Low-temperature protein metabolism: seasonal changes in protein synthesis and RNA dynamics in the Antarctic limpet Nacella concinna Strebel 1908
Natural Environment Research Council, British Antarctic Survey, High Cross, Madingley Road, Cambridge, CB3 0ET, UK
* Author for correspondence (e-mail: kppf{at}pcmail.nerc-bas.ac.uk)
Accepted 9 July 2002
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Summary |
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Key words: protein synthesis, RNA:protein ratio, Antarctic limpet, Nacella concinna, temperature
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Introduction |
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Recent analyses of resting respiration rates in fish and bivalves have
demonstrated a positive non-linear relationship with temperature
(Clarke and Johnston, 1999;
Peck and Conway, 2000
).
Therefore, polar animals have low resting metabolic rates but must still
generate sufficient ATP for the major cellular energy sinks (protein
synthesis, RNA/DNA synthesis, proton leak, Na+/K+-ATPase
and Ca2+-ATPase) while balancing their metabolic budgets
(Buttgereit and Brand, 1995
).
Some cellular parameters, such as the cycling of microtubules and red muscle
mitochondrial densities, are cold adapted in polar species
(Detrich et al., 1989
;
Johnston et al., 1994
).
Interestingly, the temperate fish Gadus morhua and the isopod
Saduria entomon increase tissue RNA concentrations when acclimated to
low temperatures in the laboratory (Foster
et al., 1992
; Robertson et
al., 2001a
). If a major cellular process is cold adapted in a
polar species (i.e. the rate is maintained) then it seems likely that the
process will require a larger proportion of the metabolic budget and there
will therefore be a shift in the balance of ATP consumption between cellular
processes.
In vivo protein synthesis rates have only been measured in a few
Antarctic species e.g. the giant isopod Glyptonotus antarcticus and
the sea urchin Sterechinus neumayeri
(Whiteley et al., 1996;
Marsh et al., 2001
;
Robertson et al., 2001b
).
Earlier studies have examined protein synthesis rates in a range of fish
species (Smith and Haschemeyer,
1980
; Haschemeyer,
1983
). Evidence so far suggests that protein synthesis rates in
polar species are similar to those in temperate species. All of these studies
have concentrated on answering specific physiological questions relating to
protein synthesis, but none has examined seasonal changes in protein synthesis
related to the highly variable Antarctic environment
(Clarke, 1983
).
Several studies have examined various aspects of the seasonal physiology
and ecology of Antarctic marine invertebrates, including feeding
(Barnes and Clarke, 1994;
Brêthes et al., 1994
),
oxygen consumption and nitrogen excretion
(Brockington and Clarke, 2001
;
Brockington and Peck, 2001
),
reproduction (Picken, 1980
;
Kim, 2001
) and growth
(Barnes, 1995
;
Peck et al., 1997
). In all
cases, a strongly seasonal pattern was evident. The primary aims of the
current study were to rigorously validate the flooding dose methodology for
measurements of protein synthesis at subzero water temperatures and to answer
the following questions using the Antarctic limpet Nacella concinna
as a model species: (1) does protein synthesis vary seasonally; (2) do tissue
RNA concentrations and RNA:protein ratios change with season and do they show
temperature adaptation; and (3) does N. concinna allocate a similar
proportion of its metabolic budget to protein synthesis as do temperate and
tropical species?
