Biophysical Characterization of Oxidized Phospholipids : Implications for Altered Membrane Structure and Function

The present study aims to elucidate the modifications in the structure and functionality of the phospholipid matrix of biological membranes brought about by free radical-mediated oxidative damage of its molecular constituents. To this end, the surface properties of two oxidatively modified phospholipids bearing an aldehyde or carboxyl function at the end of truncated sn-2 acyl chain were studied using a Langmuir balance. The results obtained reveal both oxidized species to have a significant impact on the structural dynamics of phospholipid monolayers, as illustrated by the progressive changes in force-area isotherms with increasing mole fraction of the oxidized lipid component. Moreover, surface potential measurements revealed considerable modifications in the electric properties of oxidized phospholipid containing monolayers during film compression, suggesting a packing state-controlled reorientation of the intramolecular electric dipoles of the lipid headgroups and acyl chains. Based on the above findings, a model describing the conformational state of oxidized phospholipid molecules in biological membranes is proposed, involving the protrusion of the acyl chains bearing the polar functional groups out from the hydrocarbon phase to the surrounding aqueous medium. Oxidative modifications alter profoundly the physicochemical properties of unsaturated phospholipids and are therefore readily anticipated to have important implications for their interactions with membrane-associating molecules. Along these lines, the carboxyl group bearing lipid was observed to bind avidly the peripheral membrane protein cytochrome c. The binding was reversed following increase in ionic strength or addition of polyanionic ATP, thus suggesting it to be driven by electrostatic interactions between cationic residues of the protein and the deprotonated lipid carboxyl exposed to the aqueous phase. The presence of aldehyde function bearing oxidized phospholipid was observed to enhance the intercalation of four antimicrobial peptides into phospholipid monolayers and liposomal bilayers. Partitioning of the peptides to monolayers was markedly attenuated by the aldehyde scavenger methoxyamine, revealing it to be mediated by the carbonyl moiety possibly through efficient hydrogen bonding or, alternatively, formation of covalent adduct in form of a Schiff base between the lipid aldehydes and primary amine groups of the peptide molecules. Lastly, both oxidized phospholipid species were observed to bind with high affinity three small membrane-partitioning therapeutic agents, viz. chlorpromazine, haloperidol, and doxorubicin. In conclusion, the results of studies conducted using biomimetic model systems support the notion that oxidative damage influences the molecular architecture as well as the bulk physicochemical properties of phospholipid membranes. Further, common polar functional groups carried by phospholipids subjected to oxidation were observed to act as molecular binding sites at the lipid-water interface. It is thus plausible that oxidized phospholipid species may elicit cellular level effects by modulating integration of various membrane-embedded and surface-associated proteins and peptides, whose conformational state, oligomerization, and functionality is known to be controlled by highly specific lipid-protein interactions and proper physical state of the membrane environment.

Characterization of the mitochondrial active-site serine protein LACTB: filaments in the mitochondrial intermembrane space

Mitochondria have evolved from endosymbiotic alpha-proteobacteria. During the endosymbiotic process early eukaryotes dumped the major component of the bacterial cell wall, the peptidoglycan layer. Peptidoglycan is synthesized and maintained by active-site serine enzymes belonging to the penicillin-binding protein and the β-lactamase superfamily. Mammals harbor a protein named LACTB that shares sequence similarity with bacterial penicillin-binding proteins and β-lactamases. Since eukaryotes lack the synthesis machinery for peptidoglycan, the physiological role of LACTB is intriguing. Recently, LACTB has been validated in vivo to be causative for obesity, suggesting that LACTB is implicated in metabolic processes. The aim of this study was to investigate the phylogeny, structure, biochemistry and cell biology of LACTB in order to elucidate its physiological function. Phylogenetic analysis revealed that LACTB has evolved from penicillin binding-proteins present in the bacterial periplasmic space. A structural model of LACTB indicates that LACTB shares characteristic features common to all penicillin-binding proteins and β-lactamases. Recombinat LACTB protein expressed in E. coli was recovered in significant quantities. Biochemical and cell biology studies showed that LACTB is a soluble protein localized in the mitochondrial intermembrane space. Further analysis showed that LACTB preprotein underwent proteolytic processing disclosing an N-terminal tetrapeptide motif also found in a set of cell death-inducing proteins. Electron microscopy structural studies revealed that LACTB can polymerize to form stable filaments with lengths ranging from twenty to several hundred nanometers. These data suggest that LACTB filaments define a distinct microdomain in the intermembrane space. A possible role of LACTB filaments is proposed in the intramitochondrial membrane organization and microcompartmentation. The implications of these findings offer novel insight into the evolution of mitochondria. Further studies of the LACTB function might provide a tool to treat mitochondria-related metabolic diseases.

