Purification and characterization of winter flounder antifreeze peptide messenger ribonucleic acid

The Thr- and Ala-rich hyperactive antifreeze protein from inchworm folds as a flat silk-like β-helix

Inchworm larvae of the pale beauty geometer moth, Campaea perlata, exhibit strong (6.4 °C) freezing point depression activity, indicating the presence of hyperactive antifreeze proteins (AFPs). We have purified two novel Thr- and Ala-rich AFPs from the larvae as small (∼3.5 kDa) and large (∼8.3 kDa) variants and have cloned the cDNA sequences encoding both. They have no homology to known sequences in current BLAST databases. However, these proteins and the newly characterized AFP from the Rhagium inquisitor beetle both contain stretches rich in alternating Thr and Ala residues. On the basis of these repeats, as well as the discontinuities between them, a detailed structural model is proposed for the 8.3 kDa variant. This 88-residue protein is organized into an extended parallel-stranded β-helix with seven strands connected by classic β-turns. The alternating β-strands form two β-sheets with a thin core composed of interdigitating Ala and Ser residues, similar to the thin hydrophobic core proposed for some silks. The putative ice-binding face of the protein has a 4 × 5 regular array of Thr residues and is remarkably flat. In this regard, it resembles the nonhomologous Thr-rich AFPs from other moths and some beetles, which contain two longer rows of Thr in contrast to the five shorter rows in the inchworm protein. Like that of some other hyperactive AFPs, the spacing between these ice-binding Thr residues is a close match to the spacing of oxygen atoms on several planes of ice.

Low temperature persistence of type I antifreeze protein is mediated by cold-specific mRNA stability

In winter flounder, the levels of type I antifreeze protein (AFP) and its mRNA vary seasonally by as much as 1000-fold. Elevated levels in the fall are prompted by the loss of long day-lengths, while higher spring temperatures correlate with AFP clearance. We have investigated the role of temperature on AFP accumulation using transgenic Drosophila melanogaster by expressing multiple AFP genes under control of the heat-inducible hsp70 promoter. AFP and AFP mRNA persisted far longer in flies reared at 10 degrees C compared to 22 degrees C. This difference appears to be mediated by cold-specific mRNA stability since no such temperature effect was observed with either an endogenous heat-inducible mRNA or a constitutively expressed mRNA.

Conservation of antifreeze protein-encoding genes in tandem repeats

The antifreeze protein (AFP) multigene family in winter flounder is made up of genes in tandem repeats and others that are linked, but irregularly spaced. The close spacing of the tandemly repeated genes has provided an opportunity to determine how well nearest-neighbour AFP genes from the tandem repeats resemble each other, the genes elsewhere in the tandem repeats, and the genes outside the tandem repeats. Four pairs of tandemly repeated genes were sequenced. Two pairs were identical and probably represent independent clones of the same chromosomal region. The six unique genes coded for one or other of the two major AFPs, HPLC-6 and HPLC-8. Their comparison over a region of 900 bp. from just after the CAAT box to just before the polyadenylation signal, showed a maximum of 26 single-bp changes and no major insertions or deletions. Nearest-neighbour genes had almost as many changes as genes elsewhere in the tandem repeats. However, these six genes were much more homogeneous than five AFP genes from outside the tandem repeats, none of which encode a major AFP component, and all but one had large insertions, deletions, or rearrangements. Despite the similarity of the genes in tandem repeats, three variants due to short insertions or deletions in the 3'-flanking DNA were interspersed, and genes coding for HPLC-6 and HPLC-8 were nearest neighbours. It is suggested that enhanced opportunities for recombination between repeats have helped keep these genes more uniform than those outside the tandem repeats.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential translatability of antifreeze protein mRNAs in a transgenic host

The expression of fusion gene constructs containing Drosophila regulatory sequences and the structural portions of fish antifreeze protein genes have been examined by transfer into Drosophila melanogaster using P elements. A fusion gene, containing the enhancer, promoter, and cap site of the yolk polypeptide 1 gene, joined in the 5'-untranslated region to the structural portion of the winter flounder type I antifreeze gene, was transcribed in mature female transformants to give an mRNA of the predicted size, but no antifreeze protein was detected by Western blotting. When the same antifreeze protein gene was fused to a Drosophila hsp 70 gene regulatory region and placed downstream of the yolk polypeptide gene enhancer, appropriate expression of mRNA was directed by both gene regulatory elements. However, a translation product from this mRNA was only observed under heat shock conditions and was present at low levels. It is suggested that type I antifreeze mRNA, with its high content of alanine codons and their grouping into clusters of up to seven in a row, is poorly translated when in competition with other host mRNAs. In agreement with this hypothesis, a fusion gene construct between the yolk protein gene regulatory region and two type III antifreeze protein genes produced sub-mmolar concentrations of antifreeze protein in mature females from each of several transgenic lines analysed. The type III antifreeze protein does not have an imbalanced amino acid composition or sequence irregularities, and may be an appropriate choice for conferring freeze protection to frost-susceptible hosts by gene transfer.

