Sequence and structure comparison of ATP synthase F0 subunits 6 and 8 in notothenioid fish

Mitochondrial changes such as tight coupling of the mitochondria have facilitated sustained oxygen and respiratory activity in haemoglobin-less icefish of the Channichthyidae family. We aimed to characterise features in the sequence and structure of the proteins directly involved in proton transport, which have potential physiological implications. ATP synthase subunit a (ATP6) and subunit 8 (ATP8) are proteins that function as part of the F0 component (proton pump) of the F0F1complex. Both proteins are encoded by the mitochondrial genome and involved in oxidative phosphorylation. To explore mitochondrial sequence variation for ATP6 and ATP8 we analysed sequences from C. gunnari and C. rastrospinosus and compared them with their closely related red-blooded species and eight other vertebrate species. Our comparison of the amino acid sequence of these proteins reveals important differences that could underlie aspects of the unique physiology of the icefish. In this study we find that changes in the sequence of subunit a of the icefish C. gunnari at position 35 where there is a hydrophobic alanine which is not seen in the other notothenioids we analysed. An amino acid change of this type is significant since it may have a structural impact. The biology of the haemoglobin-less icefish is necessarily unique and any insights about these animals will help to generate a better overall understanding of important physiological pathways.

A unique life-strategy of an endophytic yeast Rhodotorula mucilaginosa JGTA-S1-a comparative genomics viewpoint

Endophytic yeasts of genus Rhodotorula are gaining importance for their ability to improve plant growth. The nature of their interaction with plants, however, remains unknown. Rhodotorula mucilaginosa JGTA-S1 was isolated as an endophyte of Typha angustifolia and promoted growth in the host. To investigate the life-strategy of the yeast from a genomics perspective, we used Illumina and Oxford Nanopore reads to generate a high-quality annotated draft assembly of JGTA-S1 and compared its genome to three other Rhodotorula yeasts and the close relative Rhodosporidium toruloides. JGTA-S1 is a haploid yeast possessing several genes potentially facilitating its endophytic lifestyle such as those responsible for solubilizing phosphate and producing phytohormones. An intact mating-locus in JGTA-S1 raised the possibility of a yet unknown sexual reproductive cycle in Rhodotorula yeasts. Additionally, JGTA-S1 had functional anti-freezing genes and was also unique in lacking a functional nitrate-assimilation pathway—a feature that is associated with obligate biotrophs. Nitrogen-fixing endobacteria were found within JGTA-S1 that may circumvent this defective N-metabolism. JGTA-S1 genome data coupled with experimental evidence give us an insight into the nature of its beneficial interaction with plants.

Freeze Tolerance in Sculpins (Pisces; Cottoidea) Inhabiting North Pacific and Arctic Oceans: Antifreeze Activity and Gene Sequences of the Antifreeze Protein

Many marine species inhabiting icy seawater produce antifreeze proteins (AFPs) to prevent their body fluids from freezing. The sculpin species of the superfamily Cottoidea are widely found from the Arctic to southern hemisphere, some of which are known to express AFP. Here we clarified DNA sequence encoding type I AFP for 3 species of 2 families (Cottidae and Agonidae) belonging to Cottoidea. We also examined antifreeze activity for 3 families and 32 species of Cottoidea (Cottidae, Agonidae, and Rhamphocottidae). These fishes were collected in 2013–2015 from the Arctic Ocean, Alaska, Japan. We could identify 8 distinct DNA sequences exhibiting a high similarity to those reported for Myoxocephalus species, suggesting that Cottidae and Agonidae share the same DNA sequence encoding type I AFP. Among the 3 families, Rhamphocottidae that experience a warm current did not show antifreeze activity. The species inhabiting the Arctic Ocean and Northern Japan that often covered with ice floe showed high activity, while those inhabiting Alaska, Southern Japan with a warm current showed low/no activity. These results suggest that Cottoidea acquires type I AFP gene before dividing into Cottidae and Agonidae, and have adapted to each location with optimal antifreeze activity level.

Antarctic notothenioid fish: what are the future consequences of 'losses' and 'gains' acquired during long-term evolution at cold and stable temperatures?

Antarctic notothenioids dominate the fish fauna of the Southern Ocean. Evolution for millions of years at cold and stable temperatures has led to the acquisition of numerous biochemical traits that allow these fishes to thrive in sub-zero waters. The gain of antifreeze glycoproteins has afforded notothenioids the ability to avert freezing and survive at temperatures often hovering near the freezing point of seawater. Additionally, possession of cold-adapted proteins and membranes permits them to sustain appropriate metabolic rates at exceptionally low body temperatures. The notothenioid genome is also distinguished by the disappearance of traits in some species, losses that might prove costly in a warmer environment. Perhaps the best-illustrated example is the lack of expression of hemoglobin in white-blooded icefishes from the family Channichthyidae. Loss of key elements of the cellular stress response, notably the heat shock response, has also been observed. Along with their attainment of cold tolerance, notothenioids have developed an extreme stenothermy and many species perish at temperatures only a few degrees above their habitat temperatures. Thus, in light of today's rapidly changing climate, it is critical to evaluate how these extreme stenotherms will respond to rising ocean temperatures. It is conceivable that the remarkable cold specialization of notothenioids may ultimately leave them vulnerable to future thermal increases and threaten their fitness and survival. Within this context, our review provides a current summary of the biochemical losses and gains that are known for notothenioids and examines these cold-adapted traits with a focus on processes underlying thermal tolerance and acclimation capacity.

