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31.
Apparent Inhibition of Chloroplast Protein Import by Cold Temperatures Is Due to Energetic Considerations Not Membrane Fluidity 总被引:10,自引:2,他引:8
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The transport of proteins across virtually all types of biological membranes has been reported to be inhibited by low temperatures. Paradoxically, plants are able to acclimate to growth at temperatures below which protein import into chloroplasts is known to be blocked. In examining this incongruity, we made a number of unexpected observations. First, chloroplasts isolated from plants grown at 7/1[deg]C in light/dark and from plants grown at 25[deg]C were able to import proteins with the same efficiency over a temperature range from 5 to 21[deg]C, indicating that no functional adaptation had taken place in the protein import machinery of chloroplasts in these cold-grown plants. Second, chloroplasts from warm-grown plants were able to take up proteins at temperatures as low as 4[deg]C provided that they were illuminated. We determined that light mediates the import process at 5[deg]C by driving ATP synthesis in the stroma, the site of its utilization during protein transport. Direct measurement of the envelope phase transition temperature as well as the activity of the ATP/ADP translocator in the inner envelope membrane at 5 and 25[deg]C demonstrated that the cold block of protein import into chloroplasts observed in vitro is due primarily to energetic considerations and not to decreased membrane fluidity. 相似文献
32.
The origins of clot rheological behavior associated with network morphology and factor XIIIa-induced cross-linking were studied in fibrin clots. Network morphology was manipulated by varying the concentrations of fibrinogen, thrombin, and calcium ion, and cross-linking was controlled by a synthetic, active-center inhibitor of FXIIIa. Quantitative measurements of network features (fiber lengths, fiber diameters, and fiber and branching densities) were made by analyzing computerized three-dimensional models constructed from stereo pairs of scanning electron micrographs. Large fiber diameters and lengths were established only when branching was minimal, and increases in fiber length were generally associated with increases in fiber diameter. Junctions at which three fibers joined were the dominant branchpoint type. Viscoelastic properties of the clots were measured with a rheometer and were correlated with structural features of the networks. At constant fibrinogen but varying thrombin and calcium concentrations, maximal rigidities were established in samples (both cross-linked and noncross-linked) which displayed a balance between large fiber sizes and great branching. Clot rigidity was also enhanced by increasing fiber and branchpoint densities at greater fibrinogen concentrations. Network morphology is only minimally altered by the FXIIIa-catalyzed cross-linking reaction, which seems to augment clot rigidity most likely by the stiffening of existing fibers. 相似文献
33.
Identification and characterization of a prevacuolar compartment in stigmas of nicotiana alata 总被引:3,自引:1,他引:2
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The stigmas of the ornamental tobacco plant Nicotiana alata accumulate large quantities of a series of 6-kD proteinase inhibitors (PIs) in the central vacuole that are derived from a 40-kD precursor protein, Na-PI. The sorting information that directs Na-PI to the vacuole is likely to reside in a C-terminal propeptide domain of 25 amino acids that forms an amphipathic alpha helix. Using cell fractionation techniques, we have examined transit of Na-PI through the endomembrane system and have identified a prevacuolar compartment that contains Na-PI with an intact targeting signal. In contrast, the targeting signal is not present on the predominant form of Na-PI in the vacuole. The prevacuolar compartment is marked by the presence of homologs of both the t-SNARE, PEP12p, and the putative vacuolar sorting receptor BP-80. Cross-linking and affinity precipitation studies revealed that Na-PI associates with BP-80 within this compartment, providing in vivo evidence for the function of BP-80 as a sorting receptor for a protein with a C-terminal vacuolar targeting signal. 相似文献
34.
35.
Katarina EA Nostell Birgitta Essén-Gustavsson Johan T Bröjer 《Acta veterinaria Scandinavica》2012,54(1):7
Background
The branched chain amino acid leucine is a potent stimulator of insulin secretion. Used in combination with glucose it can increase the insulin response and the post exercise re-synthesis of glycogen in man. Decreased plasma amino acid concentrations have been reported after intravenous or per oral administration of leucine in man as well as after a single per oral dose in horses. In man, a negative correlation between the insulin response and the concentrations of isoleucine, valine and methionine have been shown but results from horses are lacking. This study aims to determine the effect of repeated per oral administration with a mixture of glucose and leucine on the free amino acid profile and the insulin response in horses after glycogen-depleting exercise. 相似文献36.
37.
Gorton HL Williams WE Binns ME Gemmell CN Leheny EA Shepherd AC 《Plant physiology》1989,90(4):1329-1334
Circadian rhythms in stomatal aperture and in stomatal conductance have been observed previously. Here we investigate circadian rhythms in apertures that persist in functionally isolated guard cells in epidermal peels of Vicia faba, and we compare these rhythms with rhythms in stomatal conductance in attached leaves. Functionally isolated guard cells kept in constant light display a rhythmic change in aperture superimposed on a continuous opening trend. The rhythm free-runs with a period of about 22 hours and is temperature compensated between 20 and 30°C. Functionally isolated guard cell pairs are therefore capable of sustaining a true circadian rhythm without interaction with mesophyll cells. Stomatal conductance in whole leaves displays a more robust rhythm, also temperature-compensated, and with a period similar to that observed for the rhythm in stomatal aperture in epidermal peels. When analyzed individually, some stomata in epidermal peels showed a robust rhythm for several days while others showed little rhythmicity or damped out rapidly. Rhythmic periods may vary between individual stomata, and this may lead to desynchronization within the population. 相似文献
38.
