Lipid structure and Ca(2+)-ATPase function

Membrane lipids and transporter function

Transport proteins are essential for cells in allowing the exchange of substances between cells and their environment across the lipid bilayer forming a tight barrier. Membrane lipids modulate the function of transmembrane proteins such as transporters in two ways: Lipids are tightly and specifically bound to transport proteins and in addition they modulate from the bulk of the lipid bilayer the function of transport proteins. This overview summarizes currently available information at the ultrastructural level on lipids tightly bound to transport proteins and the impact of altered bulk membrane lipid composition. Human diseases leading to altered lipid homeostasis will lead to altered membrane lipid composition, which in turn affect the function of transporter proteins.

Recent progress on lipid lateral heterogeneity in plasma membranes: From rafts to submicrometric domains

The concept of transient nanometric domains known as lipid rafts has brought interest to reassess the validity of the Singer-Nicolson model of a fluid bilayer for cell membranes. However, this new view is still insufficient to explain the cellular control of surface lipid diversity or membrane deformability. During the past decades, the hypothesis that some lipids form large (submicrometric/mesoscale vs nanometric rafts) and stable (>min vs s) membrane domains has emerged, largely based on indirect methods. Morphological evidence for stable submicrometric lipid domains, well-accepted for artificial and highly specialized biological membranes, was further reported for a variety of living cells from prokaryot es to yeast and mammalian cells. However, results remained questioned based on limitations of available fluorescent tools, use of poor lipid fixatives, and imaging artifacts due to non-resolved membrane projections. In this review, we will discuss recent evidence generated using powerful and innovative approaches such as lipid-specific toxin fragments that support the existence of submicrometric domains. We will integrate documented mechanisms involved in the formation and maintenance of these domains, and provide a perspective on their relevance on membrane deformability and regulation of membrane protein distribution.

Conformational changes of recombinant Ca2+-ATPase studied by reaction-induced infrared difference spectroscopy

Recombinant Ca(2+)-ATPase was expressed in Saccharomyces cerevisiae with a biotin-acceptor domain linked to its C-terminus by a thrombin cleavage site. We obtained 200 μg of ~ 70% pure recombinant sarcoendoplasmic reticulum Ca(2+)-ATPase isoform 1a (SERCA1a) from a 6-L yeast culture. The catalytic cycle of SERCA1a was followed in real time using rapid scan FTIR spectroscopy. Different intermediate states (Ca2 E1P and Ca2 E2P) of the recombinant protein were accumulated using different buffer compositions. The difference spectra of their formation from Ca2 E1 had the same spectral features as those from the native rabbit SERCA1a. The enzyme-specific activity for the active enzyme fraction in both samples was also similar. The results show that the recombinant protein obtained from the yeast-based expression system has similar structural and dynamic properties as native rabbit SERCA1a. It is now possible to apply this expression system together with IR spectroscopy to the investigation of the role of individual amino acids.

Modulation of sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase activity and oxidative modification during the development of adjuvant arthritis

Adjuvant arthritis (AA) was induced by intradermal administration of Mycobacterium butyricum to the tail of Lewis rats. In sarcoplasmic reticulum (SR) of skeletal muscles, we investigated the development of AA. SR Ca(2+)-ATPase (SERCA) activity decreased on day 21, suggesting possible conformational changes in the transmembrane part of the enzyme, especially at the site of the calcium binding transmembrane part. These events were associated with an increased level of protein carbonyls, a decrease in cysteine SH groups, and alterations in SR membrane fluidity. There was no alteration in the nucleotide binding site at any time point of AA, as detected by a FITC fluorescence marker. Some changes observed on day 21 appeared to be reversible, as indicated by SERCA activity, cysteine SH groups, SR membrane fluidity, protein carbonyl content and fluorescence of an NCD-4 marker specific for the calcium binding site. The reversibility may represent adaptive mechanisms of AA, induced by higher relative expression of SERCA, oxidation of cysteine, nitration of tyrosine and presence of acidic phospholipids such as phosphatidic acid. Nitric oxide may regulate cytoplasmic Ca(2+) level through conformational alterations of SERCA, and decreasing levels of calsequestrin in SR may also play regulatory role in SERCA activity and expression.