Interaction of ice binding proteins with ice, water and ions

Ice binding proteins (IBPs) are produced by various cold-adapted organisms to protect their body tissues against freeze damage. First discovered in Antarctic fish living in shallow waters, IBPs were later found in insects, microorganisms, and plants. Despite great structural diversity, all IBPs adhere to growing ice crystals, which is essential for their extensive repertoire of biological functions. Some IBPs maintain liquid inclusions within ice or inhibit recrystallization of ice, while other types suppress freezing by blocking further ice growth. In contrast, ice nucleating proteins stimulate ice nucleation just below 0 °C. Despite huge commercial interest and major scientific breakthroughs, the precise working mechanism of IBPs has not yet been unraveled. In this review, the authors outline the state-of-the-art in experimental and theoretical IBP research and discuss future scientific challenges. The interaction of IBPs with ice, water and ions is examined, focusing in particular on ice growth inhibition mechanisms.

Evolution and adaptive radiation of antarctic fishes

There are few instances where a knowledge of the thermal physiology, habitats and lifestyles of a group of closely related species can be mapped onto a well-supported phylogeny and a detailed climatic history. The unique fish fauna of the Southern Ocean, dominated by a single group of fish whose phylogeny is known from traditional and molecular techniques, provides one such opportunity. Furthermore, these fish are living at an extreme temperature for marine organisms. Physiological and molecular studies are revealing details of the mechanisms of temperature compensation and, combined with knowledge of the thermal history, are throwing new light on the process of evolution in this unique group of fish.

Thermodynamic Analysis of Thermal Hysteresis: Mechanistic Insights into Biological Antifreezes

Antifreeze proteins (AFPs) bind to ice crystal surfaces and thus inhibit the ice growth. The mechanism for how AFPs suppress freezing is commonly modeled as an adsorption-inhibition process by the Gibbs-Thomson effect. Here we develop an improved adsorption-inhibition model for AFP action based on the thermodynamics of impurity adsorption on the crystal surfaces. We demonstrate the derivation of a realistic relationship between surface protein coverage and the protein concentration. We show that the improved model provides a quantitatively better fit to the experimental antifreeze activities of AFPs from distinct structural classes, including fish and insect AFPs, in a wide range of concentrations. Our theoretical results yielded the adsorption coefficients of the AFPs on ice, suggesting that, despite the distinct difference in their antifreeze activities and structures, the affinities of the AFPs to ice are very close and the mechanism of AFP action is a kinetically controlled, reversible process. The applications of the model to more complex systems along with its potential limitations are also discussed.

Thermal stability properties of an antifreeze protein from the desert beetle Microdera punctipennis

An insect antifreeze protein gene Mpafp698 was cloned by the RT-PCR approach from the desert beetle Microdera punctipennis. The gene was constructed and heterogeneously expressed in Escherichia coli as fusion proteins, His-MpAFP698, glutathione S-transferase (GST)-MpAFP698, and maltose-binding protein (MBP)-MpAFP698. The thermostability and thermal hysteresis activity of these proteins were determined, with the aim of elucidating the biological characteristics of this protein. The approximate thermal hysteresis (TH) value of the purified His-MpAFP698 was 0.37 degrees C at 0.84 mg/ml, and maintained approximately 95.7% of the TH activity at 100 degrees C for 5 min. Furthermore, heat incubation showed that MBP-MpAFP698 was 10 degrees C more thermostable than MBP protein, indicating that MpAFP698 could, to some extent, improve the thermal stability of the fused partner MBP protein. This study suggests that MpAFP698 has a high thermal stability and could be used to improve the thermal stability of the less stable proteins by producing fusion proteins, which could be used for biotechnological purposes.

Determination of the expression of fish antifreeze protein (AFP) in 7th generation transgenic mice tissues and serum

In this study, the presence of antifreeze protein (AFP) gene expression through successive generations in transgenic mice carrying the chimeric gene construct of the coding sequence for the AFP protein from ocean pout was investigated. AFP transgenic hemizygote mice were used for AFP gene expression. AFP genome expressions in transgenic mice were analyzed by Western blotting, and tissue location of AFP protein was shown by immunohistochemical and immunofluorescence techniques. Seventh transgenic mice from the established founders demonstrated the expression of AFP in organs such as the skin, oviduct, lung, kidney and liver tissues and serum except for the heart. Our results demonstrate successful expression of AFP gene products in several tissues and serum of transgenic mice, the association of in vivo expressed AFP protein, for the first time. These results indicate that the coding sequence for the AFP protein gene (ocean pout type III AFP gene) could be integrated and stably transcribed and expressed in the 7th generation of transgenic mice. In conclusion transgenic mouse lines would be a good model for the cryostudy of AFP and for the determination of AFP roles in several organs and tissues.

Dissolution enhancement by bio-inspired mesocrystals: the study of racemic (R,S)-(+/-)-sodium ibuprofen dihydrate

PURPOSE

The aim of this paper is to enhance the dissolution rate of racemic (R,S)-(+/-)-sodium ibuprofen dihydrate via a bio-inspired method of growing mesocrystals.

MATERIALS AND METHODS

Mesocrystals of racemic (R,S)-(+/-)-sodium ibuprofen dihydrate were successfully prepared from a supersaturated aqueous solution of racemic (R,S)-(+/-)-sodium ibuprofen dihydrate having the initial degree of supersaturation, S ( 0 ), of 1.326 and the initial saturated concentration, C*, of 0.986 mol/l at 25 degrees C with sodium dodecyl sulfate (SDS) at a concentration of 0.10 g/l. Dynamic light scattering, scanning electron microscopy, powder X-ray diffraction, differential scanning calorimetry, and optical microscopy with cross polarizers were employed to understand the formation mechanism and to characterize the superstructures of the SDS generated mesocrystals.

