Transmembrane communication in cells chronically infected with measles virus

The transmembrane association of the measles virus hemagglutinin and hemolysin surface proteins with intracellular viral antigens was studied. Rabbit antisera monospecific for measles virus matrix and nucleocapsid proteins and a human antiserum containing specificities for both the hemagglutinin and hemolysin proteins were used to study the co-capping of these proteins in human Lu 106 cell-line, chronically infected with measles virus. Capping of the surface-associated envelope components was accompanied by co-capping of the matrix and nucleocapsid proteins, the latter being localized mainly within the inclusions. This demonstrated transmembrane communication between surface-associated envelope components and the intracellular measles virus matrix and nucleocapsid proteins. The results demonstrated the existence of a linkage between viral inclusions and viral proteins associated with cell membranes. In the presence of cytochalasin B (1--2 micrograms/ml), co-capping of the matrix protein was unchanged or slightly enhanced, whereas co-capping of the nucleocapsid protein decreased, indicating that actin filaments may mediate the communication between viral nucleocapsids and the cell membrane.     

Transmembrane proteases as disease markers and targets for therapy

Transmembrane proteases (i.e. membrane-associated proteases, ectoproteases) are present in a wide variety of tissues and cell types including endothelial, epithelial and hematopoietic cells. Natural and synthetic inhibitors have been characterized and have revealed that certain ectoenzymes are able to modulate bioactive peptide responses and to influence major biological events such as cell proliferation, survival and invasiveness. Dysregulated expression of some of them in human diseases triggers research on their role in pathophysiology, on their value as disease markers and as putative targets for therapy.     

Interferon-induced Transmembrane Protein 3 Is a Type II Transmembrane Protein

The interferon-induced transmembrane (IFITM) proteins are a family of small membrane proteins that inhibit the cellular entry of several genera of viruses. These proteins had been predicted to adopt a two-pass, type III transmembrane topology with an intracellular loop, two transmembrane helices (TM1 and TM2), and extracellular N and C termini. Recent work, however, supports an intramembrane topology for the helices with cytosolic orientation of both termini. Here we determined the topology of murine Ifitm3. We found that the N terminus of Ifitm3 could be stained by antibodies at the cell surface but that this conformation was cell type-dependent and represented a minority of the total plasma membrane pool. In contrast, the C terminus was readily accessible to antibodies at the cell surface and extracellular C termini comprised most or all of those present at the plasma membrane. The addition of a C-terminal KDEL endoplasmic reticulum retention motif to Ifitm3 resulted in sequestration of Ifitm3 in the ER, demonstrating an ER-luminal orientation of the C terminus. C-terminal, but not N-terminal, epitope tags were also degraded within lysosomes, consistent with their luminal orientation. Furthermore, epitope-tagged Ifitm3 TM2 functioned as a signal anchor sequence when expressed in isolation. Collectively, our results demonstrate a type II transmembrane topology for Ifitm3 and will provide insight into its interaction with potential targets and cofactors.     

Cadherin-Mediated Transmembrane Interactions

We show in this study that cadherin ligands, either soluble or immobilized on different surfaces, can bind to cells carrying a compatible cadherin and induce long-range signals which affect cell adhesion and dynamics. Addition of recombinant N-cadherin extracellular domain (NEC) to CHO cells expressing N-cadherin (FL4) greatly enhanced the calcium-dependent aggregation of the cells and blocked their migration into an “in vitro wound”. Monoclonal antibody which blocks cadherin interactions inhibited the aggregation of suspended FL4 cells and facilitated the “wound closure”. As previously shown (Levenberg et al., 1998) synthetic beads coupled to NEC interacted specifically with the surface of FL4 cells and significantly enhanced the formation of adherens junctions. This effect was obtained also with the parental CHO cells, which contain low levels of N-cadherin, and in additional N-cadherin expressing cells such as cultured myoblasts. We further show here that stimulation of adhesion is not affected by the geometry of the NEC-bound surface and that cells plated on flat NEC-coated substratum also develop enhanced adherens junctions. Interaction of cells expressing low levels of endogenous N-cadherin, such as CHO cells with surface-immobilized N-cadherin ligands had a prominent effect also on the total level of N-cadherin and β-catenin in the cells, probably due to stabilization of the cadherin-catenin complex by the interaction with the external surface.     