Palladin, a novel microfilament protein

Palladin is a novel actin microfilament associated protein, which together with myotilin and myopalladin forms a novel cytoskeletal IgC2 domain protein family. Whereas the expression of myotilin and myopalladin is limited mainly to striated muscle, palladin is widely expressed in both epithelial and mesenchymal tissues, including heart and the nervous system. Palladin has a complex genetic structure and it is expressed as several different sized and structured splice variants, which also display differences in their expression pattern and interactions. In muscle cells, all the family members localize to the sarcomeric Z-disc, and in non-muscle cells palladin also localizes to the stress-fiber-dense regions, lamellipodia, podosomes and focal adhesions. A common feature of this protein family is the binding to α-actinin, but other interactions are mostly unique to each member. Palladin has been shown to interact with several proteins, including VASP, profilin, Eps8, LASP-1 and LPP. Its domain structure, lack of enzymatic activity and multiple interactions define it as a molecular scaffolding protein, which links together proteins with different functional modalities into large complexes. Palladin has an important role in cytoskeletal regulation, particularly in stress fiber formation and stabilization. This assumption is supported by several experimental results. First, over-expression of palladin in non-muscle cells results in rapid reorganization of the actin cytoskeleton and formation of thick actin bundles. Second, the knock-down of palladin with anti-sense and siRNA techniques or knock-out by genetic methods leads to defective stress fiber formation. Furthermore, palladin is usually up-regulated in situations requiring a highly organized cytoskeleton, such as differentiation of dendritic cells, trophoblasts and myofibroblasts, and activation of astrocytes during glial scar formation. The protein family members have also direct disease linkages; myotilin missense mutations are the cause of LGMD1A and myofibrillar myopathy. Palladin mutations and polymorphisms, on the other hand, have been linked to hereditary pancreatic cancer and myocardial infarction, respectively. In this study we set out to characterize human palladin. We identified several palladin isoforms, studied their tissue distribution and sub-cellular localization. Four novel interaction partners were identified; ezrin, ArgBP2, SPIN90 and Src-kinase.The previously identified interaction between palladin and α-actinin was also characterized in detail. All the identified new binding partners are actin cytoskeleton associated proteins; ezrin links the plasma membrane to the cytoskeleton, ArgBP2 and SPIN90 localize, among other structures, to the lamellipodia and in cardiomyocytes to the Z-disc. Src is a transforming tyrosine kinase, which besides its role in oncogenesis has also important cytoskeletal associations. We also studied palladin in myofibroblasts, which are specialized cells involved in diverse physiological and pathological processes, such as wound healing and tissue fibrosis. We demonstrated that palladin is up-regulated during the differentiation of myofibroblasts in an isoform specific manner, and that this up-regulation is induced by TGF-β via activation of both the SMAD and MAPK signalling cascades. In summary, the results presented here describe the initial characterization of human palladin and offer a basis for further studies.

Acyclic peptides as conformational models Crystal structure of Boc-Aib-Leu-Pro-NHMe± 2H2O

The tripeptide Boc-Aib-Leu-Pro-NHMe crystallizes in the orthorhombic space group P212121 with a = 9.542, b = 15.200, c = 18.256 Å and Z = 4. Each peptide is associated wth two water molecules in the asymmetric unit of the crystal. The structure has been solved by direct methods and refined to an R-value of 0.069. The peptide adopts a structure without any intramolecular hydrogen bond. The three residues occupy distinctly different regions of the Ramachandran map: Aib in the left-handed 310-helical region (± = 67°, ± = 23°), Leu in the β-sheet region (± = - 133°, ± = 142°) and Pro in the poly (Pro) II region (± = - 69°, ± = 151°). An interesting observation is that each water molecule participates in four hydrogen bonds with distorted tetrahedral coordination about the oxygen atom.