RESULTS

The SDS generated mesocrystals were the assembly of the oriented attachment of racemic (R,S)-(+/-)-sodium ibuprofen dihydrate nano-sized platelets under the mediation of the side-to-side interaction between SDS and racemic (R,S)-(+/-)-sodium ibuprofen dihydrate. The SDS generated mesocrystals contained a mixture of the racemic compounds in alpha- and beta-forms and the resolved racemic conglomerate in gamma-form with no detectable amount of SDS. The dissolution rate of the SDS generated mesocrystals was more rapid than the one of its counterpart made by conventional crystallization pathway.

CONCLUSIONS

The crystallization of racemic (R,S)-(+/-)-sodium ibuprofen dihydrate in the presence of SDS yielded well-faceted, well-separated, but almost perfectly three-dimensionally aligned nano-sized platelets. This kind of bio-inspired mesocrystal superstructure has definitely opened a new doorway for crystal engineering and pre-formulation design in pharmaceutical industry. The future work is to study the mesocrystal formation of some other active pharmaceutical ingredients in organic solvent systems and to develop an efficient method for screening the additives.

Cryogenic effect of antifreeze protein on transgenic mouse ovaries and the production of live offspring by orthotopic transplantation of cryopreserved mouse ovaries

The purpose of the present study was to evaluate the cryogenic effect of antifreeze protein (AFP) on transgenic mouse ovaries which is expressed AFP type III from Ocean pout and the production of live offspring by orthotopic transplantation of cryopreserved mouse ovaries. In this study, whole transgenic and nontransgenic mouse ovaries were vitrified with 20% DMSO and 20% EG in M2 medium supplemented with 0.5 M sucrose. All vitrified and toxicity control and fresh ovaries were transplanted orthotopically into ovariectomized recipients bilaterally. For fresh ovaries transplantation, 5 mice delivered litters of 18 and 19 live pups in first and second matings, respectively. For toxicity control of chemicals, 6 mice delivered litters of 22 and 23 live pups. For nontransgenic mouse ovaries (vitrified) transplantation, 7 mice delivered litters of 22 and 23 live pups. For transgenic mouse ovaries (vitrified) transplantation, 10 mice delivered litters of 35 and 37 live pups. Litter sizes from pups of freshly transplanted ovaries were not significantly different from AFP-transplanted transgenic ovaries but those from nontransgenic-transplanted ovaries were significantly different from the AFP-transplanted transgenic ovaries group (P < 0.05). In this study, for the first time, it was shown that the ovarian tissue of AFP transgenic mice was protected from cryopreservation by vitrification. These results demonstrate that a normal reproductive lifespan can be restored by orthotopic transplantation of AFP transgenic-vitrified ovary.

A falsification of the thermal specialization paradigm: compensation for elevated temperatures in Antarctic fishes

Specialization to a particular environment is one of the main factors used to explain species distributions. Antarctic fishes are often cited as a classic example to illustrate the specialization process and are regarded as the archetypal stenotherms. Here we show that the Antarctic fish Pagothenia borchgrevinki has retained the capacity to compensate for chronic temperature change. By displaying astounding plasticity in cardiovascular response and metabolic control, the fishes maintained locomotory performance at elevated temperatures. Our falsification of the specialization paradigm indicates that the effect of climate change on species distribution and extinction may be overestimated by current models of global warming.

Stable transmission and transcription of newfoundland ocean pout type III fish antifreeze protein (AFP) gene in transgenic mice and hypothermic storage of transgenic ovary and testis

Here we describe the generation of transgenic mice carrying type III fish antifreeze protein (AFP) gene and evaluate whether AFP type III protects transgenic mouse ovaries and testes from hypothermic storage. AFPs exist in many different organisms. In fish, AFPs protect the host from freezing at temperatures below the colligative freezing point by adsorbing to the surface of nucleating ice crystals and inhibiting their growth. The transgenic expression of AFP holds great promise for conferring freeze-resistant plant and animal species. AFP also exhibits a potential for the cryopreservation of tissues and cells. In this study, we have generated 42 founder mice harboring the Newfoundland ocean pout (OP5A) type III AFP transgene and established one transgenic line (the line #6). This study demonstrated that AFP gene construct has been stably transmitted to the mouse progeny in the F3 generations in the line #6. Furthermore, the presence of AFP transcripts was confirmed by RT-PCR analysis on cDNAs from liver, kidney, ovarian, and testis tissues of the mouse from F3 generation in this line. These results indicate that ocean pout type III AFP gene could be integrated and transmitted to the next generation and stably transcribed in transgenic mice. In histological analysis of testis and ovarian tissues of nontransgenic control and AFP transgenic mice it has been shown that both tissues of AFP transgenic mice were protected from hypothermic storage (+4 degrees C). The AFP III transgenic mice obtained for the first time in this study would be useful for investigating the biological functions of AFP in mammalian systems and also its potential role in cryopreservation.

