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971.
The involvement of RNA in ribosome function 总被引:1,自引:0,他引:1
The ribosome is a particle made of RNA and protein that is found in abundance in all cells that are actively making protein. It catalyses the messenger RNA-directed synthesis of proteins. Recent structural work has demonstrated a profound involvement of the ribosome's RNA component in all aspects of its function, supporting the hypothesis that proteins were added to the ribosome late in its evolution. 相似文献
972.
The structure of a sunspot is determined by the local interaction between magnetic fields and convection near the Sun's surface. The dark central umbra is surrounded by a filamentary penumbra, whose complicated fine structure has only recently been revealed by high-resolution observations. The penumbral magnetic field has an intricate and unexpected interlocking-comb structure and some field lines, with associated outflows of gas, dive back down below the solar surface at the outer edge of the spot. These field lines might be expected to float quickly back to the surface because of magnetic buoyancy, but they remain submerged. Here we show that the field lines are kept submerged outside the spot by turbulent, compressible convection, which is dominated by strong, coherent, descending plumes. Moreover, this downward pumping of magnetic flux explains the origin of the interlocking-comb structure of the penumbral magnetic field, and the behaviour of other magnetic features near the sunspot. 相似文献
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The trans-Golgi network (TGN) is a major secretory pathway sorting station that directs newly synthesized proteins to different
subcellular destinations. The TGN also receives extracellular materials and recycled molecules from endocytic compartments.
In this review, we summarize recent progress on understanding TGN structure and the dynamics of trafficking to and from this
compartment. Protein sorting into different transport vesicles requires specific interactions between sorting motifs on the
cargo molecules and vesicle coat components that recognize these motifs. Current understanding of the various targeting signals
and vesicle coat components that are involved in TGN sorting are discussed, as well as the molecules that participate in retrieval
to this compartment in both yeast and mammalian cells. Besides proteins, lipids and lipid-modifying enzymes also participate
actively in the formation of secretory vesicles. The possible mechanisms of action of these lipid hydrolases and lipid kinases
are discussed. Finally, we summarize the fundamentally different apical and basolateral cell surface delivery mechanisms and
the current facts and hypotheses on protein sorting from the TGN into the regulated secretory pathway in neuroendocrine cells.
Received 2 November 2000; received after revision 19 February 2001; accepted 19 February 2001 相似文献
975.
Veverka J Farquhar B Robinson M Thomas P Murchie S Harch A Antreasian PG Chesley SR Miller JK Owen WM Williams BG Yeomans D Dunham D Heyler G Holdridge M Nelson RL Whittenburg KE Ray JC Carcich B Cheng A Chapman C Bell JF Bell M Bussey B Clark B Domingue D Gaffey MJ Hawkins E Izenberg N Joseph J Kirk R Lucey P Malin M McFadden L Merline WJ Peterson C Prockter L Warren J Wellnitz D 《Nature》2001,413(6854):390-393
The NEAR-Shoemaker spacecraft was designed to provide a comprehensive characterization of the S-type asteroid 433 Eros (refs 1,2,3), an irregularly shaped body with approximate dimensions of 34 x 13 x 13 km. Following the completion of its year-long investigation, the mission was terminated with a controlled descent to its surface, in order to provide extremely high resolution images. Here we report the results of the descent on 12 February 2001, during which 70 images were obtained. The landing area is marked by a paucity of small craters and an abundance of 'ejecta blocks'. The properties and distribution of ejecta blocks are discussed in a companion paper. The last sequence of images reveals a transition from the blocky surface to a smooth area, which we interpret as a 'pond'. Properties of the 'ponds' are discussed in a second companion paper. The closest image, from an altitude of 129 m, shows the interior of a 100-m-diameter crater at 1-cm resolution. 相似文献
976.
Host origin of marrow stromal cells following allogeneic bone marrow transplantation 总被引:15,自引:0,他引:15
Although it is generally agreed that stromal cells are important in the regulation of haematopoietic cell development, the origin of these phenotypically diverse cells has been a subject for debate for more than 50 years. Data which support the concept of a separate origin for the haematopoietic stem cell and the marrow stroma are derived from cytogenetic or enzyme marker studies of explanted and expanded stromal cells grown under conditions that do not allow haematopoiesis in vitro. Recent evidence in man and in mouse suggesting that the stromal cells capable of transferring the haematopoietic microenvironment in vitro are transplantable seemingly questions this dichotomy, one interpretation being the existence of a common haematopoietic/stromal 'stem cell'. We used in situ hybridization to discriminate donor cells from host in blood and bone marrow samples obtained from patients with functioning sex-mismatched but HLA-identical allografts. Without exception, marrow-derived stromal cells that proliferate in long-term cultures were found to be of host genotype, whereas the macrophage component of the adherent layer in these cultures originated from the donor. 相似文献
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