The Myopathic Protein Myotilin in Developing Mouse and in Muscle Function

Skeletal muscle cells are highly specialised in order to accomplish their function. During development, the fusion of hundreds of immature myoblasts creates large syncytial myofibres with a highly ordered cytoplasm filled with packed myofibrils. The assembly and organisation of contractile myofibrils must be tightly controlled. Indeed, the number of proteins involved in sarcomere building is impressive, and the role of many of them has only recently begun to be elucidated. Myotilin was originally identified as a high affinity a-actinin binding protein in yeast twohybrid screen. It was then found to interact also with filamin C, actin, ZASP and FATZ-1. Human myotilin is mainly expressed in striated muscle and induces efficient actin bundling in vitro and in cells. Moreover, mutations in myotilin cause different forms of muscle disease, now collectively known as myotilinopathies. In this thesis, consisting of three publications, the work on the mouse orthologue is presented. First, the cloning and molecular characterisation of the mouse myotilin gene showed that human and mouse myotilin share high sequence homology and a similar expression pattern and gene regulation. Functional analysis of the mouse promoter revealed the myogenic factor-binding elements that are required for myotilin gene transcription. Secondly, expression of myotilin was studied during mouse embryogenesis. Surprisingly, myotilin was expressed in a wide array of tissues at some stages of development; its expression pattern became more restricted at perinatal stages and in adult life. Immunostaining of human embryos confirmed broader myotilin expression compared to the sarcomeric marker titin. Finally, in the third article, targeted deletion of myotilin gene in mice revealed that it is not essential for muscle development and function. These data altogether indicate that the mouse can be used as a model for human myotilinopathy and that loss of myotilin does not alter significantly muscle structure and function. Therefore, disease-associated mutant myotilin may act as a dominant myopathic factor.

On the Fourier refinement of protein structures

The situation normally encountered in the high-resolution refinement of protein structures is one in which the inaccurate positions of P out of a total of N atoms are known whereas those of the remaining atoms are unknown. Fourier maps with coefficients (FN -- F'P) × exp (i[alpha]'P) and (mFN -- nF'P) exp (i[alpha]'P), where FN is the observed structure factor and F'P and [alpha]'P are the magnitude and the phase angle of the calculated structure factor corresponding to the inaccurate atomic positions, are often used to correct the positions of the P atoms and to determine those of the Q unknown atoms. A general theoretical approach is presented to elucidate the effect of errors in the positions of the known atoms on the corrected positions of the known atoms and the positions of the unknown atoms derived from such maps. The theory also leads to the optimal choice of parameters used in the different syntheses. When the errors in the positions of the input atoms are systematic, their effects are not taken care of automatically by the syntheses.

X-Ray Studies On Crystalline Complexes Involving Amino-Acids And Peptides .11. Peptide Aggregation And Water-Structure In The Crystals Of A Hydrated 1-1 Complex Between L-Histidyl-L-Serine And Glycyl-L-Glutamic Acid

The hexahydrate of a 1:1 complex between L-histidyl-L-serine and glycyl-L-glutamic acid crystallizes in space group P1 with a = 4.706(1), b= 8.578(2), c= 16.521(3) ÅA; α= 85.9(1), β= 89.7(1)°, = 77.4(1). The crystal structure, solved by direct methods, has been refined to an R value of 0.046 for 2150 observed reflections. The two peptide molecules in the structure have somewhat extended conformations. The unlike molecules aggregate into separate alternating layers. Each layer is stabilized by hydrogen bonded head-to-tail sequences as well as sequences of hydrogen bonds involving peptide groups. The arrangement of molecules in each layer is similar to one of the plausible idealized arrangements of L-alanyl-L-alanine worked out from simple geometrical considerations. Adjacent layers in the structure are held together by interactions involving side chains as well as water molecules. The water structure observed in the complex provides a good model, at atomic resolution, for that in protein crystals. An interesting feature of the crystal structure is the existence of two water channels in the interfaces between adjacent peptide layers.

A fast method of comparing protein structures

Comparative studies on protein structures form an integral part of protein crystallography. Here, a fast method of comparing protein structures is presented. Protein structures are represented as a set of secondary structural elements. The method also provides information regarding preferred packing arrangements and evolutionary dynamics of secondary structural elements. This information is not easily obtained from previous methods. In contrast to those methods, the present one can be used only for proteins with some secondary structure. The method is illustrated with globin folds, cytochromes and dehydrogenases as examples.

