全文获取类型
收费全文 | 308篇 |
免费 | 40篇 |
专业分类
348篇 |
出版年
2022年 | 4篇 |
2021年 | 3篇 |
2017年 | 4篇 |
2016年 | 5篇 |
2015年 | 10篇 |
2014年 | 7篇 |
2013年 | 15篇 |
2012年 | 11篇 |
2011年 | 15篇 |
2010年 | 15篇 |
2009年 | 15篇 |
2008年 | 6篇 |
2007年 | 10篇 |
2006年 | 12篇 |
2005年 | 10篇 |
2004年 | 10篇 |
2003年 | 11篇 |
2002年 | 10篇 |
2001年 | 10篇 |
2000年 | 8篇 |
1999年 | 6篇 |
1998年 | 8篇 |
1997年 | 2篇 |
1996年 | 5篇 |
1995年 | 4篇 |
1994年 | 3篇 |
1993年 | 9篇 |
1992年 | 4篇 |
1991年 | 7篇 |
1990年 | 9篇 |
1989年 | 7篇 |
1988年 | 10篇 |
1987年 | 7篇 |
1986年 | 8篇 |
1984年 | 2篇 |
1983年 | 2篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1978年 | 5篇 |
1976年 | 5篇 |
1975年 | 4篇 |
1973年 | 6篇 |
1972年 | 6篇 |
1971年 | 4篇 |
1970年 | 2篇 |
1969年 | 3篇 |
1968年 | 3篇 |
1962年 | 4篇 |
1960年 | 2篇 |
1955年 | 2篇 |
排序方式: 共有348条查询结果,搜索用时 12 毫秒
31.
BACKGROUND: ALDH-bright (ALDH(br)) cell populations sorted from freshly collected umbilical cord blood (UCB) on the basis of their high aldehyde dehydrogenase (ALDH) activity are highly enriched for HPC. HPC with low ALDH activity (ALDH(dim)) are primarily short-term progenitors, whereas progenitors that initiate long-term cultures or establish long-term grafts in xenograft models are ALDH(br). We examined the multilineage hematopoietic and platelet progenitor activities of ALDH(br) cells recovered from cryopreserved UCB units typically employed in the practice of clinical transplantation. METHODS: Frozen UCB units were thawed, washed, immunomagnetically depleted of cells expressing glycophorin A and CD14, reacted for flow cytometric detection of ALDH, and sorted to yield ALDH(br) and ALDH(dim) populations. We measured surface Ag expression and viability of cells in the ALDH(br) and ALDH(dim) populations by flow cytometry and hematopoietic (CFC-H) and megakaryocytic (CFC-Mk) colony-forming cells in each population. RESULTS: ALDH(br) populations isolated from thawed UCB cells were highly enriched for CD34(+) and CD133(+) cells. Flow-sorted ALDH(br) populations were enriched 1116-fold in CFC-H, 10-fold in multilineage GEMM colonies and 2015-fold in CFC-Mk compared with the ALDH(dim) population. All progenitors giving rise to large Mk colonies were derived from ALDH(br) populations. DISCUSSION: ALDH(br) populations recovered from thawed, banked UCB with the method we describe have HPC activity and may be useful in the clinic to facilitate reconstitution of erythroid, myeloid and megakaryocytic blood elements. 相似文献
32.
Ryan M. Johnson Ngoc T. Vu Brian P. Griffin Amanda E. Gentry Kellie J. Archer Charles E. Chalfant Margaret A. Park 《The Journal of biological chemistry》2015,290(42):25717-25727
Triple negative breast cancer (TNBC) represents an anomalous subset of breast cancer with a greatly reduced (30%) 5-year survival rate. The enhanced mortality and morbidity of TNBC arises from the high metastatic rate, which requires the acquisition of AnR, a process whereby anchorage-dependent cells become resistant to cell death induced by detachment. In this study TNBC cell lines were selected for AnR, and these cell lines demonstrated dramatic enhancement in the formation of lung metastases as compared with parental cells. Genetic analysis of the AnR subclones versus parental cells via next generation sequencing and analysis of global alternative RNA splicing identified that the mRNA splicing of cytoplasmic polyadenylation element binding 2 (CPEB2), a translational regulator, was altered in AnR TNBC cells. Specifically, increased inclusion of exon 4 into the mature mRNA to produce the CPEB2B isoform was observed in AnR cell lines. Molecular manipulations of CPEB2 splice variants demonstrated a key role for this RNA splicing event in the resistance of cells to anoikis. Specifically, down-regulation of the CPEB2B isoform using siRNA re-sensitized the AnR cell lines to detachment-induced cell death. The ectopic expression of CPEB2B in parental TNBC cell lines induced AnR and dramatically increased metastatic potential. Importantly, alterations in the alternative splicing of CPEB2 were also observed in human TNBC and additional subtypes of human breast cancer tumors linked to a high metastatic rate. Our findings demonstrate that the regulation of CPEB2 mRNA splicing is a key mechanism in AnR and a driving force in TNBC metastasis. 相似文献
33.
