A fluorescence quenching study of tryptophanyl residues of (Ca2+ + Mg2+)-ATPase from sarcoplasmic reticulum

Interaction of a dirhamnolipid biosurfactant with sarcoplasmic reticulum calcium ATPase (SERCA1a)

The interaction of a dirhamnolipid biosurfactant secreted by Pseudomonas aeruginosa with calcium ATPase from sarcoplasmic reticulum (SR) was studied by means of different approaches, such as enzyme activity, fluorescence spectroscopy, Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and molecular docking simulations. The ATP hydrolysis activity was fully inhibited by incubation with dirhamnolipid (diRL) up to 0.1 mM concentration, corresponding to a surfactant concentration below membrane solubilization threshold. Surfactant-protein interaction induced conformational changes in the protein observed by an increase in the accessibility of tryptophan residues to the aqueous phase and by changes in the secondary structure of the protein as seen by fluorescence and FTIR spectroscopy. As a consequence, the protein become more unstable and denatured at lower temperatures, as seen by enzyme activity and DSC studies. Finally, these results were explained at molecular level throughout molecular docking simulations. It is concluded that there is a specific dirhamnolipid-protein interaction not related to the surface activity of the surfactant but to the particular physicochemical properties of the biosurfactant molecule.

Conformation of the C-terminal domain of the pro-apoptotic protein Bax and mutants and its interaction with membranes

The C-terminal domain of the pro-apoptotic protein Bax is a hydrophobic stretch which, it has been predicted, anchors this protein to the outer mitochondrial membrane when apoptosis is induced in the cell. A 21mer peptide imitating this domain has been synthesized together with two mutants, one with a S184 substituted by K and the other with the S184 deleted. When their structures were studied by infrared spectroscopy, it was seen that the three peptides formed aggregates both in solution and within lipid membranes, and that the peptide changed its secondary structure as a consequence of these two mutations. It was also observed that the wild-type peptide and the two mutants became membrane-integral molecules and changed their conformation when they were incorporated into model membranes with the same composition as the outer mitochondrial membrane. With the peptides incorporated in the membranes the location of W188 was studied by fluorescence quenching using the water soluble quencher acrylamide and different doxyl-PC located in the membrane, this residue being found at different membrane depths in each of the three peptides. The fact that the three peptides were able to perturb the motion of the fluorescent probe diphenylhexatriene confirmed their insertion in the membrane. However, whereas the wild type and the DeltaS184 mutant peptides were very efficient in releasing encapsulated carboxyfluorescein from liposomes, the mutant S184K was less efficient. Taken together, these results showed that the mutation tested changed the conformation of the C-terminal domain of Bax and the positions that they adopted when inserted in membranes, confirming the importance of S184 determining the conformation of this domain. At the same time, these results confirmed that the C-terminal domain of Bax participates in disrupting the barrier properties of biomembranes.

Intrinsic fluorescence as a probe of structure-function relationships in Ca(2+)-transport ATPases

Applications of intrinsic fluorescence measurements in the study of Ca(2+)-transport ATPases are reviewed. Since the initial reports showing that the fluorescence emission was sensitive to Ca2+ binding, a substantial amount of work has focused on the use of both steady-state and time-resolved fluorescence spectroscopy to investigate structure-function relationships in sarcoplasmic reticulum and plasma membrane Ca(2+)-ATPases. These studies have revealed ligand-induced conformational changes, as well as provided information on protein-protein, protein-solvent and/or protein-lipid interactions in different functional states of these proteins. The main results of these studies, as well as possible future prospects are discussed.

Different classes of tryptophan residues involved in the conformational changes characteristic of the sarcoplasmic reticulum Ca2(+)-ATPase cycle

