全文获取类型
收费全文 | 19895篇 |
免费 | 1703篇 |
国内免费 | 9篇 |
出版年
2023年 | 74篇 |
2022年 | 161篇 |
2021年 | 329篇 |
2020年 | 203篇 |
2019年 | 262篇 |
2018年 | 295篇 |
2017年 | 289篇 |
2016年 | 484篇 |
2015年 | 802篇 |
2014年 | 883篇 |
2013年 | 1097篇 |
2012年 | 1533篇 |
2011年 | 1587篇 |
2010年 | 975篇 |
2009年 | 842篇 |
2008年 | 1246篇 |
2007年 | 1302篇 |
2006年 | 1218篇 |
2005年 | 1196篇 |
2004年 | 1182篇 |
2003年 | 1098篇 |
2002年 | 1015篇 |
2001年 | 207篇 |
2000年 | 164篇 |
1999年 | 201篇 |
1998年 | 276篇 |
1997年 | 184篇 |
1996年 | 151篇 |
1995年 | 184篇 |
1994年 | 179篇 |
1993年 | 150篇 |
1992年 | 124篇 |
1991年 | 124篇 |
1990年 | 118篇 |
1989年 | 103篇 |
1988年 | 104篇 |
1987年 | 115篇 |
1986年 | 100篇 |
1985年 | 100篇 |
1984年 | 99篇 |
1983年 | 116篇 |
1982年 | 112篇 |
1981年 | 112篇 |
1980年 | 90篇 |
1979年 | 56篇 |
1978年 | 56篇 |
1977年 | 59篇 |
1976年 | 51篇 |
1975年 | 36篇 |
1974年 | 24篇 |
排序方式: 共有10000条查询结果,搜索用时 22 毫秒
991.
Simian-human immunodeficiency virus escape from cytotoxic T-lymphocyte recognition at a structurally constrained epitope 下载免费PDF全文
Peyerl FW Barouch DH Yeh WW Bazick HS Kunstman J Kunstman KJ Wolinsky SM Letvin NL 《Journal of virology》2003,77(23):12572-12578
Virus-specific cytotoxic T lymphocytes (CTL) exert intense selection pressure on replicating simian immunodeficiency virus (SIV) and human immunodeficiency virus type 1 (HIV-1) in infected individuals. The immunodominant Mamu-A(*)01-restricted Gag p11C, C-M epitope is highly conserved among all sequenced isolates of SIV and therefore likely is structurally constrained. The strategies used by virus isolates to mutate away from an immunodominant epitope-specific CTL response are not well defined. Here we demonstrate that the emergence of a position 2 p11C, C-M epitope substitution (T47I) in a simian-human immunodeficiency virus (SHIV) strain 89.6P-infected Mamu-A(*)01(+) monkey is temporally correlated with the emergence of a flanking isoleucine-to-valine substitution at position 71 (I71V) of the capsid protein. An analysis of the SIV and HIV-2 sequences from the Los Alamos HIV Sequence Database revealed a significant association between any position 2 p11C, C-M epitope mutation and the I71V mutation. The T47I mutation alone is associated with significant decreases in viral protein expression, infectivity, and replication, and these deficiencies are restored to wild-type levels with the introduction of the flanking I71V mutation. Together, these data suggest that a compensatory mutation is selected for in SHIV strain 89.6P to facilitate the escape of that virus from CTL recognition of the dominant p11C, C-M epitope. 相似文献
992.
Cell killing by avian leukosis virus subgroup B (ALV-B) in cultures has been extensively studied, but the molecular basis of this process has not been established. Here we show that superinfection, which has been linked to cell killing by ALV-B, plays no crucial role in cell death induction. Instead, we show that signaling by the ALV-B receptor, TVB(S3), a member of the tumor necrosis factor receptor family, is essential for ALV-B-mediated cell death. TVB(S3) activated caspase-dependent apoptosis during ALV-B infection. Strikingly, apoptosis induction occurred predominantly in uninfected cells, while ALV-B-infected cells were protected against cell death. This bystander killing phenomenon was reproduced in a virus-free system by cocultivating ALV-B Env-expressing cells with TVB(S3)-expressing cells. Taken together, our results indicated that ALV-B-mediated apoptosis is triggered by ALV-B Env-TVB(S3) interactions. 相似文献
993.