34.
Elisabeth APM Romme Piet Geusens Willem F Lems Erica PA Rutten Frank WJM Smeenk Joop PW van den Bergh Peter ThW van Hal Emiel FM Wouters 《Respiratory research》2015,16(1)
Although osteoporosis and its related fractures are common in patients with COPD, patients at high risk of fracture are poorly identified, and consequently, undertreated. Since there are no fracture prevention guidelines available that focus on COPD patients, we developed a clinical approach to improve the identification and treatment of COPD patients at high risk of fracture. We organised a round-table discussion with 8 clinical experts in the field of COPD and fracture prevention in the Netherlands in December 2013. The clinical experts presented a review of the literature on COPD, osteoporosis and fracture prevention. Based on the Dutch fracture prevention guideline, they developed a 5-step clinical approach for fracture prevention in COPD. Thereby, they took into account both classical risk factors for fracture (low body mass index, older age, personal and family history of fracture, immobility, smoking, alcohol intake, use of glucocorticoids and increased fall risk) and COPD-specific risk factors for fracture (severe airflow obstruction, pulmonary exacerbations and oxygen therapy). Severe COPD (defined as postbronchodilator FEV1 < 50% predicted) was added as COPD-specific risk factor to the list of classical risk factors for fracture. The 5-step clinical approach starts with case finding using clinical risk factors, followed by risk evaluation (dual energy X-ray absorptiometry and imaging of the spine), differential diagnosis, treatment and follow-up. This systematic clinical approach, which is evidence-based and easy-to-use in daily practice by pulmonologists, should contribute to optimise fracture prevention in COPD patients at high risk of fracture. 相似文献
35.
36.
Howard Scott Gentry 《Economic botany》1961,15(4):326-331
The farmers of South Africa hare provided the first steps in the domestication of a medicinal herb endemic to the Cape Province. 相似文献
37.
A novel assay for protein phosphatase 2A (PP2A) complexes in vivo reveals differential effects of covalent modifications on different Saccharomyces cerevisiae PP2A heterotrimers
下载免费PDF全文
![点击此处可从《Eukaryotic cell》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Protein phosphatase 2A (PP2A) catalytic subunit can be covalently modified at its carboxy terminus by phosphorylation or carboxymethylation. Determining the effects of these covalent modifications on the relative amounts and functions of different PP2A heterotrimers is essential to understanding how these modifications regulate PP2A-controlled cellular processes. In this study we have validated and used a novel in vivo assay for assessing PP2A heterotrimer formation in Saccharomyces cerevisiae: the measurement of heterotrimer-dependent localization of green fluorescent protein-PP2A subunits. This assay relies on the fact that the correct cellular localization of PP2A requires that it be fully assembled. Thus, reduced localization would occur as the result of the inability to assemble a stable heterotrimer. Using this assay, we determined the effects of PP2A C-subunit phosphorylation mimic mutations and reduction or loss of PP2A methylation on the formation and localization of PP2A(B/Cdc55p) and PP2A(B'/Rts1p) heterotrimers. Collectively, our findings demonstrate that phosphorylation and methylation of the PP2A catalytic subunit can influence its function both by regulating the total amount of specific PP2A heterotrimers within a cell and by altering the relative proportions of PP2A(B/Cdc55p) and PP2A(B'/Rts1p) heterotrimers up to 10-fold. Thus, these posttranslational modifications allow flexible, yet highly coordinated, regulation of PP2A-dependent signaling pathways that in turn modulate cell growth and function. 相似文献
38.
