Vasculogenic mimicry (VM), a newly-defined pattern of tumor blood supply, provides a special passage without endothelial cells and is conspicuously different from angiogenesis and vasculogenesis. The biological features of the tumor cells that form VM remain unknown. Cancer stem cells (CSCs) are believed to be tumor-initiating cells, capable of self-renewal and multipotent differentiation, which resemble normal stem cells in phenotype and function. Recently CSCs have been shown to contribute to VM formation as well as angiogenesis. These findings challenge the previous understanding of the cellular basis of VM formation. In this review, we present evidence for participation of CSCs in VM formation. We also discuss the potential mechanisms and possible interaction of CSCs with various elements in tumor microenvironment niche. Based on the importance of VM in tumor progression, it constitutes a novel therapeutic target for cancer.

RNA silencing is a conserved eukaryotic pathway involved in the suppression of gene expression via sequence-specific interactions that are mediated by 21–23 nt RNA molecules. During infection, RNAi can act as an innate immune system to defend against viruses. As a counter-defensive strategy, silencing suppressors are encoded by viruses to inhibit various stages of the silencing process. These suppressors are diverse in sequence and structure and act via different mechanisms. In this review, we discuss whether RNAi is a defensive strategy in mammalian host cells and whether silencing suppressors can be encoded by mammalian viruses. We also review the modes of action proposed for some silencing suppressors.

The worldwide outbreak of the severe acute respiratory syndrome (SARS) in 2003 was due to the transmission of SARS coronavirus (SARS-CoV). The main protease (M^pro) of SARS-CoV is essential for the viral life cycle, and is considered to be an attractive target of anti-SARS drug development. As a key enzyme for proteolytic processing of viral polyproteins to produce functional non-structure proteins, M^pro is first auto-cleaved out of polyproteins. The monomeric form of M^pro is enzymatically inactive, and it is activated through homo-dimerization which is strongly affected by extra residues to both ends of the mature enzyme. This review provides a summary of the related literatures on the study of the quaternary structure, activation, and self-maturation of M^pro over the past years.

Bidirectional trafficking of macromolecules between the cytoplasm and the nucleus is mediated by the nuclear pore complexes (NPCs) embedded in the nuclear envelope (NE) of eukaryotic cell. The NPC functions as the sole pathway to allow for the passive diffusion of small molecules and the facilitated translocation of larger molecules. Evidence shows that these two transport modes and the conformation of NPC can be regulated by calcium stored in the lumen of nuclear envelope and endoplasmic reticulum. However, the mechanism of calcium regulation remains poorly understood. In this review, we integrate data on the observations of calcium-regulated structure and function of the NPC over the past years. Furthermore, we highlight challenges in the measurements of dynamic conformational changes and transient transport kinetics in the NPC. Finally, an innovative imaging approach, single-molecule super-resolution fluorescence microscopy, is introduced and expected to provide more insights into the mechanism of calcium-regulated nucleocytoplasmic transport.

Genome duplication in E. coli is carried out by DNA polymerase III, an enzyme complex consisting of ten subunits. Investigations of the biochemical and structural properties of DNA polymerase III require the expression and purification of subunits including α, ?, θ, γ, δ′, δ, and β separately followed by in vitro reconstitution of the pol III core and clamp loader. Here we propose a new method for expressing and purifying DNA polymerase III components by utilizing a protein co-expression strategy. Our results show that the subunits of the pol III core and those of the clamp loader can be co-expressed and purified based on inherent interactions between the subunits. The resulting pol III core, clamp loader and sliding clamp can be reconstituted effectively to perform DNA polymerization. Our strategy considerably simplifies the expression and purification of DNA polymerase III and provides a feasible and convenient method for exploring other multi-subunit systems.

The guanine-nucleotide exchange factor (GEF) RalGPS1a activates small GTPase Ral proteins such as RalA and RalB by stimulating the exchange of Ral bound GDP to GTP, thus regulating various downstream cellular processes. RalGPS1a is composed of an N-terminal Cdc25-like catalytic domain, followed by a PXXP motif and a C-terminal pleckstrin homology (PH) domain. The Cdc25 domain of RalGPS1a, which shares about 30% sequence identity with other Cdc25-domain proteins, is thought to be directly engaged in binding and activating the substrate Ral protein. Here we report the crystal structure of the Cdc25 domain of RalGPS1a. The bowl shaped structure is homologous to the Cdc25 domains of SOS and RasGRF1. The most remarkable difference between these three Cdc25 domains lies in their active sites, referred to as the helical hairpin region. Consistent with previous enzymological studies, the helical hairpin of RalGPS1a adopts a conformation favorable for substrate binding. A modeled RalGPS1a-RalA complex structure reveals an extensive binding surface similar to that of the SOS-Ras complex. However, analysis of the electrostatic surface potential suggests an interaction mode between the RalGPS1a active site helical hairpin and the switch 1 region of substrate RalA distinct from that of the SOS-Ras complex.

Proteins of the complement system are known to interact with many charged substances. We recently characterized binding of C1q and factor H to immobilized and liposomal anionic phospholipids. Factor H inhibited C1q binding to anionic phospholipids, suggesting a role for factor H in regulating activation of the complement classical pathway by anionic phospholipids. To extend this finding, we examined interactions of C1q and factor H with lipid A, a well-characterized activator of the classical pathway. We report that C1q and factor H both bind to immobilized lipid A, lipid A liposomes and intact Escherichia coli TG1. Factor H competes with C1q for binding to these targets. Furthermore, increasing the factor H: C1q molar ratio in serum diminished C4b fixation, indicating that factor H diminishes classical pathway activation. The recombinant forms of the C-terminal, globular heads of C1q A, B and C chains bound to lipid A and E. coli in a manner qualitatively similar to native C1q, confirming that C1q interacts with these targets via its globular head region. These observations reinforce our proposal that factor H has an additional complement regulatory role of down-regulating classical pathway activation in response to certain targets. This is distinct from its role as an alternative pathway downregulator. We suggest that under physiological conditions, factor H may serve as a downregulator of bacterially-driven inflammatory responses, thereby fine-tuning and balancing the inflammatory response in infections with Gram-negative bacteria.

Estrogen is implicated as playing an important role in aging and tumorigenesis of estrogen responsive tissues; however the mechanisms underlying the mitogenic actions of estrogen are not fully understood. Here we report that estrogen deficiency in mice caused by tar-geted disruption of the aromatase gene results in a signi-ficant inhibition of telomerase maintenance of telomeres in mouse ovaries in a tissue-specific manner. The inhibition entails a significant shortening of telomeres and compromised proliferation in the follicular granulosa cell compartment of ovary. Gene expression analysis showed decreased levels of proto-oncogene c-Myc and the telomerase catalytic subunit, telomerase reverse transcriptase (TERT), in response to estrogen deficiency. Estrogen replacement therapy led to increases in TERT gene expression, telomerase activity, telomere length and ovarian tissue growth, thereby reinstating ovary development to normal in four weeks. Our data demonstrate for the first time that telomere maintenance is the primary mechanism mediating the mitogenic effect of estrogen on ovarian granulosa cell proliferation by up-regulating the genes of c-Myc and TERT in vivo. Estrogen deficiency or over-activity may cause ovarian tissue aging or tumorigenesis, respectively, through estrogen regulation of telomere remodeling.