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Materials and methods |
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Measurement of fractional protein synthesis rates, tissue RNA
concentrations, RNA:protein ratios and RNA translational efficiencies
Limpet fractional protein synthesis rates were measured using a
modification of the flooding dose technique
(Garlick et al., 1980;
Houlihan et al., 1986
). The
length of each animal was measured to the nearest 0.05 mm using vernier
calipers. After surface drying with paper tissues, the animal (including
shell) was weighed to the nearest mg. Each limpet was then individually
labelled with a plastic number glued to the shell using cyanoacrylate
adhesive. A preliminary sample of limpets was dissected to establish the
relationship between flesh mass, y (mass excluding shell) and total
mass, x (y=0.84x-0.14,
r2=99.2%, P<0.001, N=88). This
scaling relationship was used to estimate the flesh mass of the experimental
animals used in protein synthesis measurements and, hence, to adjust the
isotope dose. A preliminary group of limpets was injected in the pedal sinus
with a solution of Alcian blue (Sigma, Poole, UK) and dissected to examine
whether the injectate was successfully delivered to the pedal sinus. In all
cases, the injections were successful in delivering the injectate to the pedal
sinus. Each experimental animal was injected in the pedal sinus with a
solution containing unlabelled and 3H-labelled phenylalanine [10
µl g-1 flesh mass of 135 mmol l-1
L-[2,6-3H] phenylalanine at 3.6 MBq ml-1 (100 µCi
ml-1); Amersham, Little Chalfont, UK]. After injection, limpets
were placed in a beaker containing 41 of seawater, maintained at a temperature
of -1.1±0.6°C. After 1 h, 2 h and 4 h, six or seven limpets were
removed from the beaker, had their shells removed, and the flesh mass weighed
and homogenised (X120 Status homogeniser) in a known volume of ice-cold 0.2
mol l-1 perchloric acid (PCA, 2 ml per 100 mg flesh mass).
Homogenised limpets were stored at 4°C prior to analysis. A group of ten
non-injected limpets were analysed to measure baseline phenylalanine
concentrations and to allow calculation of phenylalanine flooding levels.
Fractional protein synthesis and RNA content: sample analysis
PCA samples containing individual homogenised limpets were mixed and a 2 ml
sub-sample removed for further analysis. The sub-sample was centrifuged
(Hermle Z 323 K centrifuge, 3500 g, 20 min, 4°C, fixed
rotor) to separate the precipitated protein pellet, RNA and DNA from the
intracellular free-pool (Houlihan et al.,
1995a). The amount of NaOH-soluble protein in the protein pellet
was measured (Lowry et al.,
1951
) using bovine serum albumin as the standard. Total RNA was
measured by comparing the sample concentrations to known RNA standard (Type
IV, calf liver, Sigma) concentrations determined spectrophotomically at 665 nm
after reaction with an acidified orcinol reagent
(Mejbaum, 1939
). The protein
pellet was washed twice in 0.2 mol l-1 PCA before being hydrolysed
in 6 mol l-1 HCl for 18 h. Phenylalanine concentrations in the
intracellular free-pool, hydrolysed protein pellet and injection solution were
measured using a flourometric assay, after the enzymatic conversion of the
phenylalanine to ß-phenylethylamine (PEA); these procedures are described
in detail by Houlihan et al.
(1995a
). Known phenylalanine
standards were also enzymatically converted to PEA to assess the conversion
efficiency. Specific radioactivities of the intracellular free-pools, protein
pellet and injection solution were measured using scintillation counting
(3H counting efficiency 34%, Hionic Fluor scintillation fluid,
LKB-Wallac Rack Beta scintillation counter). Intracellular free-pool, protein
and injection solution specific radioactivities were expressed as d.p.m.