Various classes of tryptophan residues in the Ca2(+)-ATPase of sarcoplasmic reticulum membranes have been distinguished on the basis of their sensitivities to certain fluorescence quenchers: the brominated phospholipid 1,2-bis(9,10-dibromostearoyl)-sn-glycero(3)phosphocholine, the calcium ionophore calcimycin (A23187) and its brominated analog (4-bromo-A23187), and the nucleotide analog 2'(3')-O-(2,4,6-trinitrophenyl)-adenosine 5'-triphosphate. We show that tryptophans located at the protein-lipid interface are the main contributors to the well-known fluorescence intensity change occurring in parallel with the conformational rearrangement induced by addition of calcium to the ATPase or its removal; Trp-794 on the ATPase chain may be one of these tryptophans. We also show that tryptophans more deeply embedded in the transmembrane protein structure contribute to the fluorescence change observed upon phosphorylation from inorganic phosphate of the calcium-free ATPase. This phosphorylation step involves opposite changes in the fluorescence quantum yield of tryptophans located in the membrane and in the cytoplasmic regions of the ATPase. This result is in agreement with models in which phosphorylation from inorganic phosphate not only changes the ATPase conformation locally around the catalytic center, but also reorganizes the membrane portion of the ATPase by long-range action, allowing, for instance, the calcium sites to become accessible from the luminal medium.

Effect of protease digestion on the secondary structure of sarcoplasmic reticulum Ca2(+)-ATPase as seen by FT-i.r. spectroscopy

1. Upon controlled protein cleavage the catalytic activity of the Ca2(+)-ATPase from sarcoplasmic reticulum is drastically reduced concomitantly with small but significant changes in secondary structure as seen by Fourier transformed infrared (FT-i.r.) spectroscopy, although no loss of protein bound to the membrane is found. 2. FT-i.r. band fitting procedures show a reduction in the beta-sheet and turns content of the protein which is accompanied by an increase in alpha-helix and/or random structure. 3. These changes in the secondary structure of the protein appear to be well correlated to the tryptic digestion pattern and also to changes in the ATP hydrolysis rate of the Ca2(+)-ATPase. 4. It is concluded that these small changes reflect the disruption of key domains of the protein, which lie outside of the membrane matrix, leading to loss of enzymatic activity. 5. FT-i.r. spectroscopy appears to be a very useful technique to study changes in secondary structure of proteins.

Effect of micelle diameter on tryptophan dynamics in an amphipathic helical peptide in phosphatidylcholine

The effect of dimyristoylphosphatidylcholine (DMPC) on the conformation and environment of the single tryptophan residue of a model amphipathic helical polypeptide has been investigated by fluorescence quenching with a water-soluble, neutral quencher (acrylamide) and multiple-frequency phase fluorometry. The peptide H-Ser-Ser-Ala-Asp-Trp-Leu-Lys-Ala-Phe-Tyr-Asp-Lys-Val-Ala-Glu-Lys-Leu-Ly s-Glu- Ala-Phe-Ser-Ser-Ser-OH [18As; Kanellis, P., Romans, A.Y., Johnson, B.J., Kercret, H., Chiovetti, R., Jr., Allen, T.M., & Segrest, S.P. (1980) J. Biol. Chem. 255, 11464] was synthesized by solid-phase techniques. Peptide was incubated at 26 degrees C with DMPC at various peptide:lipid weight ratios. The diameter of the resulting disk-shaped micelles increases with increasing lipid concentration from 12.0 +/- 0.4 nm at a 1:1 weight ratio of peptide to lipid to a maximum of 48.7 +/- 1.0 nm at a 1:13 ratio. At a weight ratio of 1:5, the average diameter is 22.7 +/- 0.6 nm. Decreasing the peptide:lipid ratio of the micelle resulted in a blue-shift in the fluorescence emission maximum (from 337 nm at 1:1 to 334 nm at 1:5), an increase in the fluorescence lifetime of the tryptophan measured by the phase shift method at 18 MHz (from 3.12 ns at 1:1 to 3.61 ns at 1:5), a decrease in the rate of fluorescence quenching by acrylamide (from 0.87 x 10(9) M-1 s-1 at 1:1 to 0.42 x 10(9) M-1 s-1 at 1:5), and an increase in the activation energy for quenching (from 6.7 kcal/mol at 1:1 to 12.7 kcal/mol at 1:5).(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of lipid fluidity upon the activity and structure of the 39 kDa porin from Enterobacter cloacae 908S

The 39 kDa porin from Enterobacter cloacae 908S was isolated in a lipopolysaccharide-free form using the non-ionic detergent, octylpentaoxyethylene, and reconstituted into vesicles of dimyristoylphosphatidylcholine (DMPC) and dioleoylphosphatidylcholine (DOPC), respectively. Porin activity, measured by the rate of hydrolysis of the lipid-impermeant beta-lactam cephazoline by entrapped lactamase, could be demonstrated for porin-DMPC but not for porin-DOPC vesicles, and for the former was significantly lower in the gel than in the liquid-crystalline phase. The fluorescence changes are thought to arise from lipid phase-induced structural/dynamic changes of the porin structure.