Transmembrane segment proteases

Summary Transmembrane segment proteases comprise a novel class of proteases that cleave substrates within hydrophobic membrane-spanning segments. They cleave in parts of proteins that upon first glance should be protected by the hydrophobic environment of the lipid bilayer. At present, no such protease has been isolated and biochemically characterized. They are defined according to the appearance of the respective cleavage products. All trans-membrane segment proteases seem to participate in a regulated two-step proteolytic process that plays a central role in cellular regulation or is part of a protein degradation pathway.Abbreviations -APP -amyloid precursor protein - S1P site-1 protease - S2P site-2 protease - SPase signal peptidase - SPPase signal peptide peptidase - SREBP sterol regulatory element-binding protein - SCAP SREBP cleavage-activating protein     

Transmembrane signaling in cilia and flagella

Summary Ciliary and flagellar membranes are dynamic. Ciliary and flagellar membranes have diverged widely during evolution and perform many specialized functions. Transmembrane signaling is an important component of the function of ciliary and flagellar surfaces in general. In this review, I discuss some of the functions performed by ciliary and flagellar surfaces and I present three different ciliary and flagellar signaling systems associated with rather different dynamic events performed by ciliary and flagellar surfaces. Two of these are associated withChlamydomonas flagella and one is associated with vertebrate olfactory cilia. Calcium regulation of protein phosphorylation appears to be important in regulating glycoprotein movements in theChlamydomonas flagellar membrane. Changes in levels of cAMP and cAMP-dependent protein phosphorylation are clearly central to the signaling associated with mating events in gametic flagella ofChlamydomonas, although calcium clearly has an important, if poorly understood, role to play. There is no known role for G proteins in flagellar membrane events inChlamydomonas. In contrast, mammalian olfactory cilia possess an odorant activated, G protein regulated adenylate cyclase and conductance channels that are directly gated by cyclic nucleotides. A second class of odorants that do not affect adenylate cyclase activity appear to act through G protein activated phospholipase C and changes in IP₃ second messenger levels. These examples demonstrate the diversity in the signaling pathways associated with ciliary and flagellar membranes.Abbreviations CaPK-2 calcium-dependent protein kinase - db-cAMP dibutyryl cAMP - Fab fragment antigen binding - IgE immunoglobulin E - IP₃ myo-inositol trisphosphate - IP₄ myo-inositol tetrakisphosphate - OBP odorant binding protein - PIP₂ phosphoinositol bisphosphate - TFP trifluoperazine - WGA wheat germ agglutinin     

Organelle communication: Signaling crossroads between homeostasis and disease

Cellular organelles do not function as isolated or static units, but rather form dynamic contacts between one another that can be modulated according to cellular needs. The physical interfaces between organelles are important for Ca²⁺ and lipid homeostasis, and serve as platforms for the control of many essential functions including metabolism, signaling, organelle integrity and execution of the apoptotic program. Emerging evidence also highlights the importance of organelle communication in disorders such as Alzheimer's disease, pulmonary arterial hypertension, cancer, skeletal and cardiac muscle dysfunction. Here, we provide an overview of the current literature on organelle communication and the link to human pathologies.     

Strong Correlation Between Statistical Transmembrane Tendency and Experimental Hydrophobicity Scales for Identification of Transmembrane Helices

Direct physical chemistry measurements of the hydrophobicity of amino acids or their derivatives have often been used to estimate the propensity of amino acids to participate in transmembrane helices. In this short note, it is found that there is a very high degree of correlation (r = 0.944–0.965) between an average physical chemistry hydrophobicity scale (an average of scales derived, e.g., from the solubility of amino acid derivatives in organic solvents versus water or their binding to hydrophobic particles) and the statistically based transmembrane tendency scale (derived from the relative abundance of residues in known transmembrane and soluble protein sequences (Zhao and London, Protein Sci 15:1987–2001, 2006)). This correlation indicates that, other than hydrophobicity, amino acid properties/interactions that promote or inhibit transmembrane helix formation in a specific membrane protein largely cancel out when averaged over all transmembrane sequences. In other words, other than hydrophobicity, there are no properties of a specific amino acid residue within a hydrophobic segment that have a strong systematic effect upon transmembrane helix formation independent of the remainder of the sequence in that hydrophobic segment. However, proline is an exception to this rule.     

