A Second Look at Mini-Protein Stability: Analysis of FSD-1 Using Circular Dichroism, Differential Scanning Calorimetry, and Simulations

Mini-proteins that contain <50 amino acids often serve as model systems for studying protein folding because their small size makes long timescale simulations possible. However, not all mini-proteins are created equal. The stability and structure of FSD-1, a 28-residue mini-protein that adopted the ββα zinc-finger motif independent of zinc binding, was investigated using circular dichroism, differential scanning calorimetry, and replica-exchange molecular dynamics. The broad melting transition of FSD-1, similar to that of a helix-to-coil transition, was observed by using circular dichroism, differential scanning calorimetry, and replica-exchange molecular dynamics. The N-terminal β-hairpin was found to be flexible. The FSD-1 apparent melting temperature of 41°C may be a reflection of the melting of its α-helical segment instead of the entire protein. Thus, despite its attractiveness due to small size and purposefully designed helix, sheet, and turn structures, the status of FSD-1 as a model system for studying protein folding should be reconsidered.     

Postdispersal Seed Fate of Tropical Montane Trees in an Agricultural Landscape, Southern Costa Rica

Variation in postdispersal seed fate is an important factor driving patterns of forest regeneration. Because most previous studies have not tracked final seed fate and have commonly equated seed removal with predation without considering the possibility of secondary dispersal, little is known about individual seed mortality factors in successional and degraded habitats. This study tracked the postdispersal fate of large-seeded tropical montane trees in abandoned pasture, young secondary forests, and small forest fragments during two consecutive years in an agricultural landscape in southern Costa Rica. The incidence of secondary dispersal by animals, scatterhoarding in particular, and the effects of seed burial on germination were measured. Overall, seeds survived through germination more often in secondary forests with high levels of mortality occurring in abandoned pastures and forest fragments. The majority of seed mortality was caused by rodent predation in forest fragments, insects and fungal pathogens in secondary forests, and a combination of desiccation, insects, and fungal pathogens in pastures. Seeds were frequently secondarily dispersed in larger forest fragments, whereas they were only rarely moved in pastures and secondary forests. Burial tended to improve germination in pastures and was important for an often scatterhoarded species, Otoba novogranatensis, in all habitats. The results of this study suggest that: (1) seed mortality factors differ in response to the type of habitat degradation; (2) large-seeded species have high potential for survival when dispersed to young secondary forests; and (3) seed removal is not a reliable proxy for seed predation, particularly in less degraded forest fragments.     

No neural evidence for dynamic auditory tuning of the A1 receptor in the ear of the noctuid moth, <Emphasis Type="Italic">Noctua pronuba</Emphasis>

By examining the mechanical properties of the tympanum of the noctuid moth, Noctua pronuba, Windmill et al. (2006) suggested that this insect increases (up-tunes) the frequencies of its best hearing when exposed to high intensity sounds (HIS) resembling the echolocation calls of attacking bats. We tested whether this biophysical phenomenon was encoded in the neural responses of this moth’s most sensitive auditory receptor (A1 cell) before and after exposure to HIS. We measured: (1) the number of A1 action potentials (spikes) per stimulus pulse; (2) the proportion of A1 spike periods below that determined to elicit evasive flight maneuvers and, (3) the change in A1 cell firing (spike number, interspike interval, stimulus/spike latency) over a duration of time similar to that in which up-tuning lasts. We observed no significant spiking response changes in the predicted direction to any of the frequencies tested following exposure to HIS and we observed only two of the 24 predicted time-dependent changes to A1 firing. These results indicate that tympanal up-tuning does not result in a change to this moth’s auditory frequency sensitivity and we suggest either sensillar resonances or increases in thoracic muscle tension following exposure to HIS as alternative explanations.     

Nutritional Ecology of <Emphasis Type="Italic">Ateles chamek</Emphasis> in lowland Bolivia: How Macronutrient Balancing Influences Food Choices