nmol-1 phenylalanine. Whole-animal fractional protein synthesis
rates (expressed as a percentage of the protein mass synthesised per day) were
calculated using the following equation
(Garlick et al., 1980
):
![]() | (1) |
The absolute rates of protein synthesis (As) were
calculated using the following equation:
![]() | (2) |
Calculation of RNA concentrations, RNA:protein ratios and RNA
translational efficiencies
Tissue RNA concentrations were expressed as µg RNA mg-1 fresh
mass, and RNA:protein ratios (µg RNA mg-1 protein). The
translational efficiency of the RNA (kRNA; expressed as mg
protein mg-1 RNA day-1) was calculated using the
following equation (Preedy et al.,
1988):
![]() | (3) |
Measurement of growth rates
12months after protein synthesis rates were measured in N. concinna, in
situ growth rates were measured. During December, limpets were collected
from the same site used to collect animals for the protein synthesis
measurements and were returned to the aquarium. The animals were gently dried
with a paper tissue, and the mass of each limpet was measured to the nearest
mg. Each animal was numbered on the shell using enamel paint (Humbrol, Hull,
UK). Marked animals were returned to the site of capture within 48 h. After 64
days, as many marked limpets as possible were collected and reweighed. Wet
specific-growth rates (SGR, expressed as % body mass-1) were
calculated for each individual using the following equation
(Ricker, 1979):
![]() | (4) |
Statistical analysis
All data are expressed as means ± S.E.M. Data were tested for
normality prior to statistical testing using the AndersonDarling test
(Sokal and Rohlf, 1995). If
data were normally distributed, they were analysed using analysis of variance
(ANOVA); otherwise, the non-parametric KruskalWallis test was used.
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Results |
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The successful application of the flooding dose technique to measure fractional protein synthesis rates requires several criteria to be met, namely that the intracellular free-pool specific radioactivities increase rapidly and are stable over the course of the protein synthesis measurement, the intracellular free-pools are flooded by the injected unlabelled phenylalanine and, finally, the increase in protein radiolabelling with time is significant and linear. In our studies, the intracellular free-pool specific radioactivities increased rapidly after the flooding dose injection at all seasonal sampling points (Fig. 1). There were no significant differences in intracellular free-pool specific radioactivities over time up to 4.5 h after injection at any seasonal sampling point (KruskalWallis, all P>0.05), indicating that free-pools had increased rapidly and were stable during the time course of measurement.
|
The second criteria of the flooding dose technique is that the intracellular free-pools are completely flooded by the unlabelled phenylalanine. Injection of the limpets resulted in a 4.4-fold increase in whole body phenylalanine concentrations above the baseline (0.34 nmol mg-1 fresh mass). After the flooding dose injection of 1.35 nmol mg-1 fresh mass, phenylalanine concentrations should have increased to 1.69 nmol mg-1. At all sampling points, phenylalanine concentrations increased to 87-100% of the theoretical post-injection concentration (February = 1.48 nmol mg-1; July = 1.70 nmol mg-1, October = 1.58 nmol mg-1, December = 1.54 nmol mg-1). The close agreement between the predicted and actual phenylalanine concentrations after injection suggests that the injected phenylalanine had equilibrated within the limpet tissues.
Intracellular free-pool specific radioactivities were pooled within each sampling point and compared with the injection solution specific radioactivity. The overall mean intracellular free-pool specific radioactivity for all sampling points (1013±30 d.p.m. nmol-1 phenylalanine) was significantly lower than the injection solution specific radioactivity (1622±43 d.p.m. nmol-1 phenylalanine), owing to dilution by tissue baseline phenylalanine concentrations. If the total tissue phenylalanine concentrations were increased 4.4-fold after the flooding dose injection, then the specific radioactivity of the intracellular free-pool would be expected to be approximately 25% lower than the injection solution specific radioactivity. In fact, the mean free-pool specific radioactivity was 37% lower than the injection solution. The mean free-pool specific radioactivity was probably slightly lower than predicted due to some loss of the injection solution from the injection site after injection. The fact that both unlabelled and radiolabelled phenylalanine concentrations in the animal approximate to predicted levels after injection suggest that the intracellular free-pools have successfully flooded with phenylalanine; thus, the second criteria of the flooding dose technique is satisfied.