线粒体跨膜电位与细胞凋亡

细胞凋亡作为细胞固有的、受机体严密调控的细胞死亡形式,在多细胞生物体清除衰老细胞及无能细胞等方面发挥重要的作用.近年来,细胞凋亡的研究重点已从细胞核转向线粒体.各种死亡信号诱导线粒体膜通透性改变孔（permeability transition pore, PT pore）开放,引起线粒体跨膜电位下降,导致促凋亡物质释放,继而激活caspase,最终使细胞凋亡.Bcl-2和Bcl-X_L通过对线粒体作用而抑制细胞凋亡,而Bax、Bak与Bad通过调节线粒体而诱导细胞凋亡.     

Transmembrane helix predictions revisited

Methods that predict membrane helices have become increasingly useful in the context of analyzing entire proteomes, as well as in everyday sequence analysis. Here, we analyzed 27 advanced and simple methods in detail. To resolve contradictions in previous works and to reevaluate transmembrane helix prediction algorithms, we introduced an analysis that distinguished between performance on redundancy-reduced high- and low-resolution data sets, established thresholds for significant differences in performance, and implemented both per-segment and per-residue analysis of membrane helix predictions. Although some of the advanced methods performed better than others, we showed in a thorough bootstrapping experiment based on various measures of accuracy that no method performed consistently best. In contrast, most simple hydrophobicity scale-based methods were significantly less accurate than any advanced method as they overpredicted membrane helices and confused membrane helices with hydrophobic regions outside of membranes. In contrast, the advanced methods usually distinguished correctly between membrane-helical and other proteins. Nonetheless, few methods reliably distinguished between signal peptides and membrane helices. We could not verify a significant difference in performance between eukaryotic and prokaryotic proteins. Surprisingly, we found that proteins with more than five helices were predicted at a significantly lower accuracy than proteins with five or fewer. The important implication is that structurally unsolved multispanning membrane proteins, which are often important drug targets, will remain problematic for transmembrane helix prediction algorithms. Overall, by establishing a standardized methodology for transmembrane helix prediction evaluation, we have resolved differences among previous works and presented novel trends that may impact the analysis of entire proteomes.     

Transmembrane signalling by integrins

Recent work has shown that integrin receptors serve not only as structural receptors that connect the extracellular matrix to the cytoskeleton, but also as signalling receptors that regulate intracellular pH, intracellular free calcium, phosphorylation of proteins on tyrosine and inositol lipid turnover. The ability of extracellular matrix to influence growth, differentiation and other cell functions is very likely related to their effects on signaling pathways inside the cell.     

Transmembrane movement of heme

Evidence for CO-heme partitioning into and across lipid bilayers was obtained by kinetic and chromatographic studies. Biphasic time courses were observed when CO-heme was rapidly mixed with unilamellar lipid vesicles in a stopped-flow spectrometer. The initial rapid phase depended linearly on lipid concentration and was assigned to heme partitioning between the external solvent phase and the outer lipid layer of the membranes. The rate of the second, much slower phase was independent of both heme and lipid concentration. The fraction of absorbance change associated with this slower phase increased with increasing heme to lipid ratios and reached a maximum of approximately 45%. A similar slow phase was observed when membrane-bound heme was reacted with apomyoglobin. In the presence of excess globin, all of the CO-heme was extracted from the membranes to form native CO myoglobin. Under these conditions, the fractional amount of absorbance change associated with the slow dissociation phase was approximately 45%, regardless of the heme to lipid ratio. These results suggest strongly that the slow phases represent transmembrane movement of heme, from the outer to the inner lipid layer in the association reactions and from the inner to the outer layer in dissociation reactions. The temperature dependence of the rate of CO-heme binding to the outer lipid layer was markedly different from that of transmembrane movement. The rate of the latter, slower process decreased greatly with increasing acyl chain length, whereas the rate of the initial binding process varied little with vesicle composition, as long as the membranes were examined above their melting temperatures. Finally, the two kinetically distinct bound heme fractions could be isolated directly by column chromatography.     