Structure-Function Studies of GDNF Family Ligand-RET signalling

Glial cell line-derived neurotrophic factor (GDNF) and its family members neurturin (NRTN), artemin (ARTN) and persephin (PSPN) are growth factors, which are involved in the development, differentiation and maintenance of many neuron types. In addition, they function outside of the nervous system, e.g. in the development of kidney, testis and liver. GDNF family ligand (GFL) signalling happens through a tetrameric receptor complex, which includes two glycosylphosphatidylinositol (GPI)-anchored GDNF family receptor (GFRα) molecules and two RET (rearranged during transfection) receptor tyrosine kinases. Each of the ligands binds preferentially one of the four GFRα receptors: GDNF binds to GFRα1, NRTN to GFRα2, ARTN to GFRα3 and PSPN to GFRα4. The signal is then delivered by RET, which cannot bind the GFLs on its own, but can bind the GFL-GFRα complex. Under normal cellular conditions, RET is only phosphorylated on the cell surface after ligand binding. At least the GDNF-GFRα1 complex is believed to recruit RET to lipid rafts, where downstream signalling occurs. In general, GFRαs consist of three cysteine-rich domains, but all GFRα4s except for chicken GFRα4 lack domain 1 (D1). We characterised the biochemical and cell biological properties of mouse PSPN receptor GFRα4 and showed that it has a significantly weaker capacity than GFRα1 to recruit RET to the lipid rafts. In spite of that, it can phosphorylate RET in the presence of PSPN and contribute to neuronal differentiation and survival. Therefore, the recruitment of RET to the lipid rafts does not seem to be crucial for the biological activity of all GFRα receptors. Secondly, we demonstrated that GFRα1 D1 stabilises the GDNF-GFRα1 complex and thus affects the phosphorylation of RET and contributes to the biological activity. This may be important in physiological conditions, where the concentration of the ligand or the soluble GFRα1 receptor is low. Our results also suggest a role for D1 in heparin binding and, consequently, in the biodistribution of released GFRα1 or in the formation of the GFL-GFRα-RET complex. We also presented the crystallographic structure of GDNF in the complex with GFRα1 domains 2 and 3. The structure differs from the previously published ARTN-GFRα3 structure in three significant ways. The biochemical data verify the structure and reveal residues participating in the interactions between GFRα1 and GDNF, and preliminarily also between GFRα1 and RET and heparin. Finally, we showed that, the precursor of the oncogenic MEN 2B (multiple endocrine neoplasia type 2) form of RET gets phosphorylated already during its synthesis in the endoplasmic reticulum (ER). We also demonstrated that it associates with Src homology 2 domain-containing protein (SHC) and growth factor receptor-bound protein (GRB2) in the ER, and has the capacity to activate several downstream signalling molecules.

The influence of esterification of carboxyl groups of ribonuclease-a on its structure and immunological activity

The effect of modification of carboxyl groups of Ribonuclease-Aa on the enzymatic activity and the antigenic structure of the protein has been studied. Modification of four of the eleven free carboxyl groups of the protein by esterification in anhydrous methanol/0.1 M hydrochloric acid resulted in nearly 80% loss in enzymatic activity but had very little influence on the antigenic structure of the protein. Further increases in the modification of the carboxyl groups caused a progressive loss in immunological activity, and the fully methylated RNase-A exhibited nearly 30% immunological activity. Concomitant with this change in the antigenic structure of the protein, the ability of the molecule to complement with RNase-S-protein increased, clearly indicating the unfolding of the peptide "tail" from the remainder of the molecule. The susceptibility to proteolysis, accessibility of methionine residues for orthobenzoquinone reaction and the loss in immunological activity of the more extensively esterified derivatives of RNase-A are suggestive of the more flexible conformation of these derivatives as compared with the compact native conformation. The fact that even the fully methylated RNase-A retains nearly 30% of its immunological activity suggested that the modified protein contained antibody recognizable residual native structure, which presumably accommodates some antigenic determinants.

