Membrane Proteins: Structure, Function and Motion.

Cell membranes are intricate multicomponent supramolecular structures, with a complex variable morphology and chemical composition [...

New Insights into the Chloroplast Outer Membrane Proteome and Associated Targeting Pathways.

Plastids are a dynamic class of organelle in plant cells that arose from an ancient cyanobacterial endosymbiont. Over the course of evolution, most genes encoding plastid proteins were transferred to the nuclear genome. In parallel, eukaryotic cells evolved a series of targeting pathways and complex proteinaceous machinery at the plastid surface to direct these proteins back to their target organelle.

Uncoupling Proteins and Regulated Proton Leak in Mitochondria.

Higher concentration of protons in the mitochondrial intermembrane space compared to the matrix results in an electrochemical potential causing the back flux of protons to the matrix. This proton transport can take place through ATP synthase complex (leading to formation of ATP) or can occur via proton transporters of the mitochondrial carrier superfamily and/or membrane lipids. Some mitochondrial proton transporters, such as uncoupling proteins (UCPs), transport protons as their general regulating function; while others are symporters or antiporters, which use the proton gradient as a driving force to co-transport other substrates across the mitochondrial inner membrane (such as phosphate carrier, a symporter; or aspartate/glutamate transporter, an antiporter).

Biphasic Proton Transport Mechanism for Uncoupling Proteins.

It has been suggested that uncoupling proteins (UCPs) transport protons interconversion between two conformational states: one in the "cytoplasmic state" and the other in the "matrix state". Matrix and cytoplasmic salt-bridge networks are key controllers of these states. This study proposes a mechanism for proton transport in tetrameric UCP2, with focus on the role of the matrix network.

Systematic Design and Validation of Ion Channel Stabilization of Amphipathic α-Helical Peptides Incorporating Tryptophan Residues.

Aromatic interactions such as π-π interaction and cation-π interaction are present in membrane proteins and play important roles in both structure and function. To systematically investigate the effect of aromatic residues on the structural stability and ion permeability of peptide-formed ion channels, we designed several peptides with one or two tryptophan (Trp) residues incorporated at different positions in amphipathic α-helical peptides. Circular dichroism (CD) studies revealed the preferable position of Trp residues for self-association in these designed peptides.

Functional Oligomeric Forms of Uncoupling Protein 2: Strong Evidence for Asymmetry in Protein and Lipid Bilayer Systems.

Stoichiometry of uncoupling proteins (UCPs) and their coexistence as functional monomeric and associated forms in lipid membranes remain intriguing open questions. In this study, tertiary and quaternary structures of UCP2 were analyzed experimentally and through molecular dynamics (MD) simulations. UCP2 was overexpressed in the inner membrane of , then purified and reconstituted in lipid vesicles.

Conformational Changes and Association of Membrane-Interacting Peptides in Myelin Membrane Models: A Case of the C-Terminal Peptide of Proteolipid Protein and the Antimicrobial Peptide Melittin.

Model membranes composed of various lipid mixtures can segregate into liquid-ordered (Lo) and liquid-disordered (Ld) phases. In this study, lipid vesicles composed of mainly Lo or Ld phases as well as complex lipid systems representing the cytosolic leaflet of the myelin membrane were characterized by fluorescence resonance energy transfer with a donor/acceptor pair that preferentially partitioned into Lo or Ld phases, respectively. The fluidity of the lipid systems containing >30% cholesterol was modulated in the presence of the amphipathic peptide melittin.

A biophysical study on molecular physiology of the uncoupling proteins of the central nervous system.

Mitochondrial inner membrane uncoupling proteins (UCPs) facilitate transmembrane (TM) proton flux and consequently reduce the membrane potential and ATP production. It has been proposed that the three neuronal human UCPs (UCP2, UCP4 and UCP5) in the central nervous system (CNS) play significant roles in reducing cellular oxidative stress. However, the structure and ion transport mechanism of these proteins remain relatively unexplored.

Role of positively charged residues of the second transmembrane domain in the ion transport activity and conformation of human uncoupling protein-2.

Residing at the inner mitochondrial membrane, uncoupling protein-2 (UCP2) mediates proton transport from the intermembrane space (IMS) to the mitochondrial matrix and consequently reduces the rate of ATP synthesis in the mitochondria. The ubiquitous expression of UCP2 in humans can be attributed to the protein's multiple physiological roles in tissues, including its involvement in protective mechanisms against oxidative stress, as well as glucose and lipid metabolisms. Currently, the structural properties and ion transport mechanism of UCP2 and other UCP homologues remain poorly understood.

Folding and self-association of atTic20 in lipid membranes: implications for understanding protein transport across the inner envelope membrane of chloroplasts.

Background: The Arabidopsis thaliana protein atTic20 is a key component of the protein import machinery at the inner envelope membrane of chloroplasts. As a component of the TIC complex, it is believed to form a preprotein-conducting channel across the inner membrane.

Results: We report a method for producing large amounts of recombinant atTic20 using a codon-optimized strain of E.

Expression, folding, and proton transport activity of human uncoupling protein-1 (UCP1) in lipid membranes: evidence for associated functional forms.

Uncoupling protein-1 (UCP1) is abundantly expressed in the mitochondrial inner membrane of brown adipose tissues and has an important role in heat generation, mediated by its proton transport function. The structure and function of UCP1 are not fully understood, partially due to the difficulty in obtaining native-like folded proteins in vitro. In this study, using the auto-induction method, we have successfully expressed UCP1 in Escherichia coli membranes in high yield.

Dynamic turn conformation of a short tryptophan-rich cationic antimicrobial peptide and its interaction with phospholipid membranes.

