Lateral redistribution of transmembrane proteins and liquid-ordered domains in lipid membranes with inhomogeneous curvature

A number of processes in living cells are accompanied by significant changes of the geometric curvature of lipid membranes. In turn, heterogeneity of the lateral curvature can lead to spatial redistribution of membrane components, most important of which are transmembrane proteins and liquid-ordered lipid-protein domains. These components have a so-called hydrophobic mismatch: the length of the transmembrane domain of the protein, or the thickness of the bilayer of the domain differ from the thickness of the surrounding membrane. In this work we consider redistribution of membrane components with hydrophobic mismatch in membranes with non-uniform geometric curvature. Dependence of the components’ energy on the curvature is calculated in terms of theory of elasticity of liquid crystals adapted to lipid membranes. According to the calculations, transmembrane proteins prefer regions of the membrane with zero curvature. Liquid-ordered domains having a size of a few nm distribute mainly into regions of the membrane with small negative curvature appearing in the cell plasma membrane in the process of endocytosis. The distribution of domains of a large radius is determined by a decrease of their perimeter upon bending; these domains distribute into membrane regions with relatively large curvature.     

Dynamic aspects of the incorporation of proteins into biological membranes

The contributions of intramembranous and extramembranous segments of transmembrane proteins to frictional forces have been studied by covalently attached ¹⁴N- and ¹⁵N-indane dione and maleimide spin labels using saturation transfer electron spin resonance spectroscopy. The role of molecular size and membrane viscosity is discussed in determining rotational mobilities of proteins. By comparing the measured rotational correlation times with the predictions of hydrodynamic models the aggregation states of transmembrane proteins is estimated. On increasing the viscosity of the aqueous phase by polyols the viscous drag of the extramembranous segments of proteins is increased and from systematic hydrodynamic measurements the size of the protruding segments can be estimated. The role of slowed molecular diffusion is briefly discussed in the inhibition of enzymatic activity. © 1997 John Wiley & Sons, Ltd.     

The role of spontaneous lipid curvature in the interaction of interfacially active peptides with membranes

Research on antimicrobial peptides is in part driven by urgent medical needs such as the steady increase in pathogens being resistant to antibiotics. Despite the wealth of information compelling structure–function relationships are still scarce and thus the interfacial activity model has been proposed to bridge this gap. This model also applies to other interfacially active (membrane active) peptides such as cytolytic, cell penetrating or antitumor peptides. One parameter that is strongly linked to interfacial activity is the spontaneous lipid curvature, which is experimentally directly accessible. We discuss different parameters such as H-bonding, electrostatic repulsion, changes in monolayer surface area and lateral pressure that affect induction of membrane curvature, but also vice versa how membrane curvature triggers peptide response. In addition, the impact of membrane lipid composition on the formation of curved membrane structures and its relevance for diverse mode of action of interfacially active peptides and in turn biological activity are described. This article is part of a Special Issue entitled: Interfacially Active Peptides and Proteins. Guest Editors: William C. Wimley and Kalina Hristova.     

The influence of lysolipids on the spontaneous curvature and bending elasticity of phospholipid membranes.

The effects of lysolipids on phospholipid layer curvature and bending elasticity were examined using x-ray diffraction and the osmotic stress method. Lysolipids with two different head groups, phosphatidylcholine (PC) and phosphatidylethanolamine (PE), and differing hydrocarbon chains were mixed with the hexagonal-forming lipid, dioleoylphosphatidylethanolamine (DOPE). With up to 30 mole% lysolipid in DOPE, the mixture maintains the inverted hexagonal (H(II)) phase in excess water, where increasing levels of lysolipid result in a systematic increase in the H(II) lattice dimension. Analysis of the structural changes imposed by lysolipids show that, opposite to DOPE itself, which has an spontaneous radius of curvature (R(0)) of -30 A, PC lysolipids add high positive curvature, with R(0) = +38 to +60 A, depending on chain length. LysoPEs, in contrast, add very small curvatures. When both polar group and hydrocarbon chains of the added lysolipid mismatch those of DOPE, the structural effects are qualitatively different from otherwise. Such mismatched lysolipids "reshape" the effective combination molecule into a longer and more cylindrical configuration compared to those lysolipids with either matching polar group or hydrocarbon chain.     

Inhibition of fatty acid synthesis prevents the incorporation of aldosterone-induced proteins into membranes.

2-Methyl-2-[p-(1,2,3,4-tetrahydro-1-naphthyl)phenoxy]propionic acid (TPIA), an acetyl coenzyme A carboxylase inhibitor, blocks the aldosterone-induced increase in transepithelial sodium transport. To examine the requirement for ongoing fatty acid synthesis and/or elongation in the aldosterone-induced alteration of cellular protein metabolism in the toad's urinary bladder, the effect of TPIA has been examined in double-labeled amino acid incorporation experiments. TPIA itself has no effect on the pattern of protein labeling in either the "soluble" or a plasma membrane-enriched fraction. However, inhibition of fatty acid synthesis selectively inhibits the aldosterone-induced incorporation of membrane proteins without altering the labeling of soluble cell protein. These results indicate that ongoing fatty acid synthesis is required for the hormone-induced changes in plasma membrane protein metabolism.     