Lipids and the ocular lens

The unusually high levels of saturation and thus order contribute to the uniqueness of human lens membranes. In addition, and unlike in most biomembranes, most of the lens lipids are associated with proteins, thus reducing their mobility. The major phospholipid of the human lens is dihydrosphingomyelin. Found in significant quantities only in primate lenses, particularly human ones, this lipid is so extremely stable that it was reported to be the only lipid remaining in a frozen mammoth 40,000 years after its death. Unusually high levels of cholesterol add peculiarity to the composition of lens membranes. Beyond the lateral segregation of lipids into dynamic domains known as rafts, the high abundance of cholesterol in the human lens leads to the formation of patches of pure cholesterol. Changes in human lens lipid composition with age and disease as well as differences among species are greater than those observed for any other biomembrane. The relationships among lens membrane composition, structure, and lipid conformation reviewed in this article are unique to the mammalian lens and offer exciting insights into lens membrane function. This review focuses on findings reported over the last two decades that demonstrate the uniqueness of mammalian lens membranes regarding their morphology and composition. Because the membranes of human lenses do undergo the most dramatic changes with age and cataractogenesis, the final sections of this review address our current knowledge of the unusual composition and organization of adult human lens membranes with and without opacification. Finally, the questions that still remain to be answered are presented.

Modulation of SERCA in the chronic phase of adjuvant arthritis as a possible adaptation mechanism of redox imbalance

Adjuvant arthritis (AA) is a condition that involves systemic oxidative stress. Unexpectedly, it was found that sarcoplasmic reticulum Ca(2 +)-ATPase (SERCA) activity was elevated in muscles of rats with AA compared to controls, suggesting possible conformational changes in the enzyme. There was no alteration in the nucleotide binding site but rather in the transmembrane domain according to the tryptophan polar/non-polar fluorescence ratio. Higher relative expression of SERCA, higher content of nitrotyrosine but no increase in phospholipid oxidation in AA SR was found. In vitro treatments of SR with HOCl showed that in AA animals SERCA activity was more susceptible to oxidative stress, but SR phospholipids were more resistant and SERCA could also be activated by phosphatidic acid. It was concluded that increased SERCA activity in AA was due to increased levels of SERCA protein and structural changes to the protein, probably induced by direct and specific oxidation involving reactive nitrogen species.

Partitioning of ABA into bilayers of Di-saturated phosphatidylcholines as measured by DSC

Using differential scanning calorimetry, we have investigated partitioning of the plant hormone abscisic acid into a homologous series of di-saturated phosphatidylcholines increasing in chain length from C(14) to C(19). Partition coefficients calculated from the shift in T(m) range from 1280 for DiC(14)PC to 480 for DiC(19)PC. The free energy of transfer is chain-length independent with a value of DeltaG = -17.4 kJ/mol and an enthalpic contribution of DeltaH = -22.6 kJ/mol. The low net entropic contribution of -TDeltaS = -5.2 J/mol agrees with the concept of the bilayer effect, but differs from that of the entropy-driven classic hydrophobic effect valid for partitioning between bulk solvents. Preferential location of the hormone in the outer region of the membrane is indicated by characteristic changes in the transition profiles and by comparison with partitioning into organic solvents whose dielectric constants model the interior and exterior regions of the bilayer. Differences in partitioning and surface pKa between the biologically active ct-ABA and the inactive tt-isomer are discussed for biological relevance.

Ca(2+)-ATPase activity and lens lipid composition in reconstituted systems

Lens lipid composition and lipid hydrocarbon chain structure change with age, region and cataract. Since the lens Ca(2+)-ATPase pump is important to the maintenance of calcium homeostasis and lens clarity, muscle sarcoplasmic reticulum Ca(2+)-ATPase was reconstituted with bovine lens lipids and dihydrosphingomyelin, the rare and major phospholipid of the human lens. Ca(2+)-ATPase activity was found to be about 5 times lower when the pump was reconstituted into dihydrosphingomyelin or lens lipids compared to native sarcoplasmic reticulum lipids. The addition of cholesterol to levels ranging from 13-53 mole%, had no affect on reconstituted Ca(2+)-ATPase activity. Ca(2+)-ATPase activity correlated with the degree of hydrocarbon chain saturation. The greater levels of saturation are a consequence of the high sphingolipid content in the reconstituted systems. These data support the hypothesis that changes in lens lipid composition or structure could affect Ca(2+)-ATPase activity in human lenses. Because the mechanisms governing Ca(2+)-ATPase activity in vivo are much more complex than in these simple reconstituted systems, this study represents an initial step in the elucidation of the relationships of endogenous membrane lipid composition-structure and function.