Cationic antimicrobial peptides are promising sources for novel therapeutic agents against multi-drug-resistant bacteria. HHC-36 (KRWWKWWRR) is a simple but effective antimicrobial peptide with similar or superior activity compared with several conventional antibiotics. In this biophysical study, unique conformational properties of this peptide and some of its analogs as well as its interaction with lipid membranes are investigated in detail.

Tau-derived-hexapeptide 306VQIVYK311 aggregation inhibitors: nitrocatechol moiety as a pharmacophore in drug design.

The nitrocatechol derivatives tolcapone (1) and entacapone (2), used as adjunctive therapy in the treatment of Parkinson's disease, were investigated for their potential to inhibit the tau-derived-hexapeptide 306VQIVYK311. They were compared to small molecules that contain similar pharmacophores including the catechol derivatives (dopamine 3 and epinephrine 4), nitroderivatives (nifedipine 5 and chloramphenicol 6), nitrocatechol isomers (7 and 8), and a tolcapone derivative (13) lacking the nitrocatechol moiety. The aggregation kinetics by thioflavin S fluorescence assay indicates that both tolcapone (1) and entacapone (2) exhibit antiaggregation properties.

pH-induced changes in intrinsically disordered proteins.

Intrinsically disordered proteins are typically enriched in amino acids that confer a relatively high net charge to the protein, which is an important factor leading to the lack of a compact structure. There are many different approaches that can be used to experimentally confirm whether a protein is intrinsically disordered. One such approach takes advantage of the distinctive amino acid composition to test whether a protein is a genuine IDP.

Toward understanding the mechanism of ion transport activity of neuronal uncoupling proteins UCP2, UCP4, and UCP5.

Neuronal uncoupling proteins (UCP2, UCP4, and UCP5) have crucial roles in the function and protection of the central nervous system (CNS). Extensive biochemical studies of UCP2 have provided ample evidence of its participation in proton and anion transport. To date, functional studies of UCP4 and UCP5 are scarce.

Ion channel activity of transmembrane segment 6 of Escherichia coli proton-dependent manganese transporter.

Synthetic peptides corresponding to the sixth transmembrane segment (TMS6) of secondary-active transporter MntH (Proton-dependent Manganese Transporter) from Escherichia coli and its two mutations in the functionally important conserved histidine residue were used as a model for structure-function study of MntH. The secondary structure of the peptides was estimated in different environments using circular dichroism spectroscopy. These peptides interacted with and adopted helical conformations in lipid membranes.

The acidic domains of the Toc159 chloroplast preprotein receptor family are intrinsically disordered protein domains.

Background: The Toc159 family of proteins serve as receptors for chloroplast-destined preproteins. They directly bind to transit peptides, and exhibit preprotein substrate selectivity conferred by an unknown mechanism. The Toc159 receptors each include three domains: C-terminal membrane, central GTPase, and N-terminal acidic (A-) domains.

A comparative study on conformation and ligand binding of the neuronal uncoupling proteins.

Mitochondrial uncoupling proteins of the nervous system (UCPs 2, 4, and 5) have potential roles in the function and protection of the central nervous system (CNS). In the absence of structural information, conformations of the hexahistidine-tagged versions of all five human UCPs in liposomes were investigated for the first time, using far- and near-UV CD and fluorescence spectroscopy. Highly pure UCPs 1-5 were reconstituted in detergents and stable small unilamellar vesicles, appropriate for spectroscopic studies.

Chiral thiol-stabilized silver nanoclusters with well-resolved optical transitions synthesized by a facile etching procedure in aqueous solutions.

A novel approach of cyclic reduction in oxidative conditions has been developed to prepare a single dominant species of chiral thiol-stabilized silver nanoclusters (AgNCs). Such AgNCs, which are stable in solution for up to a few days, have been obtained for the first time. The generality of the established procedure is proven by using several enantiomeric water-soluble thiols, including glutathione, as protective ligands.

Effect of ring size on conformation and biological activity of cyclic cationic antimicrobial peptides.

In a series of cyclic peptides based on GS10, an analogue of gramicidin S (GS), the ring size was varied from 10 to 16 amino acids. Alternative addition of basic and hydrophobic amino acids to the original GS10 construct generated a variety of even-numbered rings, i.e.

Interaction of gramicidin S and its aromatic amino-acid analog with phospholipid membranes.

To investigate the mechanism of interaction of gramicidin S-like antimicrobial peptides with biological membranes, a series of five decameric cyclic cationic beta-sheet-beta-turn peptides with all possible combinations of aromatic D-amino acids, Cyclo(Val-Lys-Leu-D-Ar1-Pro-Val-Lys-Leu-D-Ar2-Pro) (Ar identical with Phe, Tyr, Trp), were synthesized. Conformations of these cyclic peptides were comparable in aqueous solutions and lipid vesicles. Isothermal titration calorimetry measurements revealed entropy-driven binding of cyclic peptides to POPC and POPE/POPG lipid vesicles.

Ion-channel formation assisted by electrostatic interhelical interactions in covalently dimerized amphiphilic helical peptides.

An ultimate goal of synthetic ion-channel peptide design is to construct stable and functional ion-conducting pores. It is expected that specific interhelical interactions would facilitate the association of helices in phospholipid membranes and the successive helix-bundle formation. In the present study, we rationally designed helix-bundle ion channels using the synthetic hybrid peptide K20E20, a disulfide dimer of cationic- and anionic-amphiphilic helices Ac-CGG-(BKBA) 5-NH 2 and Ac-CGG-(BEBA) 5-NH 2.