Theoretical study of interaction of winter flounder antifreeze protein with ice

Antifreeze proteins (AFPs) are synthesized by various organisms to enable their cells to survive subzero environment. These proteins bind to small ice crystals and inhibit their growth, which if left uncontrolled would be fatal to cells. The crystal structures of a number of AFPs have been determined; however, crystallographic analysis of AFP-ice complex is nearly impossible. Molecular modeling studies of AFPs' interaction with ice surface is therefore invaluable. Early models of AFP-ice interaction suggested H-bond as the primary driving force behind such interaction. Recent experimental evidence, however, suggested that hydrophobic interactions could be the main contributor to AFP-ice association. All computational studies published to date were carried out to verify the H-bond model, and no works attempting to verify the hydrophobic interaction model have been published. In this work, we Monte Carlo-minimized complexes of several AFPs with ice taking into account nonbonded interactions, H-bonds, and the hydration potential for proteins. Parameters of the hydration potential for ice were developed with the assumption that the free energy of the water-ice association should be close to zero at equilibrium melting temperature. Our calculations demonstrate that desolvation of hydrophobic groups in the AFPs upon their binding to the grooves at the ice surface is indeed the major stabilizing contributor to the free energy of AFP-ice binding. This study is consistent with available structural and mutation data on AFPs. In particular, it explains the paradoxical finding that substitution of Thr residues with Val does not affect the potency of winter flounder AFP whereas substitution with Ser abolished its antifreeze activity.

Structure and function of antifreeze proteins

High-resolution three-dimensional structures are now available for four of seven non-homologous fish and insect antifreeze proteins (AFPs). For each of these structures, the ice-binding site of the AFP has been defined by site-directed mutagenesis, and ice etching has indicated that the ice surface is bound by the AFP. A comparison of these extremely diverse ice-binding proteins shows that they have the following attributes in common. The binding sites are relatively flat and engage a substantial proportion of the protein's surface area in ice binding. They are also somewhat hydrophobic -- more so than that portion of the protein exposed to the solvent. Surface-surface complementarity appears to be the key to tight binding in which the contribution of hydrogen bonding seems to be secondary to van der Waals contacts.

Biotechnological applications of plant freezing associated proteins

Plants use a wide array of proteins to protect themselves against low temperature and freezing conditions. The identification of these freezing tolerance associated proteins and the elucidation of their cryoprotective functions will have important applications in several fields. Genes encoding structural proteins, osmolyte producing enzymes, oxidative stress scavenging enzymes, lipid desaturases and gene regulators have been used to produce transgenic plants. These studies have revealed the potential capacity of different genes to protect against temperature related stresses. In some cases, transgenic plants with significant cold tolerance have been produced. Furthermore, the biochemical characterization of the cold induced antifreeze proteins and dehydrins reveals many applications in the food and the medical industries. These proteins are being considered as food additives to improve the quality and shelf-life of frozen foods, as cryoprotective agents for organ and cell cryopreservation, and as chemical adjuvant in cancer cryosurgery.

Thermal hysteresis proteins

Extreme environments present a wealth of biochemical adaptations. Thermal hysteresis proteins (THPs) have been found in vertebrates, invertebrates, plants, bacteria and fungi and are able to depress the freezing point of water (in the presence of ice crystals) in a non-colligative manner by binding to the surface of nascent ice crystals. The THPs comprise a disparate group of proteins with a variety of tertiary structures and often no common sequence similarities or structural motifs. Different THPs bind to different faces of the ice crystal, and no single mechanism has been proposed to account for THP ice binding affinity and specificity. Experimentally THPs have been used in the cryopreservation of tissues and cells and to induce cold tolerance in freeze susceptible organisms. THPs represent a remarkable example of parallel and convergent evolution with different proteins being adapted for an anti-freeze role.

A comparison of plasma vitamin C and E levels in two Antarctic and two temperate water fish species

Antarctic fish have a high polyunsaturated lipid content and their muscle cells have a high mitochondria density suggesting that Antarctic fish are under greater oxidative stress than temperate water fish. To test this hypothesis, the plasma concentrations of the antioxidant vitamins E and C were measured in two Antarctic fish species, Pagothenia borchgrevinki and Trematomus bernacchii, and compared with the plasma concentrations of these vitamins in two New Zealand temperate water fish species, blue cod (Parapercis colias) and banded wrasse (Notolabrus fucicola). Neither vitamin is known to be synthesised in fish and so must be obtained from the diet. The plasma from both Antarctic fish species had vitamin E concentrations five to six times higher than those found in the two temperate water fish species. However, significantly higher levels of vitamin C were only found in the plasma of T. bernacchii, a benthic Antarctic fish. The average level of vitamin C in the plasma of the cryopelagic P. borchgrevinki was approximately one-third that of T. bernacchii. The T. bernacchii plasma yielded a high range of vitamin C values, possibly reflecting differences in nutritional status among the animals captured. No beta-carotene was found in any of the fish plasma samples studied. The data suggest that even though Antarctic fish live at -1.5 degrees C they may be exposed to greater metabolic stress from free radical mediated oxidation than temperate water species.

Molecular recognition and binding of thermal hysteresis proteins to ice

Molecular recognition and binding are two very important processes in virtually all biological and chemical processes. An extremely interesting system involving recognition and binding is that of thermal hysteresis proteins at the ice-water interface. These proteins are of great scientific interest because of their antifreeze activity. Certain fish, insects and plants living in cold weather regions are known to generate these proteins for survival. A detailed molecular understanding of how these proteins work could assist in developing synthetic analogs for use in industry. Although the shapes of these proteins vary from completely alpha-helical to globular, they perform the same function. It is the shapes of these proteins that control their recognition and binding to a specific face of ice. Thermal hysteresis proteins modify the morphology of the ice crystal, thereby depressing the freezing point. Currently there are three hypotheses proposed with respect to the antifreeze activity of thermal hysteresis proteins. From structure-function experiments, ice etching experiments, X-ray structures and computer modeling at the ice-vacuum interface, the first recognition and binding hypothesis was proposed and stated that a lattice match of the ice oxygens with hydrogen-bonding groups on the proteins was important. Additional mutagenesis experiments and computer simulations have lead to the second hypothesis, which asserted that the hydrophobic portion of the amphiphilic helix of the type I thermal hysteresis proteins accumulates at the ice-water interface. A third hypothesis, also based on mutagenesis experiments and computer simulations, suggests that the thermal hysteresis proteins accumulate in the ice-water interface and actually influence the specific ice plane to which the thermal hysteresis protein ultimately binds. The first two hypotheses emphasize the aspect of the protein 'binding or accumulating' to specific faces of ice, while the third suggests that the protein assists in the development of the binding site. Our modeling and analysis supports the third hypothesis, however, the first two cannot be completely ruled out at this time. The objective of this paper is to review the computational and experimental efforts during the past 20 years to elucidate the recognition and binding of thermal hysteresis proteins at the ice-vacuum and ice-water interface.

