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1.
A reevaluation of the secondary structure of Na, Ca and K channel proteins led to the following results. Only three segments (S1, S5 and S6) of each repeat are sufficiently hydrophobic to be predicted as transmembrane helices, if a window of 19 amino acids is used. Some of the S2 and S3 segments show higher hydrophobic values when calculated with the window of 9 amino acids and can be predicted as short helices. S4 segments are strongly hydrophilic and cannot be predicted as transmembrane helices. Some of the S2, S3 and S4 segments have an amphipathic character; however, these helices do not span a membrane. A model is proposed where 12 hydrophobic transmembrane helices surround 12 shorter helices, forming a hydrophilic pore. In addition, a unique pattern for S4 segments of voltage-gated channel proteins is defined. 相似文献
2.
A reevaluation of the secondary structure of Na, Ca and K channel proteins led to the following results. Only three segments (S1, S5 and S6) of each repeat are sufficiently hydrophobic to be predicted as transmembrane helices, if a window of 19 amino acids is used. Some of the S2 and S3 segments show higher hydrophobic values when calculated with the window of 9 amino acids and can be predicted as short helices. S4 segments are strongly hydrophilic and cannot be predicted as transmembrane helices. Some of the S2, S3 and S4 segments have an amphipathic character; however, these helices do not span a membrane. A model is proposed where 12 hydrophobic transmembrane helices surround 12 shorter helices, forming a hydrophilic pore. In addition, a unique pattern for S4 segments of voltage-gated channel proteins is defined. 相似文献
3.
Biological cells harbor a variety of molecular machines that carry out mechanical work at the nanoscale. One of these nanomachines
is the bacterial motor protein SecA which translocates secretory proteins through the protein-conducting membrane channel
SecYEG. SecA converts chemically stored energy in the form of ATP into a mechanical force to drive polypeptide transport through
SecYEG and across the cytoplasmic membrane. In order to accommodate a translocating polypeptide chain and to release transmembrane
segments of membrane proteins into the lipid bilayer, SecYEG needs to open its central channel and the lateral gate. Recent
crystal structures provide a detailed insight into the rearrangements required for channel opening. Here, we review our current
understanding of the mode of operation of the SecA motor protein in concert with the dynamic SecYEG channel. We conclude with
a new model for SecA-mediated protein translocation that unifies previous conflicting data. 相似文献
4.
Kleinschmidt JH 《Cellular and molecular life sciences : CMLS》2003,60(8):1547-1558
The biophysical principles and mechanisms by which membrane proteins insert and fold into a biomembrane have mostly been studied with bacteriorhodopsin and outer membrane protein A (OmpA). This review describes the assembly process of the monomeric outer membrane proteins of Gram-negative bacteria, for which OmpA has served as an example. OmpA is a two-domain outer membrane protein composed of a 171-residue eight-stranded -barrel transmembrane domain and a 154-residue periplasmic domain. OmpA is translocated in an unstructured form across the cytoplasmic membrane into the periplasm. In the periplasm, unfolded OmpA is kept in solution in complex with the molecular chaperone Skp. After binding of periplasmic lipopolysaccharide, OmpA insertion and folding occur spontaneously upon interaction of the complex with the phospholipid bilayer. Insertion and folding of the -barrel transmembrane domain into the lipid bilayer are highly synchronized, i.e. the formation of large amounts of -sheet secondary structure and -barrel tertiary structure take place in parallel with the same rate constants, while OmpA inserts into the hydrophobic core of the membrane. In vitro, OmpA can successfully fold into a range of model membranes of very different phospholipid compositions, i.e. into bilayers of lipids of different headgroup structures and hydrophobic chain lengths. Three membrane-bound folding intermediates of OmpA were discovered in folding studies with dioleoylphosphatidylcholine bilayers. Their formation was monitored by time-resolved distance determinations by fluorescence quenching, and they were structurally distinguished by the relative positions of the five tryptophan residues of OmpA in projection to the membrane normal. Recent studies indicate a chaperone-assisted, highly synchronized mechanism of secondary and tertiary structure formation upon membrane insertion of -barrel membrane proteins such as OmpA that involves at least three structurally distinct folding intermediates. 相似文献
5.