Regression analysis demonstrated a significant linear increase in incorporated specific radioactivity with time for all seasonal sampling points (Fig. 2). Second- and third-order regression models were also fitted to the protein radiolabelling data but these did not significantly improve the residual mean squares in any data set; the linear model was therefore used. At all seasonal sampling points, the intercepts of regression equations describing the incorporation of radiolabelled phenylalanine into protein were not significantly different from zero, suggesting that the radiolabel rapidly equilibrated with the intracellular free-pool and that incorporation of the radiolabel occurred rapidly after injection. The current work thus fulfils all the requirements of the flooding dose technique; the specific radioactivities of the intracellular free-pools increased and were stable, the intracellular free-pools were flooded with phenylalanine and, lastly, there was significant, linear incorporation of the radiolabelled amino acid into proteins. Protein synthesis rates were calculated for each animal using the animal's individual intracellular free-pool and radiolabelled protein specific radioactivity. A mean protein synthesis rate was calculated for each seasonal sampling point using the protein synthesis rate measurements for each individual animal at all three labelling time points.
|
Fractional protein synthesis rates, tissue RNA concentrations,
RNA:protein ratios and RNA translational efficiencies
Fractional protein synthesis rates during the austral summer in December
were significantly higher than during the other sampling points in February,
July and October (Table 1,
KruskalWallis, P<0.05, H=25.33, d.f.=3). The
highest rate of fractional protein synthesis (December) was twofold higher
than the lowest protein synthesis rate (July).
|
Tissue RNA concentrations also showed a similar seasonal pattern, with maximum values in summer and minimum values in winter (Table 1). Peak RNA concentrations in February were significantly higher than at all other sampling points. The RNA concentration in July was significantly lower than in October and December [analysis of variance (ANOVA), P<0.05, F=12.49, d.f.=3].
The RNA:protein ratio also decreased significantly in winter (ANOVA, P<0.05, F=40.01, d.f.=3), with winter values falling to a third of summer values (Table 1). There was also a seasonal pattern in the absolute protein synthesis rates, with the highest rates in the austral summer, although the high variance in these measurements means that the differences were not significant (KruskalWallis, P=0.147, H=5.36, d.f.=3). The seasonal pattern in RNA translational efficiencies was far less clear; the RNA translational efficiency measured in February was significantly lower (KruskalWallis, P<0.05, H=16.28, d.f.=3) than in July and December but, overall, no clear seasonal pattern was evident (Table 1). In summary, this study has revealed a clear seasonal pattern in fractional protein synthesis and RNA concentration and a less-distinct seasonality in RNA translational efficiency and absolute protein synthesis.
Growth rates
During summer, the marked limpets increased in mass from 2.07±0.15 g
to 2.30±0.16 g (N=31) with a mean SGR of 0.07±0.01%
body mass day-1. In winter, the marked animals decreased from an
initial mass of 2.42±0.01 g to a final mass of 2.35±0.01 g
(N=44), with a mean SGR of -0.01±0.01 % body mass
day-1, giving a clear seasonal difference in growth rate (Student's
t-test, P<0.001, t=5.93, d.f.=39).
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Discussion |
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Fractional protein synthesis rates
Seasonal variability in fractional protein synthesis rate
(ks) has previously been measured in only one other marine
invertebrate: the temperate bivalve Mytilus edulis. Hawkins
(1985) reported
ks in fed M. edulis of 0.29% day-1,
0.40% day-1 and 0.38% day-1 in March, June and October,
respectively. Seasonal differences in protein synthesis rates were also
evident in N. concinna in the current study. It is likely that
seasonal variation in food consumption is the main factor driving seasonal
changes in N. concinna protein synthesis. Faecal egestion data,
collected simultaneously with the current work in a separate group of limpets,
showed that food consumption in N. concinna decreased by nearly an
order of magnitude during winter, while the experimental water temperature in
which the animals were maintained only varied by approximately 2°C from
summer to winter (Fraser et al.,
2002
). N. concinna faecal egestion rates were
11.92±0.76 mg dry faeces day-1, 1.41±0.58 mg dry
faeces day-1, 7.35±0.84 mg dry faeces day-1 and
10.75±1.07 mg dry faeces day-1 in February, July, October
and December, respectively (Fraser et al.,
2002
). A long-term reduction in food consumption has previously
been shown to decrease protein synthesis rates and tissue RNA concentrations
in several species (McNurlan et al.,
1979
; Foster et al.,
1993
; Arndt et al.,
1996
). Although protein synthesis rates in N. concinna
and M. edulis were measured at water temperatures differing by a
minimum of 8°C, the protein synthesis rates were very similar
(Hawkins et al., 1983
;
Hawkins, 1985
;
Table 1).
