Who would have thought that the humble snow flea (that we used to cross country ski over in Montana) would teach us a great deal about protein chemistry turning over some beloved shibboleths in the process.
The flea contains an antifreeze protein, which stops ice crystals from forming inside the cells of the flea in the cold environment in which it lives. The protein contains 81 amino acids, is 45% glycine and contains six type II polyProline helices each 8 amino acids long (https://en.wikipedia.org/wiki/Polyproline_helix). None of the 6 polyProline helices contain proline despite the name, but all contain from 2 to 6 glycines. Also to be noted is (1) the absence of a hydrophobic core (2) the absence of alpha helices (3) the absence of beta turns (4) the protein has low sequence complexity.
Nonethless it quickly folds into a stable structure — meaning that (1), (2), and (3) are not necessary for a stable protein structure. (4) means that low sequence complexity in a protein sequence does not invariably imply an intrinsically disordered protein.
You can read all about it in Proc. Natl. Acad. Sci. vol. 114 pp. 2241 – 2446 ’17.
Time for some humility in what we thought we knew about proteins, protein folding, protein structural stability.