Ye Xiang Ulrich Baxa Ying Zhang Alasdair C. Steven Gentry L. Lewis James L. Van Etten Michael G. Rossmann 《Journal of virology》2010,84(23):12265-12273
The chloroviruses (family Phycodnaviridae), unlike most viruses, encode some, if not most, of the enzymes involved in the glycosylation of their structural proteins. Annotation of the gene product B736L from chlorovirus NY-2A suggests that it is a glycosyltransferase. The structure of the recombinantly expressed B736L protein was determined by X-ray crystallography to 2.3-Å resolution, and the protein was shown to have two nucleotide-binding folds like other glycosyltransferase type B enzymes. This is the second structure of a chlorovirus-encoded glycosyltransferase and the first structure of a chlorovirus type B enzyme to be determined. B736L is a retaining enzyme and belongs to glycosyltransferase family 4. The donor substrate was identified as GDP-mannose by isothermal titration calorimetry and was shown to bind into the cleft between the two domains in the protein. The active form of the enzyme is probably a dimer in which the active centers are separated by about 40 Å.Glycosyltransferases constitute a large family of enzymes that catalyze the transfer of sugar moieties from donor molecules to specific acceptor molecules. Unlike other enzyme families that usually share conserved features in their primary sequences, glycosyltransferases can have highly diversified sequences that have been grouped into more than 90 families (designated GTn, where n = 1, 2, …) (http://www.CAZy.org) (1, 15). However, two families, GT2 and GT4, account for about half of the total number of glycosyltransferases. Despite the large variation in the primary sequences of glycosyltransferases, their three-dimensional structures are usually conserved. There are two major glycosyltransferase structural types, named GT-A and GT-B. The GT-A members contain a single nucleotide-binding domain consisting of six parallel β-strands flanked by connecting α-helices (referred to as a “Rossmann fold” in most of the literature on these enzymes and herein). GT-A enzyme activities are usually metal ion dependent. The GT-B type glycosyltransferases have two Rossmann folds separated by a cleft that forms the substrate-binding site. Metal ions are normally not required for GT-B function. Based on their catalytic mechanism, glycosyltransferases are also classified as either retaining or inverting enzymes depending on the geometry between the sugar donor and the receptor in the product molecule (e.g., depending on whether the anomeric carbon atom is linked to the acceptor via its α or β position). If the anomeric carbon atom has the same configuration in the donor and in the product, the enzyme is classified as a retaining enzyme; if the configurations are different, the enzyme is considered to be an inverting enzyme (2).Many viruses, especially those that infect eukaryotic cells, have extensively glycosylated structural proteins. Glycans coating viral structural proteins serve multiple biological roles, e.g., they mimic host glycans to evade host cell immune reactions, aid in folding or assembly of viral structural proteins, function as a receptor recognized by cell surface proteins, or aid in stabilizing viral particles (see, e.g., reference 36).Typically, viruses use host-encoded glycosyltransferases and glycosidases located in the endoplasmic reticulum (ER) and Golgi apparatus to add and remove N-linked sugar residues from virus glycoproteins either during or shortly after translation of the protein. This posttranslational processing aids in protein folding and requires other host-encoded enzymes. After folding and assembly, virus glycoproteins are transported by host-sorting and membrane transport functions to virus-specified regions in host membranes, where they displace host glycoproteins. Progeny viruses then bud through these virus-specific target membranes, in what is usually the final step in the assembly of infectious virions (3, 14, 21, 36). Thus, nascent viruses become infectious only by budding through the target membrane, usually the plasma membrane, as they are released from the cell. Consequently, the glycan portion of virus glycoproteins is host specific. The theme that emerges is that virus glycoproteins are synthesized and glycosylated by the same mechanisms as host glycoproteins. Therefore, the only way to alter glycosylation of virus proteins is to either grow the virus in a different host or have a mutation in the virus protein that alters the protein glycosylation site.One explanation for this scenario is that, in general, viruses lack genes encoding glycosyltransferases. However, a few virus-encoded glycosyltransferases have been reported in recent years (see reference 17 for a review). Often these virus-encoded glycosyltransferases add sugars to compounds other than proteins. For instance, some phage-encoded glycosyltransferases modify virus DNA to protect it from host restriction endonucleases (see, e.g., reference 10), and a glycosyltransferase encoded by baculoviruses