The only other comparable protein synthesis rates in molluscs are those
measured in Octopus vulgaris at 22°C by Houlihan et al.
(1990). The authors reported
whole-body protein synthesis rates ranging from 2% day-1 to 7%
day-1, which are considerably higher than those in N.
concinna, even allowing for differences in water temperature. Cephalopods
are, however, notable for being semelparous and having rapid somatic growth,
so direct comparison is difficult (O'Dor
and Wells, 1987
). Larval cephalopod RNA:DNA ratios are
substantially higher than in juvenile fish, again suggesting protein synthesis
rates might be unusually high in cephalopods
(Clarke et al., 1989
).
Whole body ks has only been measured previously in the
adult stage of one Antarctic species, G. antarcticus, for which
Whiteley et al. (1996)
reported a ks of 0.24±0.04% day-1 in fed
animals held at 0°C, while Robertson et al.
(2001b
) reported
ks values ranging between 0.16% day-1 and 0.38%
day-1 in starved and fed animals at the same temperature. These
results are well within the range of those measured in N. concinna
and are very similar to those measured in M. edulis (Hawkins et al.,
1985). Protein synthesis rates in Antarctic and temperate sea urchin embryos
are also very similar (Marsh et al.,
2001
). It therefore appears that, despite the low water
temperature and highly seasonal food supply typical of Antarctic waters,
marine invertebrates are able to maintain fractional protein synthesis rates
that are comparable with related temperate species.
Previous workers have suggested that liver and white-muscle protein
synthesis rates in some species of Antarctic fish show a degree of temperature
adaptation (Smith and Haschemeyer,
1980; Haschemeyer,
1983
). Trematomus hansoni, Trematomus bernacchii, Trematomus
newnesi and Gymnodraco acuticeps have protein synthesis rates
approximately two times higher than would be predicted from extrapolation of
temperate fish protein synthesis rates to polar water temperatures
(Smith and Haschemeyer, 1980
).
However, caution does need to be exercised in interpreting these results, as
predicted protein synthesis rates are simply calculated using estimated
Q10 (2.5) values and do not consider ecological or scaling
factors. Laboratory-based studies have shown temperate fish increasing their
tissue RNA:protein ratios and decreasing their kRNA in
some tissues as water temperatures decrease
(Foster et al., 1992
). After
juvenile cod were acclimated to 5°C or 15°C for 40 days, there was no
significant difference in protein synthesis rates, suggesting that
intraspecific protein synthesis rates are maintained at some `optimum' level
by altering RNA:protein ratios to counter temperature-induced changes in
kRNA (Foster et al.,
1992
).
RNA concentration, RNA:protein ratios and RNA translational
efficiencies
There was a clear seasonal variation in both RNA concentrations and
RNA:protein ratios, closely following changes in ks
(Table 1). Seasonal changes in
tissue RNA concentrations and RNA:DNA ratios have been demonstrated in several
species (Bulow et al., 1981;
Robbins et al., 1990
;
Kent et al., 1992
), and food
consumption has been shown to influence tissue RNA concentration, which, in
turn, has a large effect on ks
(Millward et al., 1973
). As
food consumption rates change, initial alterations in protein synthesis rates
are controlled by modification of kRNA, while changes in
RNA concentration occur over longer time periods
(Millward et al., 1973
).
RNA:protein ratios measured in N. concinna appear high in comparison
with temperate species, while kRNA values seem low.