Type I 'antifreeze' proteins. Structure-activity studies and mechanisms of ice growth inhibition

The type I 'antifreeze' proteins, found in the body fluids of fish inhabiting polar oceans, are alanine-rich alpha-helical proteins that are able to inhibit the growth of ice. Within this class there are two distinct subclasses of proteins: those related to the winter flounder sequence HPLC6 and which contain 11-residue repeat units commencing with threonine; and those from the sculpins that are unique in the N-terminal region that contains established helix breakers and lacks the 11-residue repeat structure present in the rest of the protein. Although 14 type I proteins have been isolated, almost all research has focused on HPLC6, the 37-residue protein from the winter flounder Pseudopleuronectes americanus. This protein modifies both the rate and shape (or 'habit') of ice crystal growth, displays hysteresis and accumulates specifically at the {2 0 2; 1} ice plane. Until very recently, all models to explain the mechanism for this specific interaction have relied on the interaction of the four threonine hydroxyls, which are spaced equally apart on one face of the helix, with the ice lattice. In contrast, proteins belonging to the sculpin family accumulate specifically at the {2 1; 1; 0} plane. The molecular origin of this difference in specificity between the flounder and sculpin proteins is not understood. This review will summarize the structure-activity and molecular modelling and dynamics studies on HPLC6, with an emphasis on recent studies in which the threonine residues have been mutated. These studies have identified important hydrophobic contributions to the ice growth inhibition mechanism. Some 50 mutants of HPLC6 have been reported and the data is consistent with the following requirements for ice growth inhibition: (a) a minimum length of approx. 25 residues; (b) an alanine-rich sequence in order to induce a highly helical conformation; (c) a hydrophobic face; (d) a number of charged/polar residues which are involved in solubility and/or interaction with the ice surface. The emerging picture, that requires further dynamics studies including accurate modelling of the ice/water interface, suggests that a hydrophobic interaction between the surface of the protein and ice is the key to explaining accumulation at specific ice planes, and thus the molecular level mechanism for ice growth inhibition.

Structure-function relationships in a type I antifreeze polypeptide. The role of threonine methyl and hydroxyl groups in antifreeze activity

Several analogs of an alanine-rich, alpha-helical type I antifreeze polypeptide from the winter flounder were synthesized and studied to evaluate the role of threonine residues on antifreeze activity. In this series, the four Thr residues in the wild type polypeptide were substituted with from one to four Ser, allo-Thr, or Val residues. Circular dichroism studies determined that these substitutions did not significantly diminish alpha-helicity. Thermal hysteresis data showed that substitution of Thr by Ser resulted in moderate to complete loss of antifreeze activity, depending on the number and position of the substituted Thr residue(s). Replacement by Val, in confirmation of other recent reports, or by allo-Thr had a much less detrimental effect on activity though there were qualitative differences in activity between the mutants and the wild type AFP. Based on these results, we propose that both the methyl and hydroxyl groups of Thr, particularly of the central two Thr residues, Thr13 and Thr24, play key roles in the ice-binding properties of the antifreeze peptide. Specifically, the methyls participate in hydrophobic interactions with ice, which provide the driving force for binding and stability, whereas the hydroxyls and other polar residues control binding specificity and impart additional stability through hydrogen bonding.

Skin-type antifreeze protein from the shorthorn sculpin, Myoxocephalus scorpius. Expression and characterization of a Mr 9, 700 recombinant protein

A cDNA clone encoding a presumptive antifreeze protein was isolated from a skin library from shorthorn sculpin, Myoxocephalus scorpius. The clone encodes a 92-residue mature polypeptide (sssAFP-2) without any signal and prosequence, which suggests an intracellular localization. It is the largest alanine-rich, alpha-helical type I antifreeze protein known. A recombinant fusion protein containing an N-terminal-linked His-tag was produced and purified from Escherichia coli. This protein is alpha-helical at 0 degreesC and exhibits significant antifreeze activity. Northern blot and reverse transcription-polymerase chain reaction analyses indicate that sssAFP-2 mRNA has limited tissue distribution and is present in peripheral tissues such as skin and dorsal fin, but is notably absent in the liver. These studies reinforce recent evidence that indicate that the external tissues of cold water marine fishes are major organs for antifreeze protein synthesis and are likely the first line of defense against the threat of freezing.

Crystallization of ice in aqueous solutions of glycerol and dimethyl sulfoxide 2: ice crystal growth kinetics

The crystallization of ice in aqueous solutions of glycerol and dimethyl sulfoxide (Me2SO) has been studied using a combined DSC-video microscope technique. The solutions investigated were 50w/w% glycerol and 45w/w% Me2SO; both of these solutions have a solute concentration of approximately 16 mol%. The rates of growth of the external surfaces of ice crystals from both of these solutions were determined over broad temperature ranges. The growth rates were found to be generally independent of time, particularly at lower temperatures. The ice crystal growth rate in the glycerol solution became negligible at a significantly higher temperature than in the Me2SO solution. Addition of anti-freeze protein from the winter flounder at concentrations of 1.7 and 9.9 mg g-1 was found to have no significant effect on the ice crystal growth rates in 50w/w% glycerol solutions.