Diversity of Cl− Channels 总被引:5,自引:0,他引:5
Cl− channels are widely found anion pores that are regulated by a variety of signals and that play various roles. On the basis
of molecular biologic findings, ligand-gated Cl− channels in synapses, cystic fibrosis transmembrane conductors (CFTRs) and ClC channel types have been established, followed
by bestrophin and possibly by tweety, which encode Ca2+-activated Cl− channels. The ClC family has been shown to possess a variety of functions, including stabilization of membrane potential,
excitation, cellvolume regulation, fluid transport, protein degradation in endosomal vesicles and possibly cell growth. The
molecular structure of Cl− channel types varies from 1 to 12 transmembrane segments. By means of computer-based prediction, functional Cl− channels have been synthesized artificially, revealing that many possible ion pores are hidden in channel, transporter or
unidentified hydrophobic membrane proteins. Thus, novel Cl−-conducting pores may be occasionally discovered, and evidence from molecular biologic studies will clarify their physiologic
and pathophysiologic roles.
Received 28 July 2005; received after revision 25 August 2005; accepted 21 September 2005 相似文献
6.
Robert Renthal 《Cellular and molecular life sciences : CMLS》2010,67(7):1077-1088
Polytopic α-helical membrane proteins cannot spontaneously insert into lipid bilayers without assistance from polytopic α-helical
membrane proteins that already reside in the membrane. This raises the question of how these proteins evolved. Our current
knowledge of the insertion of α-helices into natural and model membranes is reviewed with the goal of gaining insight into
the evolution of membrane proteins. Topics include: translocon-dependent membrane protein insertion, antibiotic peptides and
proteins, in vitro insertion of membrane proteins, chaperone-mediated insertion of transmembrane helices, and C-terminal tail-anchored
(TA) proteins. Analysis of the E. coli genome reveals several predicted C-terminal TA proteins that may be descendents of proteins involved in pre-cellular membrane
protein insertion. Mechanisms of pre-translocon polytopic α-helical membrane protein insertion are discussed. 相似文献
7.
Sonja A. Kirsch Andreas Kugemann Armando Carpaneto Rainer A. Böckmann Petra Dietrich 《Cellular and molecular life sciences : CMLS》2018,75(20):3803-3815
Mammalian two-pore channels (TPCs) are activated by the low-abundance membrane lipid phosphatidyl-(3,5)-bisphosphate (PI(3,5)P2) present in the endo-lysosomal system. Malfunction of human TPC1 or TPC2 (hTPC) results in severe organellar storage diseases and membrane trafficking defects. Here, we compared the lipid-binding characteristics of hTPC2 and of the PI(3,5)P2-insensitive TPC1 from the model plant Arabidopsis thaliana. Combination of simulations with functional analysis of channel mutants revealed the presence of an hTPC2-specific lipid-binding pocket mutually formed by two channel regions exposed to the cytosolic side of the membrane. We showed that PI(3,5)P2 is simultaneously stabilized by positively charged amino acids (K203, K204, and K207) in the linker between transmembrane helices S4 and S5 and by S322 in the cytosolic extension of S6. We suggest that PI(3,5)P2 cross links two parts of the channel, enabling their coordinated movement during channel gating. 相似文献
8.
Sterol carrier protein-2: structure reveals function 总被引:5,自引:0,他引:5
Stolowich NJ Petrescu AD Huang H Martin GG Scott AI Schroeder F 《Cellular and molecular life sciences : CMLS》2002,59(2):193-212
The multiple actions of sterol carrier protein-2 (SCP-2) in intracellular lipid circulation and metabolism originate from its gene and protein structure. The SCP-x/pro-SCP-2 gene is a fusion gene with separate initiation sites coding for 15-kDa pro-SCP-2 (no enzyme activity) and 58-kDa SCP-x (a 3-ketoacyl CoA thiolase). Both proteins share identical cDNA and amino acid sequences for 13-kDa SCP-2 at their C-termini. Cellular 13-kDa SCP-2 derives from complete, posttranslational cleavage of the 15-kDa pro-SCP-2 and from partial posttranslational cleavage of 58-kDa SCP-x. Putative physiological functions of SCP-2 have been proposed on the basis of enhancement of intermembrane lipid transfer (e.g., cholesterol, phospholipid) and activation of enzymes involved in fatty acyl CoA transacylation (cholesterol esters, phosphatidic acid) in vitro, in transfected cells, and in genetically manipulated animals. At least four important SCP-2 structural domains have been identified and related to specific functions. First, the 46-kDa N-terminal presequence present in 58-kDa SCP-x is a 3-ketoacyl-CoA thiolase specific for branched-chain acyl CoAs. Second, the N-terminal 20 amino acid presequence in 15-kDa pro-SCP-2 dramatically modulates the secondary and tertiary structure of SCP-2 as well as potentiating its intracellular targeting coded by the C-terminal peroxisomal targeting sequence. Third, the N-terminal 32 amino