Animals studied in the current work had effectively undergone a natural acclimatisation to seasonal environmental conditions. Therefore, RNA:protein ratios were likely to have reached optimum levels for variables such as the rate of food consumption and water temperature. The lack of any consistent seasonal pattern in kRNA in the current work is therefore perhaps not surprising. The significant differences in kRNA that do exist probably reflect short-term nutritional variations.
Inter-specific comparison of the effect of temperature on RNA:protein
ratios and RNA translational efficiencies
Comparisons of inter-specific temperature-induced changes in RNA:protein
ratios and kRNA are complicated by mass-specific changes
in RNA:protein ratios (Goldspink and
Kelly, 1984; Houlihan et al.,
1995a
; Tesseraud et al.,
1996
). To allow valid inter-specific comparisons of RNA:protein
ratios, suitable data were compiled from the literature and standardised to
the mean body mass of all the animals used in the analysis using a scaling
coefficient of -0.24 (Fig. 3).
The scaling coefficient was calculated by least-squares regression analysis of
log-transformed body mass and RNA:protein data for the complete data set. Data
were used only if the animals in the studies were fed and experiments were
conducted at temperatures within the environmental range encountered by the
species. There was a significant inverse relationship between RNA:protein
ratios and temperature across a wide range of species
(Fig. 3). It should be noted
that the r2 value for the relationship is low, only
explaining 20% of the variation in RNA:protein ratios. Much of the unexplained
variation in the relationship is probably due to nutritional differences
between the studies (Millward et al.,
1973
). It is probable that a much stronger relationship could be
demonstrated in animals maintained at differing temperatures but under the
same nutritional regimes. The rat data of Goldspink and Kelly
(1984
) were not included
within the data set used to fit the regression line, as the rat was the only
non-ectotherm included in Fig.
3. However, the rat data do clearly lie very near to the
regression line fitted to the ectotherm data.
|
|
There was no significant relationship between body mass and
kRNA in the overall data set (P=0.187,
F=1.85, N=26), and kRNA was therefore
not standardised to a mean body mass. Whole-body kRNA did
however increase significantly with temperature
(Fig. 4), although the
r2 was comparatively low. The kRNA is
very sensitive to the nutritional state of an animal, and variations in
nutrition are likely to explain the low r2 value
(Millward et al., 1973).
|
|
The limited amount of suitable data in the literature meant that several species in these analyses (Fig. 3, 4) are represented by more than one data point. This can lead to a biased analysis, which could unduly influence conclusions through overestimation of the degrees of freedom. Both RNA:protein ratio and kRNA data sets were therefore re-analyzed after reducing each species to a single data point by calculation of a species mean. There was still a significant relationship between RNA:protein ratio and temperature (y=14.0-0.455x, r2=0.53, P<0.05, F=10.29, N=11). However, the relationship relating kRNA and temperature just failed to reach significance (r2=0.34, P=0.076, F=4.16, N=10). It is likely that a significant relationship will be obtained when data from further studies are available.
The relationship of both RNA:protein ratio and kRNA
with temperature is therefore similar in conspecifics acclimated to different
water temperatures (Foster et al.,
1992) and inter-specifically across a broad range of animal
groups. This suggests strongly that ectotherms living at low temperatures
maintain considerably elevated tissue RNA:protein ratios to counteract very
low kRNA (Figs
3,
4;
Robertson et al., 2001b
;
Whiteley et al., 1996
). The
elevation of tissue RNA:protein ratios in Antarctic species is thus a clear
evolutionary adaptation to living at low temperatures, allowing the continued
synthesis of sufficient protein at temperatures approaching the lower thermal
limits of RNA translation.
