MYC inactivation uncovers pluripotent differentiation and tumour dormancy in hepatocellular cancer

Hepatocellular carcinoma is generally refractory to clinical treatment. Here, we report that inactivation of the MYC oncogene is sufficient to induce sustained regression of invasive liver cancers. MYC inactivation resulted en masse in tumour cells differentiating into hepatocytes and biliary cells forming bile duct structures, and this was associated with rapid loss of expression of the tumour marker alpha-fetoprotein, the increase in expression of liver cell markers cytokeratin 8 and carcinoembryonic antigen, and in some cells the liver stem cell marker cytokeratin 19. Using in vivo bioluminescence imaging we found that many of these tumour cells remained dormant as long as MYC remain inactivated; however, MYC reactivation immediately restored their neoplastic features. Using array comparative genomic hybridization we confirmed that these dormant liver cells and the restored tumour retained the identical molecular signature and hence were clonally derived from the tumour cells. Our results show how oncogene inactivation may reverse tumorigenesis in the most clinically difficult cancers. Oncogene inactivation uncovers the pluripotent capacity of tumours to differentiate into normal cellular lineages and tissue structures, while retaining their latent potential to become cancerous, and hence existing in a state of tumour dormancy.     

Structure of an auxilin-bound clathrin coat and its implications for the mechanism of uncoating

Clathrin-coated pits invaginate from specific membrane compartments and pinch off as coated vesicles. These vesicles then uncoat rapidly once released. The Hsc70 molecular chaperone effects the uncoating reaction, and is guided to appropriate locations on clathrin lattices by the J-domain-containing co-chaperone molecule auxilin. This raises the question of how a local event such as ATP hydrolysis by Hsc70 can catalyse a global disassembly. Here, we have used electron cryomicroscopy to determine 12-A-resolution structures of in-vitro-assembled clathrin coats in association with a carboxy-terminal fragment of auxilin that contains both the clathrin-binding region and the J domain. We have located the auxilin fragment by computing differences between these structures and those lacking auxilin (described in an accompanying paper). Auxilin binds within the clathrin lattice near contacts between an inward-projecting C-terminal helical tripod and the crossing of two 'ankle' segments; it also contacts the terminal domain of yet another clathrin 'leg'. It therefore recruits Hsc70 to the neighbourhood of a set of critical interactions. Auxilin binding produces a local change in heavy-chain contacts, creating a detectable global distortion of the clathrin coat. We propose a mechanism by which local destabilization of the lattice promotes general uncoating.     

Visualization of release factor 3 on the ribosome during termination of protein synthesis

Termination of protein synthesis by the ribosome requires two release factor (RF) classes. The class II RF3 is a GTPase that removes class I RFs (RF1 or RF2) from the ribosome after release of the nascent polypeptide. RF3 in the GDP state binds to the ribosomal class I RF complex, followed by an exchange of GDP for GTP and release of the class I RF. As GTP hydrolysis triggers release of RF3 (ref. 4), we trapped RF3 on Escherichia coli ribosomes using a nonhydrolysable GTP analogue. Here we show by cryo-electron microscopy that the complex can adopt two different conformational states. In 'state 1', RF3 is pre-bound to the ribosome, whereas in 'state 2' RF3 contacts the ribosome GTPase centre. The transfer RNA molecule translocates from the peptidyl site in state 1 to the exit site in state 2. This translocation is associated with a large conformational rearrangement of the ribosome. Because state 1 seems able to accommodate simultaneously both RF3 and RF2, whose position is known from previous studies, we can infer the release mechanism of class I RFs.     

Structure of the ESCRT-II endosomal trafficking complex

The multivesicular-body (MVB) pathway delivers transmembrane proteins and lipids to the lumen of the endosome. The multivesicular-body sorting pathway has crucial roles in growth-factor-receptor downregulation, developmental signalling, regulation of the immune response and the budding of certain enveloped viruses such as human immunodeficiency virus. Ubiquitination is a signal for sorting into the MVB pathway, which also requires the functions of three protein complexes, termed ESCRT-I, -II and -III (endosomal sorting complex required for transport). Here we report the crystal structure of the core of the yeast ESCRT-II complex, which contains one molecule of the Vps protein Vps22, the carboxy-terminal domain of Vps36 and two molecules of Vps25, and has the shape of a capital letter 'Y'. The amino-terminal coiled coil of Vps22 and the flexible linker leading to the ubiquitin-binding NZF domain of Vps36 both protrude from the tip of one branch of the 'Y'. Vps22 and Vps36 form nearly equivalent interactions with the two Vps25 molecules at the centre of the 'Y'. The structure suggests how ubiquitinated cargo could be passed between ESCRT components of the MVB pathway through the sequential transfer of ubiquitinated cargo from one complex to the next.     