acids form an amphipathic a-helical region, one face of which represents a membrane-binding domain. Positively charged amino acid residues in one face of the amphipathic helices allow SCP-2 to bind to membrane surfaces containing anionic phospholipids. Fourth, the hydrophobic faces of the N-terminal amphipathic a helices along with beta strands 4, 5, and helix D form a ligand-binding cavity able to accommodate multiple types of lipids (e. g., fatty acids, fatty acyl CoAs, cholesterol, phospholipids, isoprenoids). Two-dimensional 1H-15N heteronuclear single quantum coherence spectra of both apo-SCP-2 and of the 1:1 oleate-SCP-2 complex, obtained at pH 6.7, demonstrated the homogenous formation of holo-SCP-2. While comparison of the apo- and holoprotein amide fingerprints revealed about 60% of the resonances remaining essentially unchanged, 12 assigned amide residues underwent significant chemical-shift changes upon oleic acid binding. These residues were localized in three regions: the juncture of helices A and B, the mid-section of the beta sheet, and the interface formed by the region of beta strands 4, 5, and helix D. Circular dichroism also showed that these chemical-shift changes, upon oleic acid binding, did not alter the secondary structure of SCP-2. The nuclear magnetic resonance chemical shift difference data, along with mapping of the nearby hydrophobic residues, showed the oleic acid-binding site to be comprised of a pocket created by the face of the beta sheet, helices A and B on one end, and residues associated with beta strands 4, 5, and helix D at the other end of the binding cavity. Furthermore, the hydrophobic nature of the previously ill-defined C-terminus suggested that these 20 amino acids may form a 'hydrophobic cap' which closes around the oleic acid upon binding. Thus, understanding the structural domains of the SCP-x/pro-SCP-2 gene and its respective posttranslationally processed proteins has provided new insights into their functions in intracellular targeting and metabolism of lipids. 相似文献
9.
Elektronenmikroskopische Untersuchung der phäochromen (chromaffinen) Granula in den Markzellen der Nebenniere 总被引:4,自引:0,他引:4
Summary The catechol-containing granules in the adrenal medullary cells (mouse, guinea pig, cat) are true cell organells, clearly distinguished from mitochondria; their size varies over a fairly wide range, the mean diameter being calculated at about 175 mµ in the mouse (osmium-tetroxide-fixation). They have a surrounding membrane of 100 Å thickness; their internal structure is finely granular. On the thin sections, many granules are surrounded by spaces which appear empty under the electron microscope and which are limited by double-membranes; it is most likely thatintra vitam these spaces contain lipids.
Ausgeführt mit Unterstützung der Friedrich-Baur-Stiftung und der Deutschen Forschungsgemeinschaft. 相似文献
Ausgeführt mit Unterstützung der Friedrich-Baur-Stiftung und der Deutschen Forschungsgemeinschaft. 相似文献
10.
Marsh D 《Cellular and molecular life sciences : CMLS》2003,60(8):1575-1580
Magnetic resonance results, principally from 2H-nuclear magnetic resonance, indicate that the mean lipid-chain ordering at the surface of transmembrane proteins is comparable to that in fluid lipid bilayers. Principally, it is the requirement for matching the hydrophobic lengths of lipid and protein that modulates the degree of chain ordering at the lipid-protein interface. The distribution of chain order parameters is, nonetheless, broader in the presence of integral proteins than in fluid lipid bilayers. The chain configurations of the phospholipids that are resolved in crystals of integral membrane proteins display considerable conformational heterogeneity. Chain C–C dihedral angles are, however, not restricted to the energetically allowable trans and gauche rotamers. This indicates that the chains of a given lipid do not have a unique configuration in protein crystals. 相似文献
11.
Sebastian Fiedler Jana Broecker Sandro Keller 《Cellular and molecular life sciences : CMLS》2010,67(11):1779-1798
Separation of cells and organelles by bilayer membranes is a fundamental principle of life. Cellular membranes contain a baffling
variety of proteins, which fulfil vital functions as receptors and signal transducers, channels and transporters, motors and
anchors. The vast majority of membrane-bound proteins contain bundles of α-helical transmembrane domains. Understanding how
these proteins adopt their native, biologically active structures in the complex milieu of a membrane is therefore a major
challenge in today’s life sciences. Here, we review recent progress in the folding, unfolding and refolding of α-helical membrane
proteins and compare the molecular interactions that stabilise proteins in lipid bilayers. We also provide a critical discussion
of a detergent denaturation assay that is increasingly used to determine membrane-protein stability but is not devoid of conceptual
difficulties. 相似文献
12.
