322
Chi-Hing C. Cheng
sequence - through repeated duplications to create a unique protein with
the novel ice-binding function, thereby creating sense from partly nonsense.
Antifreeze Glycoprotein Genes of Arctic Cod
Through the use of molecular analyses we have also addressed the
lingering mystery of how the northern cods arrived at essentially the same
AFGPs as the unrelated Antarctic notothenioids [30]. We recently reported
the first AFGP gene from a northern gadid, the Arctic cod, Boreogadus
saida [30]; its schematic structure is shown in Fig. 4A. Other Bs (B. saida)
AFGP genes have the same structure (unpublished data).
The Bs AFGP genes consist of three exons and two introns. Exons I and
2 and the two introns are small. Eland E2 encode the 5'UTR and the
translation start (Met). The large exon 3 encodes the signal peptide, an
apparent AFGP precursor, and the 3'UTR. The encoded AFGP precursor
consists of a long string of Thr-AlalPro-Ala repeats, punctuated at various
intervals by an Arg (R) or ArgAlaAlaArg (RAAR), with the Arg residue
occupying where a Thr is normally found. The percentage of Arg residues
in the precursor encoded in these genes is always much higher (~7%) than
that in mature AFGPs (0.6-1.2%) [30]. Thus, similar to the notothenioids,
Arctic cod AFGP genes appear to also encode AFGPs as a polyprotein
precursor, but with some of the Arg residues serving as cleavable spacers
and involving a trypsin-like protease instead (Fig. 4B).
Arctic Cod AFGP Genes Evolved Independently
The polyprotein gene structure of the AFGP coding region of the Arctic
cod and notothenioid AFGP genes is where their similarity ends.
Molecular evidence from detailed comparative analyses argues strongly
for independent evolution of the cod AFGP genes, and thus the nearidentical AFGPs of these two unrelated fish are a rare example of protein
sequence convergence [30]. The evidence is summarized below
Arctic Cod AFGP Gene Did Not Evolve From Trypsinogen Gene
The Arctic cod AFGP genes have none of the inherited features that
bespeak the trypsinogen origin of the notothenioid AFGP genes, i.e. the 5'
and 3' regions of high sequence identity which include identical signal
peptides. Thus, assuming an endogenous genetic origin, the cod AFGP
genes must have evolved from a different genomic locus which is
decidedly not trypsinogen.
Chi-Hing C. Cheng
sequence - through repeated duplications to create a unique protein with
the novel ice-binding function, thereby creating sense from partly nonsense.
Antifreeze Glycoprotein Genes of Arctic Cod
Through the use of molecular analyses we have also addressed the
lingering mystery of how the northern cods arrived at essentially the same
AFGPs as the unrelated Antarctic notothenioids [30]. We recently reported
the first AFGP gene from a northern gadid, the Arctic cod, Boreogadus
saida [30]; its schematic structure is shown in Fig. 4A. Other Bs (B. saida)
AFGP genes have the same structure (unpublished data).
The Bs AFGP genes consist of three exons and two introns. Exons I and
2 and the two introns are small. Eland E2 encode the 5'UTR and the
translation start (Met). The large exon 3 encodes the signal peptide, an
apparent AFGP precursor, and the 3'UTR. The encoded AFGP precursor
consists of a long string of Thr-AlalPro-Ala repeats, punctuated at various
intervals by an Arg (R) or ArgAlaAlaArg (RAAR), with the Arg residue
occupying where a Thr is normally found. The percentage of Arg residues
in the precursor encoded in these genes is always much higher (~7%) than
that in mature AFGPs (0.6-1.2%) [30]. Thus, similar to the notothenioids,
Arctic cod AFGP genes appear to also encode AFGPs as a polyprotein
precursor, but with some of the Arg residues serving as cleavable spacers
and involving a trypsin-like protease instead (Fig. 4B).
Arctic Cod AFGP Genes Evolved Independently
The polyprotein gene structure of the AFGP coding region of the Arctic
cod and notothenioid AFGP genes is where their similarity ends.
Molecular evidence from detailed comparative analyses argues strongly
for independent evolution of the cod AFGP genes, and thus the nearidentical AFGPs of these two unrelated fish are a rare example of protein
sequence convergence [30]. The evidence is summarized below
Arctic Cod AFGP Gene Did Not Evolve From Trypsinogen Gene
The Arctic cod AFGP genes have none of the inherited features that
bespeak the trypsinogen origin of the notothenioid AFGP genes, i.e. the 5'
and 3' regions of high sequence identity which include identical signal
peptides. Thus, assuming an endogenous genetic origin, the cod AFGP
genes must have evolved from a different genomic locus which is
decidedly not trypsinogen.