Vibrational analysis of conformation in peptides,polypeptides, and proteins

A vibrational force field for the polypeptide chain has been developed for normal-mode analysis of such molecules. It can reproduce observed frequencies of known structures to within about 5 cm^?1. We review the application of this technique to conformational problems in peptides (β-turns and their model compounds), polypeptides [the α_II-helix and crystalline poly(glycine II)], and proteins (bacteriorhodopsin and glucagon).     

Membrane shape changes at initial stage of membrane fusion under the action of proteins inducing spontaneous curvature

This paper studies change of membrane shape at the initial stage of the fusion process due to the fusion proteins inducing spontaneous curvature in the membrane. As protein inclusions are embedded into the membrane, a highly curved surface forms in the center of the membrane; it facilitates the formation of short-lived hydrophobic defects and leads to the merger of the contact monolayers of the membranes. Membrane is considered as continuous liquid-crystal medium subject to elastic deformations. One deformational mode of splay is taken into account; energy is calculated in the quadratic approximation on this deformation. In case of positive spontaneous curvature induced by the protein there is no bulge on the top of the membrane despite high deviation of membrane shape from the equilibrium state. In case of negative spontaneous curvature a bulge is formed and its height and curvature increase with the increase of the membrane curvature in the initial state.     

The incorporation of glucose into alpha-1,4-glucan proteins of bovine retina membranes.

—Incubation of bovine retina membranes with UDP-[¹⁴C]glucose resulted in the incorporation of [¹⁴C]glucose into endogenous α-1, 4-glucan proteins. The transferring system was concentrated in membranes that floated at 0.94 and 1.10m -sucrose when centrifuged in a discontinuous sucrose density gradient and was almost absent in the rod outer segment (ROS) and the 100, 000 g supernatant fractions. The glucan proteins labelled by incubation with the radioactive sugar nucleotide at micromolar concentrations were distinguished in two fractions by their solubilities in trichloroacetic acid (TCA): glucan protein-I (GP-I), insoluble in TCA, and glucan protein-II (GP-II), soluble in TCA and precipitable by ethanol from the TCA soluble fraction. GP-I and GP-II were precipitated by trichloroacetic acid-phosphotungstic acid (TCA-PTA). A third fraction, glucan protein-III (GP-III) was found when incubations were carried out with UDP-[¹⁴C]glucose at millimolar instead of micromolar concentrations. GP-III was soluble in TCA and in TCA-PTA and precipitable by ethanol from the TCA soluble fraction. GP-II was excluded from a Sephadex G-200 column and showed a greater size than GP-I in a Sepharose 2B column. The radioactive residues obtained from the glucan proteins after digestion with pronase were totally included in a Sephadex G-25 column and were of a greater size than the labelled residues released with salivary α-amylase. Only radioactive maltose was found after a-amylase treatment. When membranes containing labelled GP-I and GP-II were incubated with unlabelled UDP-glucose at millimolar concentrations, GP-I was converted into GP-II and GP-III was formed.     

Lateral chain packing in lipids and membranes

The aliphatic chains of many biologically important lipids are heterogeneous and often related to the functions of the molecules. Certain phospholipids containing arachidonic acid may serve as precursors for prostaglandins, certain diglycerides may serve as second messengers for certain membrane-triggered reactions (43), and other phospholipids containing a very short chain in the two position may serve as vasoactive hormones (44). The packing of such molecules is of interest. The evidence is quite clear from both the conformation of saturated and unsaturated molecules and from mixing experiments in the solid state that long and short chains don't mix well, nor do unsaturated and saturated chains, even if they are of the same chain length. There is even some evidence to indicate that some degree of chain segregation occurs even in the liquid state. However, different chains are often associated through covalent bonds, e.g., in wax esters, diacylglycerols, triacylglycerols, and phospholipids. A variety of possibilities for chain segregation are present in the neat phases of wax esters, ceramides, diacylglycerols, and triacylglycerols. However, in the unique case of membrane lipids like phospholipids or sphingolipids, the two chains are forced to lie side by side by virtue of the interaction of the polar group with water, and thus interactions between different chains must occur. Most of the evidence suggests that, when a solid phase results in these systems, the nonspecific chain packing mode (hexagonal chain packing) is preferred. In fact, for all of the phospholipids studied thus far, clearcut evidence of specific chain-chain interaction in molecules having both unsaturated and saturated chains has never been observed. However, for mixed chain triacylglycerols, evidence of specific chain-chain interactions (beta' and even beta) has been found and some suggestions have been given as to how this might occur through chain segregation mechanisms in the neat state. The literature suggests that further work needs to be done on the interaction of different chains that are covalently linked to the same molecule. Such studies will lead to a better understanding of the structure of lipid bilayers, membranes, lipoproteins, and lipid deposits.     