Vitrification of mature mouse oocytes in a 6 M Me2SO solution supplemented with antifreeze glycoproteins: the effect of temperature

Oocytes have been successfully cryopreserved using rapid and slow freezing procedures. However, variability in the success of replicates has limited its practical application. In the present study, mature mouse oocytes were vitrified in 6 M dimethyl sulfoxide supplemented with 1 mg/ml antifreeze glycoproteins (AFGP) (solution known as VSD + AFGP) from the blood of Antarctic notothenioid fish. Such AFGPs have been used to protect mammalian cells during hypothermia and cryopreservation. However, the degree of protection afforded is a contentious issue. Stepwise addition of cryoprotectant was performed either at room temperature (19-21 degreesC) or on ice (2-4 degreesC), at the final stage of which oocytes were pipetted into 0.25 ml plastic insemination straws and held in liquid nitrogen vapor at -140 degreesC for 3 min before being plunged into liquid nitrogen. Thawing involved holding the straw in the air for 10 s and then in water at 20 degreesC for 10 s before dilution of the VSD solution with 1 M sucrose. Viability was assessed by in vitro fertilization; results have been quoted as median (range). Statistical analyses were performed using Kruskall-Wallis and Mann-Whitney U tests (P < 0.05). Of the oocytes cryopreserved following exposure to VSD + AFGP at room temperature (n = 518, 15 experimental runs), 78% (0-94%) retained normal morphology and, of these, 53% (0-100%) cleaved to two cells. Of these two-cell embryos, 56% (0-100%) went on to develop to blastocyst. The overall percentage development to blastocyst, i.e., number of blastocysts/total number of oocytes treated x 100, was 20% (0-76%). Exposure of oocytes to the VSD + AFGP on ice prior to cryopreservation yielded significantly improved rates of fertilization (94%, 82-100%) and overall development to blastocyst (66%, 24-89%) when compared with oocytes cryopreserved following exposure to the VSD + AFGP at room temperature. Rates of normality (86%, 35-95%) and development to blastocyst (89%, 64-100%) were also improved. Cryopreservation in 6 M dimethyl sulfoxide supplemented with 1 mg/ml AFGP resulted in poor rates of survival which were highly variable when exposure to cryoprotective agent (CPA) was performed at room temperature. Lowering the temperature of exposure to CPA prior to cryopreservation resulted in improved viability.

Valine substituted winter flounder 'antifreeze': preservation of ice growth hysteresis

Three mutant polypeptides of the type I 37-residue winter flounder 'antifreeze' protein have been synthesized. All four threonine residues in the native peptide were been mutated to serine, valine and glycine respectively and two additional salt bridges were incorporated into the sequences in order to improve aqueous solubility. The peptides were analyzed by nanoliter osmometry, the 'ice hemisphere' test, the 'crystal habit' test, measurement of ice growth hysteresis and CD spectroscopy. While the valine and serine mutants retain the alpha-helical structure, only the valine mutant retains 'antifreeze' activity similar to that of the native protein. These data show that the threonine hydroxyl groups do not play a crucial role in the accumulation of the native 'antifreeze' protein at the ice/water interface and the inhibition of ice growth below the equilibrium melting temperature.

Evolution of antifreeze glycoprotein gene from a trypsinogen gene in Antarctic notothenioid fish

Freezing avoidance conferred by different types of antifreeze proteins in various polar and subpolar fishes represents a remarkable example of cold adaptation, but how these unique proteins arose is unknown. We have found that the antifreeze glycoproteins (AFGPs) of the predominant Antarctic fish taxon, the notothenioids, evolved from a pancreatic trypsinogen. We have determined the likely evolutionary process by which this occurred through characterization and analyses of notothenioid AFGP and trypsinogen genes. The primordial AFGP gene apparently arose through recruitment of the 5' and 3' ends of an ancestral trypsinogen gene, which provided the secretory signal and the 3' untranslated region, respectively, plus de novo amplification of a 9-nt Thr-Ala-Ala coding element from the trypsinogen progenitor to create a new protein coding region for the repetitive tripeptide backbone of the antifreeze protein. The small sequence divergence (4-7%) between notothenioid AFGP and trypsinogen genes indicates that the transformation of the proteinase gene into the novel ice-binding protein gene occurred quite recently, about 5-14 million years ago (mya), which is highly consistent with the estimated times of the freezing of the Antarctic Ocean at 10-14 mya, and of the main phyletic divergence of the AFGP-bearing notothenioid families at 7-15 mya. The notothenioid trypsinogen to AFGP conversion is the first clear example of how an old protein gene spawned a new gene for an entirely new protein with a new function. It also represents a rare instance in which protein evolution, organismal adaptation, and environmental conditions can be linked directly.

Refined solution structure of type III antifreeze protein: hydrophobic groups may be involved in the energetics of the protein-ice interaction

BACKGROUND

Antifreeze proteins are found in certain fish inhabiting polar sea water. These proteins depress the freezing points of blood and body fluids below that of the surrounding sea water by binding to and inhibiting the growth of seed ice crystals. The proteins are believed to bind irreversibly to growing ice crystals in such a way as to change the curvature of the ice-water interface, leading to freezing point depression, but the mechanism of high-affinity ice binding is not yet fully understood.