modifies a host insect ecdysteroid hormone, leading to its inactivation (22). Bovine herpesvirus 4 encodes a β-1,6-N-acetyl-glucosaminyltransferase that is localized in the Golgi apparatus and is probably involved in posttranslational modification of the virus structural proteins (32).One group of viruses differs from the scenario that viruses use the host machinery located in the ER and the Golgi apparatus to glycosylate their glycoproteins. These viruses are the large, plaque-forming, double-stranded DNA (dsDNA)-containing chloroviruses (family Phycodnaviridae) that infect eukaryotic algae (4, 34, 39, 40). The chloroviruses have up to 400 protein-encoding genes (or coding sequences [CDSs]). Annotation of six chlorovirus genomes showed that each virus encodes 3 to 6 putative glycosyltransferases (7-9, 16, 33). Three of these viruses, NY-2A, AR158, and the prototype chlorovirus Paramecium bursaria chlorella virus 1 (PBCV-1), infect Chlorella strain NC64A. Two of the viruses, MT325 and FR483, infect Chlorella Pbi, and one of them, Acanthocystis turfacea chlorella virus (ATCV-1), infects Chlorella SAG 3.83.Glycosylation of the PBCV-1 major capsid protein, Vp54, is at least partially performed by the viral glycosyltransferases (11, 20, 33, 38, 41). PBCV-1 encodes 5 putative glycosyltransferases. A previous structural study established that the N-terminal 211 amino acids of the A64R protein from PBCV-1 form a GT-A group glycosyltransferase that is a retaining enzyme belonging to the GT34 family and that UDP-glucose possibly serves as the donor sugar (41).Among the four additional PBCV-1 glycosyltransferase-encoding genes, gene a546l encodes a 396-amino-acid protein that resembles members in the GT4 family of glycosyltransferases, based on amino acid sequence comparison of members in the CAZy classification (1, 15). Homologs of this protein, A546L, are encoded by 3 other chloroviruses, NY-2A, AR158, and ATCV-1. Here, we report the crystal structure of one of these homologs, B736L, at 2.3-Å resolution. 相似文献
39.
Growth of new dendritic spines contributes to experience-dependent circuit plasticity in the cerebral cortex. Yet the signaling mechanisms leading to?new spine outgrowth remain poorly defined. Increasing evidence supports that the proteasome is an important mediator of activity-dependent neuronal signaling. We therefore tested the role of the proteasome in activity-dependent spinogenesis. Using pharmacological manipulations, glutamate uncaging, and two-photon imaging of GFP-transfected hippocampal pyramidal neurons, we demonstrate that acute inhibition of the proteasome blocks activity-induced spine outgrowth. Remarkably, mutation of serine 120 to alanine of the Rpt6 proteasomal subunit in individual neurons was sufficient to block activity-induced spine outgrowth. Signaling through NMDA receptors and CaMKII, but not PKA, is required to facilitate spine outgrowth. Moreover, abrogating CaMKII binding to the NMDA receptor abolished activity-induced spinogenesis. Our data support a model in which neural activity facilitates spine outgrowth via an NMDA receptor- and CaMKII-dependent increase in local proteasomal degradation. 相似文献
40.
Gentry GT Lambe J Forbes W Olcott B Sanders D Bondioli K Godke RA 《Theriogenology》2012,77(9):1894-1899
Control of the white-tailed doe's reproductive cycle is not well documented. The objective was to determine the effects of giving equine chorionic gonadotropin (eCG) at progesterone device removal on fixed time artificial insemination (FTAI) pregnancy rates in white-tailed does. All does (n = 74) were synchronized with a vaginal progesterone implant (CIDR; 0.3 g progesterone), inserted on Day 0 (without regard to stage of estrous cycle), removed 14 days later, and subjected to FTAI, on average, 60 h post-CIDR removal. Of these, 34 were given 200 IU (im) of eCG at CIDR removal. Overall, FTAI pregnancy rate was 50% across 2 yrs (effect of year, P = 0.35). Administration of eCG at CIDR removal did not affect (P = 0.16) pregnancy rate (eCG = 59%; no eCG = 43%). Pregnancy rates were not affected by vulva score or doe disposition. Does that were ≤ 4 yrs old were more likely (P = 0.01) to become pregnant than does > 4 yrs of age. Does inseminated ≥ 60.5 h after CIDR removal were 22 times more likely (P = 0.002) to become pregnant to FTAI than does inseminated < 60.5 h. When frozen-thawed semen was deposited in the cervix or uterus, does were 17 times more likely (P = 0.005) to become pregnant compared with those receiving intravaginal insemination. Fecundity was not different (P = 0.73) across treatment groups (1.6 ± 0.11; no eCG vs. 1.7 ± 0.10; eCG). Furthermore, fecundity of does pregnant to FTAI was not different (P = 0.72) compared with does pregnant to clean-up bucks (1.7 ± 0.08; AI does vs. 1.7 ± 0.09; clean-up bucks). In summary, white-tailed does were successfully inseminated using a 14 days FTAI protocol, eCG may not be essential for acceptable pregnancy rates, and increased pregnancy rates may result when FTAI is done ≥ 60.5 h after progesterone device removal. 相似文献