Biological membrane fusion is driven by different types of molecular fusion machines. Most of these proteins are membrane-anchored
by single transmembrane domains. SNARE proteins are essential for intracellular membrane fusion along the secretory and endocytic
pathway, while various viral fusogens mediate infection of eukaryotic cells by enveloped viruses. Although both types of fusion
proteins are evolutionarily quite distant from each other, they do share a number of structural and functional features. Their
transmembrane domains are now known to be critical for the fusion reaction. We discuss at which stages they might contribute
to bilayer mixing.
Received 5 October 2006; received after revision 14 November 2006; accepted 8 January 2007 相似文献
13.
Jonas Protze Doreen Braun Katrin Manuela Hinz Dorothea Bayer-Kusch Ulrich Schweizer Gerd Krause 《Cellular and molecular life sciences : CMLS》2017,74(12):2299-2318
Monocarboxylate transporter 8 (MCT8) mediates thyroid hormone (TH) transport across the plasma membrane in many cell types. In order to better understand its mechanism, we have generated three new MCT8 homology models based on sugar transporters XylE in the intracellular opened (PDB ID: 4aj4) and the extracellular partly occluded (PDB ID: 4gby) conformations as well as FucP (PDB ID: 3o7q) and GLUT3 (PDB ID: 4zwc) in the fully extracellular opened conformation. T3-docking studies from both sides revealed interactions with His192, His415, Arg445 and Asp498 as previously identified. Selected mutations revealed further transport-sensitive positions mainly at the discontinuous transmembrane helices TMH7 and 10. Lys418 is potentially involved in neutralising the charge of the TH substrate because it can be replaced by charged, but not by uncharged, amino acids. The side chain of Thr503 was hypothesised to stabilise a helix break at TMH10 that undergoes a prominent local shift during the transport cycle. A T503V mutation accordingly affected transport. The aromatic Tyr419, the polar Ser313 and Ser314 as well as the charged Glu422 and Glu423 lining the transport channel have been studied. Based on related sugar transporters, we suggest an alternating access mechanism for MCT8 involving a series of amino acid positions previously and newly identified as critical for transport. 相似文献
14.
Alexander Negoda Elizabeth A. Cowley Yassine El Hiani Paul Linsdell 《Cellular and molecular life sciences : CMLS》2018,75(16):3027-3038
Cystic fibrosis can be treated by potentiators, drugs that interact directly with the cystic fibrosis transmembrane conductance regulator (CFTR) Cl? channel to increase its open probability. These substances likely target key conformational changes occurring during channel opening and closing, however, the molecular bases of these conformational changes, and their susceptibility to manipulation are poorly understood. We have used patch clamp recording to identify changes in the three-dimensional organization of the extracellularly accessible parts of the CFTR protein during channel opening and closing. State-dependent formation of both disulfide bonds and Cd2+ bridges occurred for pairs of cysteine side-chains introduced into the extreme extracellular ends of transmembrane helices (TMs) 1, 6, and 12. Between each of these three TMs, we found that both disulfide bonds and metal bridges formed preferentially or exclusively in the closed state and that these inter-TM cross-links stabilized the closed state. These results indicate that the extracellular ends of these TMs are close together when the channel is closed and that they separate from each other when the channel opens. These findings identify for the first time key conformational changes in the extracellular parts of the CFTR protein that can potentially be manipulated to control channel activity. 相似文献
15.
R. Benz 《Cellular and molecular life sciences : CMLS》1990,46(2):131-137
Summary The matrix space of mitochondria is surrounded by two membranes. The mitochondrial inner membrane contains the respiration chain and a large number of highly specific carriers for the mostly anionic substrates of mitochondrial metabolism. In contrast to this the permeability properties of the mitochondrial outer membrane are by far less specific. It acts as a molecular sieve for hydrophilic molecules with a defined exclusion limit around 3000 Da. Responsible for the extremely high permeability of the mitochondrial outer membrane is the presence of a pore-forming protein termed mitochondrial porin. Mitochondrial porins have been isolated from a variety of eukaryotic cells. They are basic proteins with molecular masses between 30 and 35 kDa. Reconstitution experiments define their function as pore-forming components with a single-channel conductance of about 0.40 nS (nano Siemens) in 0.1 M KCl at low voltages. In the open state mitochondrial porin behaves as a general diffusion pore with an effective diameter of 1.7 nm. Eukaryotic porins are slightly anion-selective in the open state but become cation-selective after voltage-dependent closure. 相似文献
16.