RESULTS

The solution structure of the type III antifreeze protein was determined by multidimensional NMR spectroscopy. Twenty-two structures converged and display a root mean square difference from the mean of 0.26 A for backbone atoms and 0.62 A for all non-hydrogen atoms. The protein exhibits a compact fold with a relatively large hydrophobic core, several short and irregular beta sheets and one helical turn. The ice-binding site, which encompasses parts of the C-terminal sheet and a loop, is planar and relatively nonpolar. The site is further characterized by the low solvent accessibilities and the specific spatial arrangement of the polar side-chain atoms of the putative ice-binding residues Gln9, Asn14, Thr15, Thr18 and Gln44.

CONCLUSIONS

In agreement with the adsorption-inhibition mechanism of action, interatomic distances between active polar protein residues match the spacing of water molecules in the prism planes (¿10&1macr;0¿) of the hexagonal ice crystal. The particular side-chain conformations, however, limit the number and strength of possible proten-ice hydrogen bonds. This suggests that other entropic and enthalpic contributions, such as those arising from hydrophobic groups, could play a role in the high-affinity protein-ice adsorption.

Crystallization and preliminary X-ray crystallographic studies on Type III antifreeze protein

Type III antifreeze protein, more specifically the recombinant QAE-Sephadex-binding isoform, has been crystallized in 50-55% saturated ammonium sulfate, 0.1 M sodium acetate, pH 4.0-4.5. The resultant crystals belong to the orthorhombic space group P212121 with a = 32.60 A, b = 39.00 A, and c = 46.57 A and diffract to at least 1.7 A. A set of 1.7-A native data has been collected, with completeness 93.4% and Rsym of 0.069. Initial screening for heavy-atom derivatives has yielded a Pt-bound derivative.

Comparative modeling of the three-dimensional structure of type II antifreeze protein

Type II antifreeze proteins (AFP), which inhibit the growth of seed ice crystals in the blood of certain fishes (sea raven, herring, and smelt), are the largest known fish AFPs and the only class for which detailed structural information is not yet available. However, a sequence homology has been recognized between these proteins and the carbohydrate recognition domain of C-type lectins. The structure of this domain from rat mannose-binding protein (MBP-A) has been solved by X-ray crystallography (Weis WI, Drickamer K, Hendrickson WA, 1992, Nature 360:127-134) and provided the coordinates for constructing the three-dimensional model of the 129-amino acid Type II AFP from sea raven, to which it shows 19% sequence identity. Multiple sequence alignments between Type II AFPs, pancreatic stone protein, MBP-A, and as many as 50 carbohydrate-recognition domain sequences from various lectins were performed to determine reliably aligned sequence regions. Successive molecular dynamics and energy minimization calculations were used to relax bond lengths and angles and to identify flexible regions. The derived structure contains two alpha-helices, two beta-sheets, and a high proportion of amino acids in loops and turns. The model is in good agreement with preliminary NMR spectroscopic analyses. It explains the observed differences in calcium binding between sea raven Type II AFP and MBP-A. Furthermore, the model proposes the formation of five disulfide bridges between Cys 7 and Cys 18, Cys 35 and Cys 125, Cys 69 and Cys 100, Cys 89 and Cys 111, and Cys 101 and Cys 117.(ABSTRACT TRUNCATED AT 250 WORDS)

Structure-function relationship in the globular type III antifreeze protein: identification of a cluster of surface residues required for binding to ice

Antifreeze proteins (AFPs) depress the freezing point of aqueous solutions by binding to and inhibiting the growth of ice. Whereas the ice-binding surface of some fish AFPs is suggested by their linear, repetitive, hydrogen bonding motifs, the 66-amino-acid-long Type III AFP has a compact, globular fold without any obvious periodicity. In the structure, 9 beta-strands are paired to form 2 triple-stranded antiparallel sheets and 1 double-stranded antiparallel sheet, with the 2 triple sheets arranged as an orthogonal beta-sandwich (Sönnichsen FD, Sykes BD, Chao H, Davies PL, 1993, Science 259:1154-1157). Based on its structure and an alignment of Type III AFP isoform sequences, a cluster of conserved, polar, surface-accessible amino acids (N14, T18, Q44, and N46) was noted on and around the triple-stranded sheet near the C-terminus. At 3 of these sites, mutations that switched amide and hydroxyl groups caused a large decrease in antifreeze activity, but amide to carboxylic acid changes produced AFPs that were fully active at pH 3 and pH 6. This is consistent with the observation that Type III AFP is optimally active from pH 2 to pH 11. At a concentration of 1 mg/mL, Q44T, N14S, and T18N had 50%, 25%, and 10% of the activity of wild-type antifreeze, respectively. The effects of the mutations were cumulative, such that the double mutant N14S/Q44T had 10% of the wild-type activity and the triple mutant N14S/T18N/Q44T had no activity. All mutants with reduced activity were shown to be correctly folded by NMR spectroscopy. Moreover, a complete characterization of the triple mutant by 2-dimensional NMR spectroscopy indicated that the individual and combined mutations did not significantly alter the structure of these proteins. These results suggest that the C-terminal beta-sheet of Type III AFP is primarily responsible for antifreeze activity, and they identify N14, T18, and Q44 as key residues for the AFP-ice interaction.

Cryogenic protection of oocytes with antifreeze proteins

Proteins belonging to a family of compounds known as "antifreeze proteins" interact with oocytes and protect the oolemma from damage at cryogenic temperatures. Experiments were performed with pig oocytes rapidly cooled to cryogenic temperatures in vitrifying solutions with and without antifreeze proteins. Four different types of antifreeze polypeptides and glycoproteins were tested. The integrity of the oolemma was examined with Fluorescein Diacetate (FDA) staining and morphological examinations. Results show that the pig oocyte oolemma is a primary site of injury during exposure to low temperatures and that all the different proteins have a similar ability to interact with and protect the oolemma. Our results may be important in developing solutions for long-term preservation of oocytes at cryogenic temperatures (cryopreservation).

