand.
The liver-synthesis mindset dominated earlier studies even though results appeared to be at odds, Cheng said. The first radioactive-tracer characterization of liver AFGP biosynthesis, in fact, suggested another source of production was possible. Later on, Northern-blot studies had shown very low expression levels of antifreeze messenger RNA in the liver, but this was inconsistent with high levels of production of the protein, the researchers noted.
Cheng and colleagues used Northern blots of total RNA from various tissues to hybridize with an AFGP gene probe. A clear picture of strong AFGP mRNA expression came into focus in the pancreatic tissues in all notothenioids tested. The use of cDNA cloning and sequencing showed that the mRNA all encode secreted AFGPs.