All free-living animals must make choices regarding which foods to eat, with the choices influencing their health and fitness. An important goal in nutritional ecology is therefore to understand what governs animals’ diet selection. Despite large variation in the availability of different food items, Peruvian spider monkeys (Ateles chamek) maintain a relatively stable daily protein intake, but allow total energy intake to vary as a function of the composition of available food items. This is referred to as protein-dominated macronutrient balancing. Here we assess the influence of this nutritional strategy on daily and seasonal nutritional intakes, estimate the nutritional value of different foods, and interpret unusual food choices. We conducted continuous all-day observations of focal spider monkeys inhabiting a semideciduous forest in Bolivia. We recorded feeding events, collected foods, and analyzed their nutrient content. By using the Geometric Framework for nutrition, we show that individuals reached their daily end-point in nutrient space —balance between protein and nonprotein energy intake— by consuming nutritionally balanced foods or by alternating between nutritionally complementary foods. The macronutritionally balanced figs of Ficus boliviana were their primary staple food and therefore dominated their overall nutritional intake. Our results also demonstrate that spider monkeys consumed a diverse array of ripe fruits to overcome periods of fig scarcity rather than vice versa; they could obtain sufficient protein on a diet of pure fruit; and unripe figs constituted a nutritionally rewarding and reliable food resource. We hope that the approaches taken and the conclusions reached in this study will catalyze further inquiries into the nutritional ecology of frugivorous primates.     

Lipid body formation plays a central role in cell fate determination during developmental differentiation of Myxococcus xanthus

Cell differentiation is widespread during the development of multicellular organisms, but rarely observed in prokaryotes. One example of prokaryotic differentiation is the Gram-negative bacterium Myxococcus xanthus . In response to starvation, this gliding bacterium initiates a complex developmental programme that results in the formation of spore-filled fruiting bodies. How the cells metabolically support the necessary complex cellular differentiation from rod-shaped vegetative cells into spherical spores is unknown. Here, we present evidence that intracellular lipid bodies provide the necessary metabolic fuel for the development of spores. Formed at the onset of starvation, these lipid bodies gradually disappear until they are completely used up by the time the cells have become mature spores. Moreover, it appears that lipid body formation in M. xanthus is an important initial step indicating cell fate during differentiation. Upon starvation, two subpopulations of cells occur: cells that form lipid bodies invariably develop into spores, while cells that do not form lipid bodies end up becoming peripheral rods, which are cells that lack signs of morphological differentiation and stay in a vegetative-like state. These data indicate that lipid bodies not only fuel cellular differentiation but that their formation represents the first known morphological sign indicating cell fate during differentiation.     

Demography and population viability of polar bears in the Gulf of Boothia, Nunavut

We estimated demographic parameters and harvest risks for polar bears ( Ursus maritimus ) inhabiting the Gulf of Boothia, Nunavut, from 1976 to 2000. We computed survival and abundance from capture–recapture and recovery data (630 marks) using a Burnham joint live–dead model implemented in program MARK. Annual mean total survival (including harvest) was 0.889 ± 0.179 (± 1 SE) for cubs, 0.883 ± 0.087 for subadults (ages 1–4), 0.919 ± 0.044 for adult females, and 0.917 ± 0.041 for adult males. Abundance in the last 3 yr of study was 1,592 ± 361 bears. Mean size of newborn litters was 1.648 ± 0.098 cubs. By age 7, 0.97 ± 0.30 of available females were producing litters. Harvest averaged 38.4 ± 4.2 bears/year in the last 5 yr of study; however, the 2002–2007 kill averaged 56.4 bears/yr. We used a harvested Population Viability Analysis (PVA) to examine impacts of increasing rates of harvest. We estimated the current population growth rate, λ _H , to be 1.025 ± 0.032. Although this suggests the population is growing, progressive environmental changes may require more frequent population inventory studies to maintain the same levels of harvest risk.     

Elevated endogenous testosterone concentrations potentiate muscle androgen receptor responses to resistance exercise

The purpose of this study was to determine the influence of endogenous circulating testosterone (T) on muscle androgen receptor (AR) responses to acute resistance exercise (RE). Six healthy men (26 ± 4 years; 176 ± 5 cm; 75.8 ± 11.4 kg) performed a knee extension exercise protocol on two occasions separated by 1–3 weeks. Rest preceded one trial (i.e., control [CON] trial) and a high-volume upper-body RE protocol designed to increase circulating T preceded the other trial (i.e., high T [HT] trial). Serial blood samples were obtained throughout each trial to determine circulating T concentrations. Biopsies of the vastus lateralis were obtained pre-RE (REST), 10-min post-RE (+10), and 180-min post-RE (+180) to determine muscle AR content. Circulating T concentrations remained stable during CON. Alternately, HT significantly (p ≤ 0.05) increased T concentrations above resting values (+16%). Testosterone area-under-the-time curve during HT exceeded CON by 14%. AR content remained stable from REST to +10 in both trials. Compared to the corresponding +10 value, muscle AR content at +180 tended to decrease during CON (−33%; p = 0.10) but remained stable during HT (+40%; p = 0.17). Muscle AR content at +180 during the HT trial exceeded the corresponding CON value. In conclusion, acute elevations in circulating T potentiated muscle AR content following RE.