Transmembrane signalling and modulation of neutrophil behaviour

Neutrophils are blood cells which respond to chemotactic stimuli and phagocytize invading microorganisms. The activation of oriented locomotion, the formation of pseudopods, and the production and secretion of factors toxic to the endycytized organisms all require the transmission and amplification of external stimuli by the neutrophil plasma membrane. Ca²⁺ mobilization, the transient elevation of cyclic AMP, protein phosphorylation and lipid turnover may all be involved in transmembrane signalling and the modulation of neutrophil functions.     

Transmembrane topography and evolutionary conservation of synaptophysin

Synaptophysin is the major integral membrane protein of small synaptic vesicles. Its primary structure deduced from rat and human complementary DNA sequences predicts that synaptophysin contains four transmembrane regions and a carboxyl-terminal domain having a novel repetitive structure. To elucidate the transmembrane organization of this protein in the synaptic vesicle, five antipeptide antibodies were raised. The site-specific antibodies were used to map the cognate sequences to the cytoplasmic or intravesicular side of the synaptic vesicle membrane by determining the susceptibility of the epitopes to proteolysis. The results confirm a topographic model for synaptophysin in which the protein spans the vesicle membrane four times, with both the amino and carboxyl terminus being cytoplasmic. In addition, the evolutionary conservation of the synaptophysin domains was addressed as a function of their membrane localization. To this end the primary structure of bovine synaptophysin was determined. Sequence comparisons between bovine, rat, and human synaptophysin revealed that only the intravesicular loops showed a significant number of amino acid substitutions (22%), while the transmembrane regions and cytoplasmic sequences were highly conserved (3% substitutions). These results depict synaptophysin as a protein with multiple membrane spanning regions whose functional site is likely to reside in highly conserved intramembranous and cytoplasmic sequences.     

The Spindle-Associated Transmembrane Protein Axs Identifies a New Family of Transmembrane Proteins in Eukaryotes

Axs mutations disrupt both the progression of the meiotic cell cycle and meiotic chromosome segregation in Drosophila. Axs protein co-localizes with endoplasmic reticulum components and is present within a novel structure ensheathing the meiotic spindle. We show that Axs encodes the founding member of a eukaryotic family of trans-membrane proteins.     

Intercellular (mis)communication in neurodegenerative disease

Neurodegenerative diseases have been intensively studied, but a comprehensive understanding of their pathogenesis remains elusive. An increasing body of evidence suggests that non-cell-autonomous processes play critical roles during the initiation and spatiotemporal progression or propagation of the dominant pathology. Here, we review findings highlighting the importance of pathological cell-cell communication in neurodegenerative disease. We focus primarily on the accumulating evidence suggesting dysfunctional crosstalk between neurons and astroglia, neurons and innate immune system cells, as well as cellular processes leading to transmission of pathogenic proteins between cells. Insights into the complex intercellular perturbations underlying neurodegeneration will enhance our efforts to develop effective therapeutic approaches for preventing or reversing symptomatic progression in this devastating class of human diseases.     

Type II Transmembrane Serine Proteases

Analysis of genome and expressed sequence tag data bases at the turn of the millennium unveiled a new protease family named the type II transmembrane serine proteases (TTSPs) in a Journal of Biological Chemistry minireview (Hooper, J. D., Clements, J. A., Quigley, J. P., and Antalis, T. M. (2001) J. Biol. Chem. 276, 857–860). Since then, the number of known TTSPs has more than doubled, and more importantly, our understanding of the physiological functions of individual TTSPs and their contribution to human disease has greatly increased. Progress has also been made in identifying molecular substrates and endogenous inhibitors. This minireview summarizes the current knowledge of the rapidly advancing TTSP field.