R Benz 《Experientia》1990,46(2):131-137
The matrix space of mitochondria is surrounded by two membranes. The mitochondrial inner membrane contains the respiration chain and a large number of highly specific carriers for the mostly anionic substrates of mitochondrial metabolism. In contrast to this the permeability properties of the mitochondrial outer membrane are by far less specific. It acts as a molecular sieve for hydrophilic molecules with a defined exclusion limit around 3000 Da. Responsible for the extremely high permeability of the mitochondrial outer membrane is the presence of a pore-forming protein termed mitochondrial porin. Mitochondrial porins have been isolated from a variety of eukaryotic cells. They are basic proteins with molecular masses between 30 and 35 kDa. Reconstitution experiments define their function as pore-forming components with a single-channel conductance of about 0.40 nS (nano Siemens) in 0.1 M KCl at low voltages. In the open state mitochondrial porin behaves as a general diffusion pore with an effective diameter of 1.7 nm. Eukaryotic porins are slightly anion-selective in the open state but become cation-selective after voltage-dependent closure. 相似文献
17.
Antonenkov VD Rokka A Sormunen RT Benz R Hiltunen JK 《Cellular and molecular life sciences : CMLS》2005,62(23):2886-2895
Mouse liver peroxisomes were isolated by centrifugation in a self-generated Percoll gradient followed by an Optiprep density
gradient centrifugation. Peroxisomes contributed 90–96% of the total protein content in the fraction, as confirmed by marker
enzyme assays, protein pattern in SDS-PAGE, immunoblotting, and electron microscopy. Solubilized peroxisomal membrane proteins
were reconstituted into a planar lipid bilayer. A single-channel conductance monitoring of the reconstituted lipid bilayer
revealed the presence of two pore-forming components with a conductance in 1 M KCl of 1.3 nS and 2.5 nS. Control experiments
with fractions enriched in mitochondria, lysosomes, and fragments of endoplasmic reticulum showed that the peroxisomal channel-forming
activities were not due to admixture of isolated peroxisomes with other cellular organelles. The peroxisomal channels were
well preserved in membrane preparations but became unstable after solubilization from the membranes by detergent.
Received 27 May 2005; received after revision 23 September 2005; accepted 11 October 2005 相似文献
18.
S Mutaftschiev J Olive E Azoulay 《Comptes rendus des séances de l'Académie des sciences. Série D, Sciences naturelles》1976,283(7):825-828
By freeze-fracturing it is shown that the vesicles reconstituted by complementation of the chlA and chlB mutants of E. coli K 12 extracts are characterized by an asymmetric membrane bilayer. In a feature quite similar to the original intact plasma membranes, the membrane splits in two halves and the intramembranous particles are asymmetrically distributed on the two facture faces. It is proposed that the process of membrane reconstitution, which is also associated with the restoration of nitrate-reductase activity, relies on a sequence of increasing complexity of the molecular organisation. 相似文献
19.
C. N. Sun 《Cellular and molecular life sciences : CMLS》1964,20(9):497-498
Summary The leaves ofAbutilon striatum v. Thompson infected with chlorosis proved to contain spheroid virus particles of about 800 Å diameter. Each particle consisted of a central dark staining core of about 160 Å. This central core is surrounded by an inner and an outer envelope. The particles are found in the cytoplasm, and there is evidence to show that virus particles are able to penetrate into the chloroplasts. 相似文献
20.
M S Felter A Stahl 《Comptes rendus des séances de l'Académie des sciences. Série D, Sciences naturelles》1975,280(16):1903-1906
The polyphosphate-synthetase, isolated from a homogenate of phosphate starved cells, catalyses the synthesis of linear polyphosphates from orthophosphate. It is localized in the membrane fraction which deposits between 400 and 1000 X g; its optimal pH is 7.1; its KM toward orthophosphate is 4.0 X 10(-4) M; ATP stimulates the reaction. The enzyme synthezises especially polyphosphates with short chain length. 相似文献