The GTPase superfamily: conserved structure and molecular mechanism

GTPases are conserved molecular switches, built according to a common structural design. Rapidly accruing knowledge of individual GTPases--crystal structures, biochemical properties, or results of molecular genetic experiments--support and generate hypotheses relating structure to function in other members of the diverse family of GTPases.     

Deformed autoregulatory element from Drosophila functions in a conserved manner in transgenic mice.

The striking similarities in the structure, organization and anterior-posterior expression patterns between the murine Hox gene system and the Drosophila homeotic gene complexes, called HOM-C (ref. 3), may point to highly conserved mechanisms for specifying positional identities (reviewed in ref. 4). Strong support for this concept lies in the observation of conserved colinearity between the genomic order of the Hox/HOM genes and their unique successive expression domains along the anterior-posterior axes of both mouse and fly embryos. These unique and precise expression patterns appear to be facilitated by multiple cis-regulatory elements (reviewed in ref. 5). One of the few elements characterized in detail is the autoregulatory enhancer of the homeotic gene Deformed (Dfd), which supports expression in subregions of posterior head segments of Drosophila embryos. Here we present evidence that this enhancer is capable of conferring reporter gene expression to a discrete subregion of the hindbrain in transgenic mouse embryos. Remarkably, this anterior-posterior subregion lies within the common anterior expression domain of the Dfd cognate Hox genes in the postotic hindbrain. Our results indicate that the Dfd autoregulatory enhancer is part of a highly conserved mechanism for establishing region-specific gene expression along the anterior-posterior axis of the embryo.     

The essential bacterial cell-division protein FtsZ is a GTPase.

Cytokinesis defines the last stage in the division cycle, in which cell constriction leads to the formation of daughter cells. The biochemical mechanisms responsible for this process are poorly understood. In bacteria, the ftsZ gene product, FtsZ, is required for cell division, playing a prominent role in cytokinesis. The cellular concentration of FtsZ regulates the frequency of division and genetic studies have indicated that it is the target of several endogenous division inhibitors. At the time of onset of septal invagination, the FtsZ protein is recruited from the cytoplasm to the division site, where it assembles into a ring that remains associated with the leading edge of the invaginating septum until septation is completed. Here we report that FtsZ specifically binds and hydrolyses GTP. The reaction can be dissociated into a GTP-dependent activation stage that is markedly affected by the concentration of FtsZ, and a hydrolysis stage in which GTP is hydrolysed to GDP. The results indicate that GTP binding and hydrolysis are important in enabling FtsZ to support bacterial cytokinesis, either by facilitating the assembly of the FtsZ ring and/or by catalysing an essential step in the cytokinetic process itself.     

UV light-induced immunoglobulin heavy-chain class switch in a human lymphoblastoid cell line

During attempts to select nonsecretory variants from 0.467.3, and Epstein-Barr virus-transformed human lymphoblastoid cell line that secretes small amounts of IgM lambda, we exposed the cells to UV light. Cells that survived the irradiation were subcultured and their supernatants were screened for immunoglobulin production by an enzyme-linked immunosorbent assay (ELISA). Although stable nonsecretory variants were not isolated, we report here that an immunoglobulin class switch occurred in the UV-treated cell population. All survivors were found to produce large quantities of IgG lambda. Some cell cultures also produced the original IgM lambda. The UV-light-induced class switch was regularly reproducible with this target cell line.     

细胞不对称分裂机制--芽殖酵母接合型不对称转换

芽列酵母的母细胞与子细胞呈不对称接合型转换，其原因是只有母细胞可表达编码核酸内切酶的基因HO，使相反接合型的缄默基因转位到活动位点取代了原来的接合型基因。HO的不对称表达是因在细胞分裂的末期至G1早期，子细胞核中存在有Ashlp转录抑制因子。Ashlp的不对称分布是由其mRNA的定向转运而实现的：ASH1 mRNA在有丝分裂期被转录出之后，通过接头蛋白She2p和She3p与肌球蛋白Myo4p结合成核糖核蛋白颗粒，经肌动蛋白纤维转运到子细胞远端皮层而锚定并翻译。     

一类非线性泛函最小元的必要条件

研究了一类非线性泛函最小元.这类泛函是一维含杂质超导模型的Ginzburg-Landau泛函当Ginzburg-Landau参数趋于无穷时的极限.这个泛函在不同的参数条件下具有多个临界点,我们给出了判断这类泛函临界点是最小元的必要条件.     