The spontaneous insertion of proteins into and across membranes: The helical hairpin hypothesis

We propose that the initial event in the secretion of proteins across membranes and their insertion into membranes is the spontaneous penetration of the hydrophobic portion of the bilayer by a helical hairpin. Energetic considerations of polypeptide structures in a nonpolar, lipid environment compared with an aqueous environment suggest that only α and 3₁₀ helices will be observed in the hydrophobic interior of membranes. Insertion of a polypeptide is accomplished by a hairpin structure composed of two helices, which will partition into membranes if the free energy arising from burying hydrophobic helical surfaces exceeds the free energy “cost” of burying potentially charged and hydrogen-bonding groups. We suggest, for example, that the hydrophobic leader peptide found in secreted proteins and in many membrane proteins forms one of these helices and is oriented in the membrane with its N terminus inside. In secreted proteins, the leader functions by pulling polar portions of a protein into the membrane as the second helix of the hairpin. The occurrence of all categories of membrane proteins can be rationalized by the hydrophobic or hydrophilic character of the two helices of the inserted hairpin and, for some integral membrane proteins, by events in which a single terminal helix is inserted. We propose that, because of the distribution of polar and nonpolar sequences in the polypeptide sequence, secretion and the insertion of membrane proteins are spontaneous processes that do not require the participation of additional specific membrane receptors or transport proteins.     

Nonbilayer lipids affect peripheral and integral membrane proteins via changes in the lateral pressure profile

Nonbilayer lipids can be defined as cone-shaped lipids with a preference for nonbilayer structures with a negative curvature, such as the hexagonal phase. All membranes contain these lipids in large amounts. Yet, the lipids in biological membranes are organized in a bilayer. This leads to the question: what is the physiological role of nonbilayer lipids? Different models are discussed in this review, with a focus on the lateral pressure profile within the membrane. Based on this lateral pressure model, predictions can be made for the effect of nonbilayer lipids on peripheral and integral membrane proteins. Recent data on the catalytic domain of Leader Peptidase and the potassium channel KcsA are discussed in relation to these predictions and in relation to the different models on the function of nonbilayer lipids. The data suggest a general mechanism for the interaction between nonbilayer lipids and membrane proteins via the membrane lateral pressure.     

CAP: conformation angles package-displaying the conformation angles of side chains in proteins

SUMMARY: A graphics package has been developed to display all side chain conformation angles of the user selected residue in a given protein structure. The proposed package is incorporated with all the protein structures (solved using X-ray diffraction and NMR spectroscopy) available in the Protein Data Bank. The package displays the multiple conformations adopted by a single amino acid residue whose structure is solved and refined at very high resolution. In addition, it shows the percentage distribution of the side chain conformation angles in different rotameric states. AVAILABILITY: http://144.16.71.146/cap/     

Investigating diproline segments in proteins: occurrences, conformation and classification

The covalent linkage between the side‐chain and the backbone nitrogen atom of proline leads to the formation of the five‐membered pyrrolidine ring and hence restriction of the backbone torsional angle ? to values of ?60 °± 30° for the L ‐proline. Diproline segments constitute a chain fragment with considerably reduced conformational choices. In the current study, the conformational states for the diproline segment ( ^L Pro‐ ^L Pro) found in proteins has been investigated with an emphasis on the cis and trans states for the Pro‐Pro peptide bond. The occurrence of diproline segments in turns and other secondary structures has been studied and compared to that of Xaa‐Pro‐Yaa segments in proteins which gives us a better understanding on the restriction imposed on other residues by the diproline segment and the single proline residue. The study indicates that P_II–P_II and P_II–α are the most favorable conformational states for the diproline segment. The analysis on Xaa‐Pro‐Yaa sequences reveals that the Xaa‐Pro peptide bond exists preferably as the trans conformer rather than the cis conformer. The present study may lead to a better understanding of the behavior of proline occurring in diproline segments which can facilitate various designed diproline‐based synthetic templates for biological and structural studies. © 2011 Wiley Periodicals, Inc. Biopolymers 97: 54–64, 2012.     

Lateral phase separations and perpendicular transport in membranes

A valinomycin-mediated potassium conductivity has been studied using a glass U-tube in which two aqueous compartments are separated by a fritted glass filter impregnated with valinomycin and one or more pure phospholipids. This system can be used to detect the beginning and end of lateral phase separations in binary lipid mixtures, and also demonstrates a pronounced maximum in electrical conductivity of dipalmitoyl phosphatidylcholine at the transition temperature, 41°C.