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71.
Protein interactions between MAP kinases and substrates, activators, and scaffolding proteins are regulated by docking site motifs, one containing basic residues proximal to Leu-X-Leu (DEJL) and a second containing Phe-X-Phe (DEF). Hydrogen exchange mass spectrometry was used to identify regions in MAP kinases protected from solvent by docking motif interactions. Protection by DEJL peptide binding was observed in loops spanning beta7-beta8 and alphaD-alphaE in p38alpha and ERK2. In contrast, protection by DEF binding to ERK2 revealed a distinct hydrophobic pocket for Phe-X-Phe binding formed between the P+1 site, alphaF helix, and the MAP kinase insert. In inactive ERK2, this pocket is occluded by intramolecular interactions with residues in the activation lip. In vitro assays confirm the dependence of Elk1 and nucleoporin binding on ERK2 phosphorylation, and provide a structural basis for preferential involvement of active ERK in substrate binding and nuclear pore protein interactions.  相似文献   
72.
Bird song is a sexually selected trait and females have been shown to prefer males that sing more complex songs. However, for repertoire size to be an honest signal of male quality it must be associated with some form of cost. This experiment investigates the effects of food restriction and social status during development on song complexity in the European starling (Sturnus vulgaris). Birds that experienced an unpredictable food supply early in life produced a significantly smaller repertoire of song phrases than those with a constant food supply. Social status during development was also significantly correlated with repertoire size, with dominant birds producing more phrase types. This study therefore provides novel evidence that social as well as nutritional history may be important in shaping the song signal in this species.  相似文献   
73.
Aberrant cell survival and resistance to apoptosis are hallmarks of tumor invasion and progression to metastatic disease, but the mechanisms involved are poorly understood. The epithelial-mesenchymal transition (EMT), a process that facilitates progression to invasive cancer, provides a superb model for studying such survival mechanisms. Here, we used a unique spheroid culture system that recapitulates the structure of the colonic epithelium and undergoes an EMT in response to cytokine stimulation to study this problem. Our data reveal that the EMT results in the increased expression of both VEGF and Flt-1, a tyrosine kinase VEGF receptor, and that the survival of these cells depends on a VEGF/Flt-1 autocrine pathway. Perturbation of Flt-1 function by either a blocking antibody or adenoviral expression of soluble Flt-1, which acts in a dominant-negative fashion, caused massive apoptosis only in cells that underwent EMT. This pathway was critical for the survival of other invasive colon carcinoma cell lines, and we observed a correlative upregulation of Flt-1 expression linked to in vivo human cancer progression. A role for Flt-1 in cell survival is unprecedented and has significant implications for Flt-1 function in tumor progression, as well as in other biological processes, including angiogenesis and development.  相似文献   
74.
The destruction of the immune system by progressive loss of CD4 T cells is the hallmark of AIDS. CCR5-dependent (R5) human immunodeficiency virus type 1 (HIV-1) isolates predominate in the early, asymptomatic stages of HIV-1 infection, while CXCR4-dependent (X4) isolates typically emerge at later stages, frequently coinciding with a rapid decline in CD4 T cells. Lymphocyte killing in vivo primarily occurs through apoptosis, but the importance of apoptosis of HIV-1-infected cells relative to apoptosis of uninfected bystander cells is controversial. Here we show that in human lymphoid tissues ex vivo, apoptosis of uninfected bystander CD4 T cells is a major mechanism of lymphocyte depletion caused by X4 HIV-1 strains but is only a minor mechanism of depletion by R5 strains. Further, X4 HIV-1-induced bystander apoptosis requires the interaction of the viral envelope glycoprotein gp120 with the CXCR4 coreceptor on CD4 T cells. These results emphasize the contribution of bystander apoptosis to HIV-1 cytotoxicity and suggest that in association with a coreceptor switch in HIV disease, T-cell killing evolves from an infection-restricted stage to generalized toxicity that involves a high degree of bystander apoptosis.  相似文献   
75.