The immunoglobulin superfamily protein Izumo is required for sperm to fuse with eggs

Representing the 60 trillion cells that build a human body, a sperm and an egg meet, recognize each other, and fuse to form a new generation of life. The factors involved in this important membrane fusion event, fertilization, have been sought for a long time. Recently, CD9 on the egg membrane was found to be essential for fusion, but sperm-related fusion factors remain unknown. Here, by using a fusion-inhibiting monoclonal antibody and gene cloning, we identify a mouse sperm fusion-related antigen and show that the antigen is a novel immunoglobulin superfamily protein. We have termed the gene Izumo and produced a gene-disrupted mouse line. Izumo-/- mice were healthy but males were sterile. They produced normal-looking sperm that bound to and penetrated the zona pellucida but were incapable of fusing with eggs. Human sperm also contain Izumo and addition of the antibody against human Izumo left the sperm unable to fuse with zona-free hamster eggs.     

Hebbian-based mean shift for learning the diverse shapes of V1 simple cell receptive fields

The L0-norm constraint in sparse coding has the advantage of producing the same diversity of receptive field shapes as physiology data,but is difficult for analysis.It remains a challenging issue to understand how the diverse shapes of V1 simple cell receptive fields emerge in visual cortex.This paper presents a biologically plausible learning algorithm,named Hebbian-based mean shift,for this problem.The L0-norm constraint optimizes the number of basis functions rather than their coefficients.We report that the optimization procedure is essentially a 0–1 programming of the selection of basis functions.By assuming that the basis functions are independently selected from a basis set,we find the spatial distribution of input samples containing a special basis function has a star shape and peaks at this basis function.Thus,learning the basis functions for sparse coding with the L0-norm can be interpreted as mode detection where the basis functions are the modes of the kernel density estimate.We employ mean shift to detect modes and prove that the updating rule for the mean shift is Hebbian.The simulation results demonstrate the robustness of the proposed algorithm in producing both Gabor-like and blob-like basis functions.     

一种高线性度的射频收发开关的设计

基于0.18 μm射频锗硅工艺,提出了一种可广泛应用于无线通信系统的低插入损耗和高线性度的射频开关电路.该电路利用特殊的深N阱工艺、高衬底电阻器件,采用经典的串并联堆叠结构开关电路,实现了低插入损耗和高线性度的目的.测试结果显示:在频率为2.4 GHz下,插入损耗,隔离度和1 dB压缩点分别为-1 dB,- 34 dBm和23 dBm.     

一类代数体函数的唯一性

本文在有理函数集合 R_k 上,对那些支点增长较慢的代数体函数建立了两个唯一性定理。文中所得结果可以视作亚纯函数有关定理的推广。     

一类亚纯函数的分担值问题

在亚纯函数的Nevanlinna值分布理论的基础上,讨论了整函数的唯一性问题. 将(fn(f-1))(k)和(gn(g-1))(k)分担1CM的问题推广到(fn(f m-1))(k),(gn(gm-1))(k)分担1 CM的情形.     

The MAPK kinase Pek1 acts as a phosphorylation-dependent molecular switch.

The mitogen-activated protein kinase (MAPK) pathway is a highly conserved eukaryotic signalling cascade that converts extracellular signals into various outputs, such as cell growth and differentiation. MAPK is phosphorylated and activated by a specific MAPK kinase (MAPKK): MAPKK is therefore considered to be an activating regulator of MAPK. Pmk1 is a MAPK that regulates cell integrity and which, with calcineurin phosphatase, antagonizes chloride homeostasis in fission yeast. We have now identified Pek1, a MAPKK for Pmk1 MAPK. We show here that Pek1, in its unphosphorylated form, acts as a potent negative regulator of Pmk1 MAPK signalling. Mkh1, an upstream MAPKK kinase (MAPKKK), converts Pek1 from being an inhibitor to an activator. Our results indicate that Pek1 has a dual stimulatory and inhibitory function which depends on its phosphorylation state. This switch-like mechanism could contribute to the all-or-none physiological response mediated by the MAPK signalling pathway.