Growth rates
Growth in Antarctic ectotherms is generally considered to be variable and
reduced in comparison to temperate and tropical species with similar ecology
and size (Barnes, 1995),
although there are a few species that do demonstrate relatively high growth
rates (Dayton et al., 1974
;
Dayton, 1989
;
Rauschert, 1991
). Specific
growth rates measured in the current work suggest that overall growth rates in
N. concinna are also slow in summer but negative in winter. Previous
authors have reported low shell growth rates in N. concinna in
comparison with those in temperate and tropical limpets
(Shabica, 1976
).
Interestingly, overall proportional growth rates (expressed as % body mass
day-1) in N. concinna are considerably lower than
proportional protein synthesis rates (expressed as % protein mass synthesised
day-1), suggesting that protein growth rates are also likely to be
low (unless very large changes occur in animal protein content). In turn, this
suggests that protein degradation rates are considerable.
Estimated energetic cost of protein synthesis
Current estimates indicate that protein synthesis is energetically
expensive and accounts for 25-42% of total oxygen consumption in a wide range
of animal species (Houlihan et al.,
1995a; Houlihan et al.,
1995b
). Two recent studies have reported widely differing
energetic costs of protein synthesis in Antarctic marine invertebrates.
Whiteley et al. (1996
)
calculated that protein synthesis following a satiating meal accounted for 66%
of total post-prandial oxygen consumption in the giant isopod G.
antarcticus and estimated an energetic cost of 148 mmol
O2g-1 protein. In contrast, Marsh et al.
(2001
) reported that protein
synthesis accounted for 1-53% of oxygen consumption in developing embryos of
the Antarctic sea urchin S. neumayeri and estimated a cost of only 1
mmol O2 g-1 protein
(Marsh et al., 2001
). This
latter cost of protein synthesis in S. neumayeri embryos is lower
than the minimum theoretical stoichiometric cost of peptide elongation by a
factor of eight (Houlihan et al.,
1995b
; Marsh et al.,
2001
). It seems unlikely that this apparent difference of two
orders of magnitude in the cost of protein synthesis between these two species
can be real.
The reason for the widely differing costs of protein synthesis in these two
Antarctic species remains to be resolved. Reported costs of protein synthesis
in non-polar species also vary widely (reviewed in
Houlihan et al., 1995b). The
large variation in experimentally derived energetic costs of protein synthesis
is probably largely due to differences in methodology and insufficient
validation of techniques. For example, recent work has shown that high
concentrations of cycloheximide may affect cellular processes other than
protein synthesis, leading to an overestimate of protein synthesis costs
(Fuery et al., 1998
;
Wieser and Krumschnabel,
2001
).
The proportion of oxygen consumption used for protein synthesis in the
current work (34-40%) was estimated using the minimal stoichiometric cost of
protein synthesis (Table 4,
Houlihan et al., 1995b). This
technique provides a minimum cost of protein synthesis, based solely on the
cost of peptidebond formation. The N. concinna oxygen consumption
data used to make the calculations were from measurements made at the same
time as the protein synthesis measurements but in a separate group of animals
(Fraser et al., 2002
). The
proportion of oxygen consumption used for protein synthesis in N.
concinna is fairly consistent throughout the season and falls at the
upper extreme range of estimates made using the same theoretical cost of
protein synthesis in a range of temperate and tropical fish species (mean 26%;
range 11-42%) (Table 4;
Carter and Houlihan, 2001
).
Measurements of the proportion of oxygen consumption used for protein
synthesis by Marsh et al.
(2001
) and Whiteley et al.
(1996
) and from our calculated
value suggest that the proportion of the energetic budget dedicated to protein
synthesis in polar species might be higher than in temperate or tropical
species. Further experimental work is required to clarify the absolute costs
of protein synthesis in polar species and the proportion of the metabolic
budget allocated to the synthesis of proteins. If polar species use a higher
proportion of their metabolic budget for protein synthesis and for maintaining
considerably elevated tissue RNA concentrations, the overall costs of growth
may be considerably higher than in temperate and tropical species. In turn,
the energy remaining that is available for other processes, such as ion
transport, would be greatly reduced.
|
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Acknowledgments |
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