Seasonal cycle and regulation by temperature of antifreeze protein mRNA in a Long Island population of winter flounder

The seasonal cycle and regulation by temperature of antifreeze protein mRNA (AF mRNA) were investigated in a Long Island population of winter flounder (Pseudopleuronectes americanus) by Northern blot hybridization and by in vitro translation of liver RNA. AF mRNA was expressed at high levels in the fall and winter (Nov.-Feb.) and at low or undetectable levels in the summer. The time of accumulation of AF mRNA coincides with the time during which water temperature and photoperiod decrease to 4°C and 9 h of light per day, respectively. A temperature and photoperiod decrease in the laboratory during this time also resulted in high levels of AF mRNA. The levels of other mRNAs, as assayed by in vitro translation, were relatively constant during both seasonal acclimation and laboratory acclimation. The seasonal cycle of AF mRNA in Long Island winter flounder is similar to that of a more northern, Newfoundland population of winter flounder and different from that of an intermediate, New Brunswick population. These similarities and dissimilarities are discussed in light of potentially different exogenous and endogenous regulatory cues in the different populations.

Accumulation of winter flounder antifreeze messenger RNA after hypophysectomy

The effect of hypophysectomy (hypex) on winter flounder antifreeze mRNA accumulation in the liver was examined. Hypophysectomy resulted in a significant decrease in serum freezing temperature, and increases in liver weights, total liver poly(A)+ RNA and antifreeze polypeptides (AFP) mRNA accumulation. The identity of the AFP mRNA in hypex animals was confirmed by gel electrophoresis, cell-free translation, and Northern blot hybridization techniques. Cytoplasmic dot hybridization analysis indicated that the AFP mRNA level in hypex fish approximated that observed in winter animals actively synthesizing AFP. An increase in AFP mRNA was detectable as early as the first day after hypophysectomy and by Day 7 reached 25% of the level found in fish actively synthesizing AFP mRNA during the winter months. Since AFP mRNA is found at very low levels in the control flounder, we suggest that its accumulation after hypophysectomy depends on accelerated transcription. The pituitary gland appears to regulate the liver AFP mRNA level by a negative transcriptional control mechanism.

Seasonal variation in the level of antifreeze protein mRNA from the winter flounder

Livers from winter flounder collected at monthly intervals throughout the year were analyzed for their content of antifreeze protein mRNA. Putative mRNA was detected in liver RNAs from summer fish by liquid hybridization to kinetically purified antifreeze protein cDNA. These mRNA sequences were shown by Northern blot analysis to be of the same length as mature antifreeze protein mRNA isolated from winter fish, and were able to direct the incorporation of alanine into a translation product which comigrated with antifreeze preproprotein. The build-up of antifreeze protein mRNA levels in the autumn and their decline in the spring to summer levels were measured by liquid hybridization. These seasonal fluctuations match closely, but slightly precede, the rise and fall in plasma osmolality due to the presence of antifreeze protein. In mid-winter 0.5% of the total liver RNA is antifreeze protein mRNA, and although by late August the level of this mRNA has declined to 0.0007% of the total RNA, at no time during the summer is the mRNA undetectable. These results suggest that antifreeze protein production is more likely to be regulated by changing the rate of transcription of their genes than by switching them between active and inactive states.

Molecular cloning and characterization of winter flounder antifreeze cDNA

Double-stranded cDNA was synthesized from partially purified winter flounder antifreeze mRNA and inserted into the endonuclease Pst I site of plasmid pBR322 by the poly(dG).poly(dC) homopolymer extension technique. The recombinant plasmids wee used to transform Escherichia coli. Clones containing antifreeze cDNA inserts were identified by the hybridization-selection technique. One of the inserts, 380 nucleotides in length, was digested with endonucleases Sau3AI and HinfI, which cleaved the insert into three fragments. The nucleotide sequences of these fragments were determined. The cDNA contains the entire coding sequence for a possible antifreeze peptide, including the leader sequence. The predicted amino acid sequence is similar to but not identical to one of the known sequences of antifreeze peptide. Within the cDNA are three segments of repeating sequences. The basic repeating sequence of 11 amino acids is maintained in the amino acid sequence coded by the cDNA and in the antifreeze peptide.