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981.
982.
983.
Plasma transferrin is involved in iron transport within the circulatory system of vertebrates, and provides an iron source for haemoglobin synthesis and other metabolic requirements. However, despite extensive studies by spectroscopic, biochemical and physiological techniques, the nature of iron binding and the mechanisms of uptake and release of iron are not fully understood. Plasma transferrins are monomeric glycoproteins with a molecular weight of approximately 80,000 (ref. 2); they have two similar and very strong binding sites for Fe(III), together with two associated anion binding sites. Fragmentation studies on various transferrins have shown that the polypeptide chain is composed of two domains formed from the N-terminal and C-terminal halves of the polypeptide chain. Each domain contains one metal binding site. The marked sequence similarities which exist between the two halves may reflect a doubling of an ancestral structural gene during the phylogenetic development of the protein. Preliminary crystallographic investigations of diferric rabbit plasma transferrin have been reported from this laboratory. We now report initial studies of the X-ray structure determination of dife-ric rabbit plasma transferrin which have led to a 6-A resolution electron density map. 相似文献
984.
Transgenic mice were produced by microinjection of a rearranged, functional immunoglobulin kappa gene into fertilized mouse eggs and implantation of the microinjected embryos into foster mothers. Mice that integrated the injected gene were mated and the DNA, RNA and serum kappa chains of their offspring were analysed. The data from offspring of three different transgenic mice indicate that the microinjected gene is expressed in the spleen, but not the liver of mice which inherited the injected gene. 相似文献
985.
B D Gomperts 《Nature》1983,306(5938):64-66
The introduction of impermeant aqueous solutes into individual cells by microinjection has long been established but the difficulties of manipulating the cytosol composition of large populations of microscopic cells have only recently been overcome. Successful techniques include a dielectric breakdown procedure, treatment with micromolar concentrations of ATP4- (ref. 7) and also with very small (that is nonagglutinating, non-fusogenic) amounts of Sendai virus. So far, attention has been concentrated on the behaviour of the cells (generally their response to applied Ca2+ buffers) at the time when the membrane permeability lesions are open, and thus cytosol and external medium are in contact. I now report a novel technique for monitoring the state of molecular solute permeability in cell membranes and show that the lesions generated by ATP4- in the membrane of mast cells can be closed within seconds of adding Mg2+ so that a cycle of permeabilization and resealing can be used to explore the effect of foreign compounds trapped in the cytosol of effectively intact cells. I show that non-hydrolysable GTP analogues, introduced into the cytosol of mast cells, cause them to undergo exocytotic secretion in response to addition of extracellular Ca2+. This finding is discussed in the light of previous experience relating guanine nucleotide regulatory proteins as intermediaries between receptors and the transducers which they control. 相似文献
986.
987.
Effect of vitamin A deficiency on the incorporation of C-14-leucine into tissue proteins of rats 总被引:1,自引:0,他引:1
B S Narasinga Rao 《Nature》1966,210(5033):306-307
988.
Synthesis of ATP driven by a potassium gradient in mitochondria 总被引:6,自引:0,他引:6
989.
990.
Dopamine has a central role in motivation and reward. Dopaminergic neurons in the ventral tegmental area (VTA) signal the discrepancy between expected and actual rewards (that is, reward prediction error), but how they compute such signals is unknown. We recorded the activity of VTA neurons while mice associated different odour cues with appetitive and aversive outcomes. We found three types of neuron based on responses to odours and outcomes: approximately half of the neurons (type I, 52%) showed phasic excitation after reward-predicting odours and rewards in a manner consistent with reward prediction error coding; the other half of neurons showed persistent activity during the delay between odour and outcome that was modulated positively (type II, 31%) or negatively (type III, 18%) by the value of outcomes. Whereas the activity of type I neurons was sensitive to actual outcomes (that is, when the reward was delivered as expected compared to when it was unexpectedly omitted), the activity of type II and type III neurons was determined predominantly by reward-predicting odours. We 'tagged' dopaminergic and GABAergic neurons with the light-sensitive protein channelrhodopsin-2 and identified them based on their responses to optical stimulation while recording. All identified dopaminergic neurons were of type I and all GABAergic neurons were of type II. These results show that VTA GABAergic neurons signal expected reward, a key variable for dopaminergic neurons to calculate reward prediction error. 相似文献