Neuronal synchrony does not correlate with motion coherence in cortical area MT

Natural visual scenes are cluttered with multiple objects whose individual features must somehow be selectively linked (or 'bound') if perception is to coincide with reality. Recent neurophysiological evidence supports a 'binding-by-synchrony' hypothesis: neurons excited by features of the same object fire synchronously, while neurons excited by features of different objects do not. Moving plaid patterns offer a straightforward means to test this idea. By appropriate manipulations of apparent transparency, the component gratings of a plaid pattern can be seen as parts of a single coherently moving surface or as two non-coherently moving surfaces. We examined directional tuning and synchrony of area-MT neurons in awake, fixating primates in response to perceptually coherent and non-coherent plaid patterns. Here we show that directional tuning correlated highly with perceptual coherence, which is consistent with an earlier study. Although we found stimulus-dependent synchrony, coherent plaids elicited significantly less synchrony than did non-coherent plaids. Our data therefore do not support the binding-by-synchrony hypothesis as applied to this class of motion stimuli in area MT.     

Structure of the Escherichia coli ribosomal termination complex with release factor 2

Termination of protein synthesis occurs when the messenger RNA presents a stop codon in the ribosomal aminoacyl (A) site. Class I release factor proteins (RF1 or RF2) are believed to recognize stop codons via tripeptide motifs, leading to release of the completed polypeptide chain from its covalent attachment to transfer RNA in the ribosomal peptidyl (P) site. Class I RFs possess a conserved GGQ amino-acid motif that is thought to be involved directly in protein-transfer-RNA bond hydrolysis. Crystal structures of bacterial and eukaryotic class I RFs have been determined, but the mechanism of stop codon recognition and peptidyl-tRNA hydrolysis remains unclear. Here we present the structure of the Escherichia coli ribosome in a post-termination complex with RF2, obtained by single-particle cryo-electron microscopy (cryo-EM). Fitting the known 70S and RF2 structures into the electron density map reveals that RF2 adopts a different conformation on the ribosome when compared with the crystal structure of the isolated protein. The amino-terminal helical domain of RF2 contacts the factor-binding site of the ribosome, the 'SPF' loop of the protein is situated close to the mRNA, and the GGQ-containing domain of RF2 interacts with the peptidyl-transferase centre (PTC). By connecting the ribosomal decoding centre with the PTC, RF2 functionally mimics a tRNA molecule in the A site. Translational termination in eukaryotes is likely to be based on a similar mechanism.     

A camelid antibody fragment inhibits the formation of amyloid fibrils by human lysozyme

Amyloid diseases are characterized by an aberrant assembly of a specific protein or protein fragment into fibrils and plaques that are deposited in various organs and tissues, often with serious pathological consequences. Non-neuropathic systemic amyloidosis is associated with single point mutations in the gene coding for human lysozyme. Here we report that a single-domain fragment of a camelid antibody raised against wild-type human lysozyme inhibits the in vitro aggregation of its amyloidogenic variant, D67H. Our structural studies reveal that the epitope includes neither the site of mutation nor most residues in the region of the protein structure that is destabilized by the mutation. Instead, the binding of the antibody fragment achieves its effect by restoring the structural cooperativity characteristic of the wild-type protein. This appears to occur at least in part through the transmission of long-range conformational effects to the interface between the two structural domains of the protein. Thus, reducing the ability of an amyloidogenic protein to form partly unfolded species can be an effective method of preventing its aggregation, suggesting approaches to the rational design of therapeutic agents directed against protein deposition diseases.     

拉伐尔喷嘴中气粉流的数学模型

考虑粉粒体积分率及粒径分布对流动的影响，建立了描述拉伐尔喷嘴中超声速气粉流行为的数学模型。     

等直径喷嘴中气粉两相流粉粒速度的实验研究

利用粉粒光电分光器对等直径喷嘴中粉粒速度进行了实验研究，在实验的基础上，提出了用实测喷嘴内初始运动参数和出口压强确定喷嘴内粉粒初始速度的方法，结果表明，喷嘴内粉粒速度的计算结果同实测结果基本一致。