Stereochemical restrictions on the occurrence of amino acid residues in the collagen structure

The primary structure of collagen is characterized by the repeating tripeptide sequence (Gly-R2-R3)n. The results of theoretical studies, carried out using contact criteria to compute the stereochemically allowed orientations for various side chains at locations 2 and 3, are reported here. It is found that side chains with only γ-atoms, as in valine, serine and threonine, or with only one δ-methyl group, as in isoleucine, can occur equally well at locations 2 and 3, as is actually the case in collagen. Side chains with two Cδ-atoms, as in leucine and phenyl-alanine, can also be accommodated at both positions. However, if they occur as R3 their freedom of orientation is severely restricted in the presence of a proline residue as R2 in a neighbouring chain. If water molecules bound to the chains of the triple helix are assumed to be present, then location 3 is virtually impossible for leucine and phenylalanine residues. Location 2 is, however, unaffected, and their presence as R2 can help to shield the water molecules from disturbance by the solvent medium. This may be the reason for the preferential occurrence of Leu and Phe residues in location 2 in the collagen triplets, although the polypeptides (Gly-Pro-Leu)n and (Gly-Pro-Phe)n form collagen-like structures.

The structure of glycosaminoglycans and their interactions with proteins

Glycosaminoglycans (GAGs) are important complex carbohydrates that participate in many biological processes through the regulation of their various protein partners. Biochemical, structural biology and molecular modelling approaches have assisted in understanding the molecular basis of such interactions, creating an opportunity to capitalize on the large structural diversity of GAGs in the discovery of new drugs. The complexity of GAG–protein interactions is in part due to the conformational flexibility and underlying sulphation patterns of GAGs, the role of metal ions and the effect of pH on the affinity of binding. Current understanding of the structure of GAGs and their interactions with proteins is here reviewed: the basic structures and functions of GAGs and their proteoglycans, their clinical significance, the three-dimensional features of GAGs, their interactions with proteins and the molecular modelling of heparin binding sites and GAG–protein interactions. This review focuses on some key aspects of GAG structure–function relationships using classical examples that illustrate the specificity of GAG–protein interactions, such as growth factors, anti-thrombin, cytokines and cell adhesion molecules. New approaches to the development of GAG mimetics as possible new glycotherapeutics are also briefly covered.

Thermodynamics of metal ion binding and denaturation of a calcium binding protein from Entamoeba histolytica

The thermodynamics of tie binding of calcium and magnesium ions to a calcium binding protein from Entamoeba histolytica was investigated by isothermal titration calorimetry (ITC) in 20 mM MOPS buffer (pH 7.0) at 20 degrees C. Enthalpy titration curves of calcium show the presence of four Ca2+ binding sites, There exist two low-affinity sites for Ca2+, both of which are exothermic in nature and with positive cooperative interaction between them. Two other high affinity sites for Ca2+ exist of which one is endothermic and the other exothermic, again with positive cooperative interaction. The binding constants for Ca2+ at the four sites have been verified by a competitive binding assay, where CaBP competes with a chromophoric chelator 5, 5'-Br-2 BAPTA to bind Ca2+ and a Ca2+ titration employing intrinsic tyrosine fluorescence of the protein, The enthalpy of titration of magnesium in the absence of calcium is single site and endothermic in nature. In the case of the titrations performed using protein presaturated with magnesium, the amount of heat produced is altered. Further, the interaction between the high-affinity sites changes to negative cooperativity. No exchange of heat was observed throughout the addition of magnesium in the presence of 1 mM calcium, Titrations performed on a cleaved peptide comprising the N-terminus and the central linker show the existence of two Ca2+ specific sites, These results indicate that this CaBP has one high-affinity Ca-Mg site, one high-affinity Ca-specific site, and two low-affinity Ca-specific sites. The thermodynamic parameters of the binding of these metal ions were used to elucidate the energetics at the individual site(s) and the interactions involved therein at various concentrations of the denaturant, guanidine hydrochloride, ranging from 0.05 to 6.5 M. Unfolding of the protein was also monitored by titration calorimetry as a function of the concentration of the denaturant. These data show that at a GdnHCl concentration of 0.25 M the binding affinity for the Mg2+ ion is lost and there are only two sites which can bind to Ca2+, with substantial loss cooperativity. At concentrations beyond 2.5 M GdnHCl, at which the unfolding of the tertiary structure of this protein is observed by near UV CD spectroscopy, the binding of Ca2+ ions is lost. We thus show that the domain containing the two low-affinity sites is the first to unfold in the presence of GdnHCl. Control experiments with change in ionic strength by addition of KCI in the range 0.25-1 M show the existence of four sites with altered ion binding parameters.