994.
995.
Howell SA Well I Fleck SL Kettleborough C Collins CR Blackman MJ 《The Journal of biological chemistry》2003,278(26):23890-23898
Erythrocyte invasion by the malaria merozoite is accompanied by the regulated discharge of apically located secretory organelles called micronemes. Plasmodium falciparum apical membrane antigen-1 (PfAMA-1), which plays an indispensable role in invasion, translocates from micronemes onto the parasite surface and is proteolytically shed in a soluble form during invasion. We have previously proposed, on the basis of incomplete mass spectrometric mapping data, that PfAMA-1 shedding results from cleavage at two alternative positions. We now show conclusively that the PfAMA-1 ectodomain is shed from the merozoite solely as a result of cleavage at a single site, just 29 residues away from the predicted transmembrane-spanning sequence. Remarkably, this cleavage is mediated by the same membrane-bound parasite serine protease as that responsible for shedding of the merozoite surface protein-1 (MSP-1) complex, an abundant, glycosylphosphatidylinositol-anchored multiprotein complex. Processing of MSP-1 is essential for invasion. Our results indicate the presence on the merozoite surface of a multifunctional serine sheddase with a broad substrate specificity. We further demonstrate that translocation and shedding of PfAMA-1 is an actin-independent process. 相似文献
996.
997.
Curariform alkaloids competitively inhibit muscle acetylcholine receptors (AChR) by bridging the alpha and non-alpha subunits that form the ligand-binding site. Here we delineate bound orientations of d-tubocurarine (d-TC) and its methylated derivative metocurine using mutagenesis, ligand binding measurements, and computational methods. When tested against a series of lysine mutations in the epsilon subunit, the two antagonists show marked differences in the consequences of the mutations on binding affinity. The mutations epsilon L117K, epsilon Y111K, and epsilon L109K decrease affinity of metocurine by up to 3 orders of magnitude but only slightly alter affinity of d-TC. At the alpha subunit face of the binding site, the mutation alpha Y198T decreases affinity of both antagonists, but alpha Y198F preferentially enhances affinity of d-TC. Computation of antagonist docking orientations, based on our structural model of the alpha-epsilon site of the human AChR, indicates distinct orientations of each antagonist; the flatter metocurine fits into a pocket formed principally by the epsilon subunit, whereas the more compact d-TC spans the narrower crevasse between alpha and epsilon subunits. The side chains of epsilon Tyr-111 and epsilon Thr-117 juxtapose one of two quaternary nitrogens in metocurine but are remote from the equivalent quaternary nitrogen in d-TC, which instead closely approaches alpha Tyr-198. The different docked orientations arise through tilt of the curariform scaffold by approximately 60 degrees normal to the nitrogen-nitrogen axis, together with a 20 degrees rotation about the axis. The overall mutagenesis and computational results show that despite their similar structures, d-TC and metocurine bind in distinctly different orientations to the adult human AChR. 相似文献
998.
Alvestad RM Grosshans DR Coultrap SJ Nakazawa T Yamamoto T Browning MD 《The Journal of biological chemistry》2003,278(13):11020-11025
The inhibitory effect of ethanol on N-methyl-d-aspartate receptors (NMDARs) is well documented in several brain regions. However, the molecular mechanisms by which ethanol affects NMDARs are not well understood. In contrast to the inhibitory effect of ethanol, phosphorylation of the NMDAR potentiates channel currents (Lu, W. Y., Xiong, Z. G., Lei, S., Orser, B. A., Dudek, E., Browning, M. D., and MacDonald, J. F. (1999) Nat. Neurosci. 2, 331-338). We have previously shown that protein kinase C activators induce tyrosine phosphorylation and potentiation of the NMDAR (Grosshans, D. R., Clayton, D. R., Coultrap, S. J., and Browning, M. D. (2002) Nat. Neurosci. 5, 27-33). We therefore hypothesized that the ethanol inhibition of NMDARs might be due to changes in tyrosine phosphorylation of NMDAR subunits. In support of this hypothesis, we found that tyrosine phosphorylation of both NR2A and NR2B subunits was significantly reduced following in situ exposure of hippocampal slices to 100 mm ethanol. Specifically, phosphorylation of tyrosine 1472 on NR2B was reduced 23.5%. These data suggest a possible mechanism by which ethanol may inhibit the NMDAR via activation of a tyrosine phosphatase. Electrophysiological studies demonstrated that ethanol inhibited NMDAR field excitatory postsynaptic potential slope and amplitude to a similar degree as previously reported by our laboratory and others (Schummers, J., Bentz, S., and Browning, M. D. (1997) Alcohol Clin. Exp. Res. 21, 404-408). Inclusion of bpV(phen), a potent phosphotyrosine phosphatase inhibitor, in the recording chamber prior to and during ethanol exposure significantly reduced the inhibitory effect of ethanol on NMDAR field excitatory postsynaptic potentials. Taken together, these data suggest that phosphatase-mediated dephosphorylation of NMDAR subunits may play an important role in mediating the inhibitory effects of ethanol on the N-methyl-D-aspartate receptor. 相似文献
999.