Tryptophan fluorescence of mitochondrial complex III reconstituted in phosphatidylcholine bilayers

The tryptophan intrinsic fluorescence of mitochondrial complex III reconstituted in phosphatidylcholine bilayers was examined at different temperatures. Absorption and emission maxima occur at 277 and 332 nm, irrespective of temperature or lipid:protein ratio even if there are indications (from fluorescence quenching) of protein conformational changes as a function of lipid:protein ratio. Low values of Trp fluorescence quantum yield in complex III (0.008-0.010) are probably due to the neighborhood of the heme groups. The temperature-dependent decrease of fluorescence intensity is nonlinear; the corresponding Arrhenius plots show "breaks" or discontinuities that could be interpreted as thermally dependent changes in protein conformation. However, no temperature-dependent changes in fluorescence quenching have been observed that may be related to protein conformational changes. In addition, Arrhenius plots of the fluorescence intensity of simple molecules, such as Trp or 1-anilino-8-naphthalene sulfonate in the presence of aqueous phospholipid dispersions, also show breaks in the same temperature range. Stern-Volmer plots of acrylamide and iodide quenching were also nonlinear, indicating large differences in quenching constants for the various tryptophanyl residues. The quenching results also suggest that, at high lipid:protein ratios, the microviscosity of the protein matrix is higher than that in lipid-poor systems. Comparison of quenching efficiencies of iodide and acrylamide suggest that no significant fraction of the fluorophores occurs in the neighborhood of charged residues.

Structural properties of sarcoplasmic reticulum Ca2+-ATPase as studied by intrinsic protein fluorescence

1. From the intrinsic fluorescence spectral properties and fluorescence quenching experiments done with acrylamide and iodide, using native sarcoplasmic reticulum vesicles, purified ATPase and ATPase solubilized with 1% Triton X-100, it is deduced that practically all the fluorescent tryptophanyl residues of this protein belong to a single population showing similar hydrophobic microenvironments. 2. Both acrylamide and iodide seem to be able to penetrate through the sarcoplasmic reticulum membrane. 3. The intrinsic fluorescence of the Ca2+-ATPase due to tryptophan residues probably buried inside the membrane is used as a tool to follow thermotropic changes in membrane fluidity of reconstituted systems.

Distances between the functional sites of sarcoplasmic reticulum (Ca2+ + Mg2+)-ATPase and the lipid/water interface

Measurements of fluorescence energy transfer have been performed to determine the distance between the lipid-water interface and the ATP-binding site in the (Ca2+ + Mg2+)-ATPase from sarcoplasmic reticulum. The calculated distance between the donor, FITC bound to the protein (nucleotide binding-site marker), and the acceptor, rhodamine-5'-isothiocyanyldipalmitoylphosphatidylethanolamine (RITC-DPPE) incorporated in the membrane, was in the range of 34-42 A. In addition the distance between the high affinity Ca2+-binding sites and the lipid/water interface has been calculated by luminescence energy transfer from Tb3+ bound to the Ca2+ sites to RITC-DPPE included in the membrane, and it was approx. 10 A.

Lipid-protein interactions and the function of the Ca2+-ATPase of sarcoplasmic reticulum

Regardless of the nature of the protein constituents of membranes, the molecular arrangement of lipids interacting with them must satisfy hydrophobic, ionic, and steric requirements. Biological membranes have a great diversity of lipid constituents, and this diversity might have functional roles. It has been proposed, for example, that the hydrophobic regions of membrane proteins are stabilized in the membrane through interactions with lipids able to adopt configurations other than the bilayer structure. Progress in understanding at the molecular level how lipid-protein interactions control the properties of membrane proteins has been hindered by the lack of information concerning the structure of the hydrophobic regions of membrane proteins. Nevertheless, there are many examples in the literature describing how changes in the lipid environment affect physical and biochemical properties of membrane proteins. From these studies, discussed in this review, an overall picture of how lipids and proteins interact in membranes is beginning to emerge.