Use of proline mutants to help solve the NMR solution structure of type III antifreeze protein

To help understand the structure/function relationships in antifreeze proteins (AFP), and to define the motifs required for ice binding, a Type III AFP suitable for two-dimensional (2D) NMR studies was produced in Escherichia coli. A synthetic gene for one of the Type III AFP isoforms was assembled in a T7 polymerase-directed expression vector. The 67-amino acid-long gene product differed from the natural AFP by inclusion of an N-terminal methionine but was indistinguishable in activity. The NMR spectra of this AFP were complicated by cis-trans proline isomerization from the C-terminal sequence YPPA. Substitution of this sequence by YAA eliminated isomer signals without altering the activity or structure of the mutant AFP. This variant (rQAE m1.1) was selected for sequential assignment and the secondary structure determination using 2D 1H NMR spectroscopy. Nine beta-strands are paired to form two triple-stranded antiparallel sheets and one double-stranded antiparallel sheet. Two further proline replacements, P29A and P33A, were made to delineate the role of conserved prolines in Type III AFP. These mutants were valuable in clarifying ambiguous NMR spectral assignments amongst the remaining six prolines of rQAE m1.1. In contrast to the replacement of the C-terminal prolyl residues, the exchange of P29 and P33 caused some structural changes and significantly decreased protein solubility and antifreeze activity.

The cryoprotective effect of antifreeze glycopeptides from antarctic fishes

Apparently vitrified cells and tissues often fail to survive, probably from damage from growth of microscopically invisible ice crystals. Special biological antifreezes from some polar fishes have been shown to adsorb to specific faces of ice crystals and inhibit crystal growth. Vitrification in the presence of antifreezes therefore may help enhance postvitrification viability of cells and tissues. We report here that the addition of fish antifreeze glycopeptides (AFGPs) to vitrifying solutions increases post-thaw viability in cultured immature pig oocytes and two-cell stage embryos of mice and pigs after rapid cooling to cryogenic temperatures. The criterion for viability is maturation to metaphase for the oocytes and the ability to develop into the four-cell stage for the pig embryo and the blastocyst stage for the mouse embryo. Without AFGPs, or with addition of antifreeze peptides (AFPs), the particular vitrifying solution and cooling/warming/culturing regime used in this study produced zero viability. In the presence of the AFGPs (40 mg/ml), survival of pig oocytes and embryos was increased to about 25%, and that of mouse embryos to 82%. Dose-response studies for the mouse embryos showed that the protective effect of AFGPs shows saturation kinetics and levels off at 20 mg/ml. The AFGPs appeared to preserve cell membrane structural integrity; however, an intact cell membrane did not always lead to viability. The absence of protective effect by AFPs suggests that protection by the AFGPs is unrelated to their common antifreeze property, i.e., inhibition of ice crystal growth, but probably results from interaction with and stabilization of the cell membranes unique to the AFGPs.

Energy-optimized structure of antifreeze protein and its binding mechanism

A combination of Monte Carlo simulated annealing and energy minimization was utilized to determine the conformation of the antifreeze protein from the fish winter flounder. It was found from the energy-optimized structure that the hydroxyl groups of its four threonine residues, i.e. Thr2, Thr13, Thr24, Thr35, are aligned on almost the same line parallel to the helix axis and separated successively by 16.1, 16.0 and 16.2 A, respectively, very close to the 16.6 A repeat spacing along [0112] in ice. Based on such a space match, a zipper-like model is proposed to elucidate the binding mechanism of the antifreeze protein to ice crystals. According to the current model, the antifreeze protein may bind to an ice nucleation structure in a zipper-like fashion through hydrogen bonding of the hydroxyl groups of these four Thr residues to the oxygen atoms along the [0112] direction in ice lattice, subsequently stopping or retarding the growth of ice pyramidal planes so as to depress the freeze point. The calculated results and the binding mechanism thus derived accord with recent experimental observations. The mechanistic implications derived from such a special antifreeze molecule might be generally applied to elucidate the structure-function relationship of other antifreeze proteins with the following two common features: (1) recurrence of a Thr residue (or any other polar amino acid residue whose side-chain can form a hydrogen bond with water) in an 11-amino-acid period along the sequence concerned; and (2) a high percentage of Ala residue component therein. Further experiments are suggested to test the ice binding model.

Calorimetric analysis of antifreeze glycoproteins of the polar fish, Dissostichus mawsoni

Solutions of antifreeze glycoproteins 1 through 5 and 8 were analyzed for activity by differential scanning calorimetry. With a scan rate of 1 degree C min-1, antifreeze glycoproteins 1-5 (20 mg/ml) revealed antifreeze activity with a delay in the freeze exotherm during cooling in the presence of ice. Antifreeze glycoprotein 8 (60 mg/ml), however, did not reveal antifreeze activity. When a 0.1 degree C min-1 scan rate was used, glycoproteins 1-5 again yielded a delay in the freeze onset, but the exotherm consisted of multiple events. At the slower scan glycoprotein 8 revealed an initial freeze followed by multiple exothermic events resembling those of glycoproteins 1-5. Thermograms exhibiting antifreeze activity had an initial shoulder in the exotherm direction upon cooling followed by a delay before the exotherm. The shoulders were correlated with c-axis ice growth observed in visual methods. The glycoprotein antifreezes had a linear increase in activity with decreased ice content.