Lamble HJ Heyer NI Bull SD Hough DW Danson MJ 《The Journal of biological chemistry》2003,278(36):34066-34072
The hyperthermophilic Archaeon Sulfolobus solfataricus metabolizes glucose by a non-phosphorylative variant of the Entner-Doudoroff pathway. In this pathway glucose dehydrogenase and gluconate dehydratase catalyze the oxidation of glucose to gluconate and the subsequent dehydration of gluconate to 2-keto-3-deoxygluconate. 2-Keto-3-deoxygluconate (KDG) aldolase then catalyzes the cleavage of 2-keto-3-deoxygluconate to glyceraldehyde and pyruvate. The gene encoding glucose dehydrogenase has been cloned and expressed in Escherichia coli to give a fully active enzyme, with properties indistinguishable from the enzyme purified from S. solfataricus cells. Kinetic analysis revealed the enzyme to have a high catalytic efficiency for both glucose and galactose. KDG aldolase from S. solfataricus has previously been cloned and expressed in E. coli. In the current work its stereoselectivity was investigated by aldol condensation reactions between D-glyceraldehyde and pyruvate; this revealed the enzyme to have an unexpected lack of facial selectivity, yielding approximately equal quantities of 2-keto-3-deoxygluconate and 2-keto-3-deoxygalactonate. The KDG aldolase-catalyzed cleavage reaction was also investigated, and a comparable catalytic efficiency was observed with both compounds. Our evidence suggests that the same enzymes are responsible for the catabolism of both glucose and galactose in this Archaeon. The physiological and evolutionary implications of this observation are discussed in terms of catalytic and metabolic promiscuity. 相似文献
1000.
Baxa U Taylor KL Wall JS Simon MN Cheng N Wickner RB Steven AC 《The Journal of biological chemistry》2003,278(44):43717-43727
The [URE3] prion is an inactive, self-propagating, filamentous form of the Ure2 protein, a regulator of nitrogen catabolism in yeast. The N-terminal "prion" domain of Ure2p determines its in vivo prion properties and in vitro amyloid-forming ability. Here we determined the overall structures of Ure2p filaments and related polymers of the prion domain fused to other globular proteins. Protease digestion of 25-nm diameter Ure2p filaments trimmed them to 4-nm filaments, which mass spectrometry showed to be composed of prion domain fragments, primarily residues approximately 1-70. Fusion protein filaments with diameters of 14-25 nm were also reduced to 4-nm filaments by proteolysis. The prion domain transforms from the most to the least protease-sensitive part upon filament formation in each case, implying that it undergoes a conformational change. Intact filaments imaged by cryo-electron microscopy or after vanadate staining by scanning transmission electron microscopy (STEM) revealed a central 4-nm core with attached globular appendages. STEM mass per unit length measurements of unstained filaments yielded 1 monomer per 0.45 nm in each case. These observations strongly support a unifying model whereby subunits in Ure2p filaments, as well as in fusion protein filaments, are connected by interactions between their prion domains, which form a 4-nm amyloid filament backbone, surrounded by the corresponding C-terminal moieties. 相似文献