A model of the carbohydrate recognition domain CRD, residues 111-245, of
hamster galectin-3 has been made using homology modeling and dynamics
minimization methods. The model is based on the known x-ray structures of
bovine galectin-1 and human galectin-2. The oligosaccharides
NeuNAc-alpha2,3-Gal-beta1,4-Glc and GalNAc-alpha1, 3-
[Fuc-alpha1,2]-Gal-beta1,4-Glc, known to be specific high-affinity ligands
for galectin-3, as well as lactose recognized by all galectins were docked
in the galectin-3 CRD model structure and a minimized binding conformation
found in each case. These studies indicate a putative extended
carbohydrate-binding subsite in the hamster galectin- 3 involving Arg139,
Glu230, and Ser232 for NeuNAc-alpha2,3-; Arg139 and Glu160 for
fucose-alpha1,2-; and Arg139 and Ile141 for GalNAc-alpha1,3- substituents
on the primary galactose. Each of these positions is variable within the
whole galectin family. Two of these residues, Arg139 and Ser232, were
selected for mutagenesis to probe their importance in this newly identified
putative subsite. Residue 139 adopts main-chain dihedral angles
characteristic of an isolated bridge structural feature, while residue 232
is the C-terminal residue of beta- strand-11, and is followed immediately
by an inverse gamma-turn. A systematic series of mutant proteins have been
prepared to represent the residue variation present in the aligned
sequences of galectins-1, - 2, and -3. Minimized docked models were
generated for each mutant in complex with NeuNAc-alpha2,3-Gal-beta1,4-Glc,
GalNAc-alpha1, 3-[Fuc- alpha1,2]-Gal-beta1,4- Glc, and Gal-beta1,4-Glc.
Correlation of the computed protein-carbohydrate interaction energies for
each lectin- oligosaccharide pair with the experimentally determined
binding affinities for fetuin and asialofetuin or the relative potencies of
lactose and sialyllactose in inhibiting binding to asiolofetuin is
consistent with the postulated key importance of Arg139 in recognition of
the extended sialylated ligand.
相似文献
39.
Nucleotide sequence analysis of the human salivary protein genes HIS1 and HIS2, and evolution of the STATH/HIS gene family 总被引:1,自引:0,他引:1
Human histatins are a family of low-M(r), neutral to very basic,
histidine-rich salivary polypeptides. They probably function as part of the
nonimmune host defense system in the oral cavity. A 39-kb region of DNA
containing the HIS1 and HIS2 genes was isolated from two human genomic
phage libraries as a series of overlapping clones. The nucleotide sequences
of the HIS1 gene and part of the HIS2(1) gene were determined. The
transcribed region of HIS1 spans 8.5 kb and contains six exons and five
introns. The HIS1 and HIS2(1) genes exhibit 89% overall sequence identity,
with exon sequences exhibiting 95% identity. The two loci probably arose by
a gene duplication event approximately 15-30 Mya. The HIS1 sequence data
were also compared with that of STATH. Human statherin is a low-M(r) acidic
phosphoprotein that acts as an inhibitor of precipitation of calcium
phosphate salts in the oral cavity. The HIS1 and STATH genes show nearly
identical overall gene structures. The HIS1 and STATH loci exhibit 77%-81%
sequence identity in intron DNA and 80%-88% sequence identity in noncoding
exons but only 38%-43% sequence identity in the protein-coding regions of
exons 4 and 5. These unusual data suggest that HIS1, HIS2, and STATH belong
to a single gene family exhibiting accelerated evolution between the HIS
and STATH coding sequences.
相似文献
40.
Evolution of the Adh locus in the Drosophila willistoni group: the loss of an intron, and shift in codon usage 总被引:1,自引:0,他引:1
We report here the DNA sequence of the alcohol dehydrogenase gene (Adh)
cloned from Drosophila willistoni. The three major findings are as follows:
(1) Relative to all other Adh genes known from Drosophila, D. willistoni
Adh has the last intron precisely deleted; PCR directly from total genomic
DNA indicates that the deletion exists in all members of the willistoni
group but not in any other group, including the closely related saltans
group. Otherwise the structure and predicted protein are very similar to
those of other species. (2) There is a significant shift in codon usage,
especially compared with that in D. melanogaster Adh. The most striking
shift is from C to U in the wobble position (both third and first
position). Unlike the codon-usage-bias pattern typical of highly biased
genes in D. melanogaster, including Adh, D. willistoni has nearly 50% G + C
in the third position. (3) The phylogenetic information provided by this
new sequence is in agreement with almost all other molecular and
morphological data, in placing the obscura group closer to the melanogaster
group, with the willistoni group farther distant but still clearly within
the subgenus Sophophora.
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