Origin and Mechanism of Evolution of Antifreeze Glycoproleins in Polar Fishes
313
heterogeneity (size and composition).
AFGPs were subsequently found in several Arctic and north Atlantic
cods which are unrelated to notothenioids [14,15]. Cod AFGPs are
identical to notothenioid AFGPs in every way, structure and protein
heterogeneity. The only difference is that Thr in the tripeptide repeats is
occasionally replaced by an Arg [14,15].
The three types of AFPs are small peptides or proteins without attached
sugars. They were first classified based on distinctive differences in their
protein sequences. Type I AFPs are Ala-rich (>50%), a-helical peptides of
3.3 - 4.5 kDa, most of which have an II-residue repeat ending with a Thr.
They are found in the pleuronectids (flounder and plaice, order
Perciformes) [16,17] and the unrelated cottid sculpins (order
Scorpaeniformes) [18]. Type II AFPs are 11 - 24 kDa, cystine-rich, /3structured proteins found in a highly divergent group of fishes - smelt
(order Salmoniformes), herring (order Clupeiformes) [19] and sea raven
(order Scorpaeniformes) [20]. Type III AFPs are 7 kDa and 14 kDa
globular proteins with unbiased amino acid composition found in the
closely related eel pouts (zoarcids) [21-23] and wolffish (anarhichadid)
[24].
Two recurring themes are evident in the studies of the various fish
antifreezes - disparate protein structures but identical protein function, and
inconsistent correlation between the phyletic relatedness of the various
fishes and the type of antifreeze protein they synthesize. Both indicate that
fish antifreeze proteins arose independently (at least four times) and
achieved remarkable functional convergence. As a monofunctional group
of macromolecules, the diverse antifreeze proteins epitomize the creativity
of molecular evolution. Where did they come from?
Evolutionary Enigma of Near-Identical AFGPs
in Unrelated Antarctic Notothenioids and Northern Cods
For over two decades since the AFGPs were discovered, the puzzle has
persisted as to how two such geographically and phylogenetically distant
fishes, the modem Antarctic notothenioids (superorder Acanthopterygii,
order Perciformes) and the more basal northern cods (superorder
Paracanthopterygii, order Gadiformes), could have arrived at essentially
the same AFGPs. An obvious possibility which had been suggested is that
the two taxa inherited their AFGPs from a common ancestor [25], but this
is counter to their known phylogeny [26], their distinct geographic origins
[27,28], and the recent but separate glacial histories of the polar regions
[1], which collectively indicate that their ancestor is ancient and would not
need or have an antifreeze. The alternate possibility is that these two
313
heterogeneity (size and composition).
AFGPs were subsequently found in several Arctic and north Atlantic
cods which are unrelated to notothenioids [14,15]. Cod AFGPs are
identical to notothenioid AFGPs in every way, structure and protein
heterogeneity. The only difference is that Thr in the tripeptide repeats is
occasionally replaced by an Arg [14,15].
The three types of AFPs are small peptides or proteins without attached
sugars. They were first classified based on distinctive differences in their
protein sequences. Type I AFPs are Ala-rich (>50%), a-helical peptides of
3.3 - 4.5 kDa, most of which have an II-residue repeat ending with a Thr.
They are found in the pleuronectids (flounder and plaice, order
Perciformes) [16,17] and the unrelated cottid sculpins (order
Scorpaeniformes) [18]. Type II AFPs are 11 - 24 kDa, cystine-rich, /3structured proteins found in a highly divergent group of fishes - smelt
(order Salmoniformes), herring (order Clupeiformes) [19] and sea raven
(order Scorpaeniformes) [20]. Type III AFPs are 7 kDa and 14 kDa
globular proteins with unbiased amino acid composition found in the
closely related eel pouts (zoarcids) [21-23] and wolffish (anarhichadid)
[24].
Two recurring themes are evident in the studies of the various fish
antifreezes - disparate protein structures but identical protein function, and
inconsistent correlation between the phyletic relatedness of the various
fishes and the type of antifreeze protein they synthesize. Both indicate that
fish antifreeze proteins arose independently (at least four times) and
achieved remarkable functional convergence. As a monofunctional group
of macromolecules, the diverse antifreeze proteins epitomize the creativity
of molecular evolution. Where did they come from?
Evolutionary Enigma of Near-Identical AFGPs
in Unrelated Antarctic Notothenioids and Northern Cods
For over two decades since the AFGPs were discovered, the puzzle has
persisted as to how two such geographically and phylogenetically distant
fishes, the modem Antarctic notothenioids (superorder Acanthopterygii,
order Perciformes) and the more basal northern cods (superorder
Paracanthopterygii, order Gadiformes), could have arrived at essentially
the same AFGPs. An obvious possibility which had been suggested is that
the two taxa inherited their AFGPs from a common ancestor [25], but this
is counter to their known phylogeny [26], their distinct geographic origins
[27,28], and the recent but separate glacial histories of the polar regions
[1], which collectively indicate that their ancestor is ancient and would not
need or have an antifreeze. The alternate possibility is that these two