Retroviruses must gain access to the host cell nucleus for subsequent replication and viral propagation. Human immunodeficiency virus type 1 (HIV-1) and other primate lentiviruses are distinguished from the gammaretroviruses by their ability to infect nondividing cells such as macrophages, an important viral reservoir in vivo. Rather than requiring nuclear membrane breakdown during cell division, the HIV-1 preintegration complex (PIC) enters the nucleus by traversing the central aqueous channel of the limiting nuclear pore complex. The HIV-1 PIC contains three nucleophilic proteins, matrix, integrase, and Vpr, all of which have been implicated in nuclear targeting. The mechanism by which Vpr can display such nucleophilic properties and yet also be available for incorporation into virions assembling at the plasma membrane is unresolved. We recently characterized Vpr as a nucleocytoplasmic shuttling protein that contains two novel nuclear import signals and an exportin-1-dependent nuclear export signal (NES). We now demonstrate that mutation of this NES impairs the incorporation of Vpr into newly formed virions. Furthermore, we find that the Vpr NES is required for efficient HIV replication in tissue macrophages present in human spleens and tonsils. These findings underscore how the nucleocytoplasmic shuttling of Vpr not only contributes to nuclear import of the HIV-1 PIC but also enables Vpr to be present in the cytoplasm for incorporation into virions, leading to enhancement of viral spread within nondividing tissue macrophages.  相似文献   
76.
The biotin-binding tetrameric proteins, streptavidin from Streptomyces avidinii and chicken egg white avidin, are excellent models for the study of subunit-subunit interactions of a multimeric protein. Efforts are thus being made to prepare mutated forms of streptavidin and avidin, which would form monomers or dimers, in order to examine their effect on quaternary structure and assembly. In the present communication, we compared the crystal structures of binding site W-->K mutations in streptavidin and avidin. In solution, both mutant proteins are known to form dimers, but upon crystallization, both formed tetramers with the same parameters as the native proteins. All of the intersubunit bonds were conserved, except for the hydrophobic interaction between biotin and the tryptophan that was replaced by lysine. In the crystal structure, the binding site of the mutated apo-avidin contains 3 molecules of structured water instead of the 5 contained in the native protein. The lysine side chain extends in a direction opposite that of the native tryptophan, the void being partially filled by an adjacent lysine residue. Nevertheless, the binding-site conformation observed for the mutant tetramer is an artificial consequence of crystal packing that would not be maintained in the solution-phase dimer. It appears that the dimer-tetramer transition may be concentration dependent, and the interaction among subunits obeys the law of mass action.  相似文献   
77.
78.
The ultrastructure of Hendra and Nipah viruses is described in cultured cells, pigs, horses and humans. Differences in ultrastructure between the viruses are evident within infected cell cultures and lungs from infected amplifier hosts. These differences are important in viral identification and differentiation and understanding the pathogenesis of disease.  相似文献   
79.
The synapse loss and neuronal cell death characteristic of Alzheimer's disease (AD) are believed to result in large part from the neurotoxic effects of beta-amyloid peptide (Abeta), a 40-42 amino acid peptide(s) derived proteolytically from beta-amyloid precursor protein (APP). However, APP is also cleaved intracellularly to generate a second cytotoxic peptide, C31, and this cleavage event occurs in vivo as well as in vitro and preferentially in the brains of AD patients (Lu et al. 2000). Here we show that APPC31 is toxic to neurons in primary culture, and that like APP, the APP family members APLP1 and possibly APLP2 are cleaved by caspases at their C-termini. The carboxy-terminal peptide derived from caspase cleavage of APLP1 shows a degree of neurotoxicity comparable to APPC31. Our results suggest that even though APLP1 and APLP2 cannot generate Abeta, they may potentially contribute to the pathology of AD by generating peptide fragments whose toxicity is comparable to that of APPC31.  相似文献   
80.
A method has been developed for the determination of the efficiency of fluorescence resonance energy transfer efficiency between moieties located on cell surfaces by performing individual cell fluorescence polarization (FP) measurements. The absolute value of energy transfer efficiency (E) is calculated on an individual cell basis. The examination of this methodology was carried out using model experiments on human T lymphocyte cells. The cells were labeled with fluorescein-conjugated Concanavalin A (ConA) as donor, or rhodamine-conjugated ConA as acceptor. The experiments and results clearly indicate that determination of E via FP measurements is possible, efficient, and more convenient than other methods.  相似文献   
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