Lipid composition, membrane structure relationships in lens and muscle sarcoplasmic reticulum membranes

Membrane lipid composition varies in different tissues and species. Since a defined lipid composition is essential to the function of many membranes, the relationship between membrane lipid composition and structure was determined using infrared and Raman spectroscopy in four membranes containing a calcium pump: rabbit fast and slow twitch muscle sarcoplasmic reticulum and human and bovine lens fiber cell membranes. We found that membrane sphingolipid and phosphatidylcholine content were correlated to a decrease and increase, respectively, in the infrared lipid CH2 symmetric stretching band frequency. We interpret the change in frequency as a change in lipid hydrocarbon chain structural order. This was confirmed by Raman order parameters. The high degree of hydrocarbon chain saturation found in the variable amide chains of sphingolipids is likely to account for this correlation. Lipid phase transition temperature and cooperativity also correlated to sphingolipid and phosphatidylcholine content, and are the forces defining the order in at physiological temperature in the samples studied. Ca(2+)-ATPase caused an increase in the CH2 symmetric stretching frequency in fast twitch muscle sarcoplasmic reticulum (interpreted as an increase in hydrocarbon chain disorder), but had no effect on slow twitch muscle sarcoplasmic reticulum lipid hydrocarbon chain structure. In the natural systems studied, we find that it is the lipid hydrocarbon chain saturation that defines lipid hydrocarbon chain order.

How lipids interact with an intrinsic membrane protein: the case of the calcium pump

Ca2+-ATPase can be purified from the skeletal muscle of sarcoplasmic reticulum and reconstituted into phospholipid bilayers of defined composition. This allows a detailed study of the interactions between phospholipid molecules and the ATPase, and of the effects of phospholipid structure on the activity of the ATPase. It has been shown that the thickness of the lipid bilayer, its physical phase and the lipid headgroup structure can all be important. The interaction between phospholipids and the ATPase is not structurally specific in that the strength of the phospholipid-ATPase interaction does not depend on headgroup structure or on fatty acyl chain length, but the strength of binding is different for liquid crystalline and gel phase lipid. There are also 'specific' sites for some lipids on the ATPase. There is no unique mechanism explaining the effects of phospholipid on the function of the ATPase; the changes observed with any particular phospholipid follow from a distinct set of changes in the conformational state of the ATPase. The changes in activity are likely to follow from tilting of trans-membrane alpha-helices in the ATPase. In simple model systems it has been shown that the extent to which lipids can distort to match the protein is limited.

Mutational analysis of trans-membrane helices M3, M4, M5 and M7 of the fast-twitch Ca2+-ATPase

Mutational analysis of trans-membrane helices M3, M4, M5 and M7 of the Ca2+-ATPase revealed a novel phenotypic variant, M4 [Y295A (the one-letter symbols are used for amino acid residues throughout)], displaying an increased affinity for Pi and decreased affinity for MgATP, while retaining the ability to translocate Ca2+ ions across the endoplasmic reticulum membrane. The properties of this mutant suggest that the E1-E2 equilibrium is shifted towards E2, and indicate a key role for this aromatic residue (Y295) at the end of trans-membrane helix M4. A mutant containing three amino acid residue substitutions at the end of the seventh trans-membrane helix, M7 (F834A, F835A, T837F), showed a complete loss of ATPase activity and a reduced ability to phosphorylate with Pi, although MgATP-initiated phosphorylation was unaffected. The observation that single mutations in this cluster of residues had no effect on Ca2+ transport suggests that correct anchoring of the helix at the lipid-water interface by these aromatic residues is important in the functioning of the ATPase. Mutation of polar residues in helix M3 did not affect inhibition of the ATPase by thapsigargin, thapsivillosin A or t-butyl hydroquinone, suggesting that hydrogen-bonding partners for the essential -OH groups on these inhibitors lie elsewhere in the ATPase.

Decreased conformational stability of the sarcoplasmic reticulum Ca-ATPase in aged skeletal muscle

Sarcoplasmic reticulum (SR) membranes purified from young adult (4-6 months) and aged (26-28 months) Fischer 344 male rat skeletal muscle were compared with respect to the functional and structural properties of the Ca-ATPase and its associated lipids. While we find no age-related alterations in (1) expression levels of Ca-ATPase protein, and (2) calcium transport and ATPase activities, the Ca-ATPase isolated from aged muscle exhibits more rapid inactivation during mild (37 degrees C) heat treatment relative to that from young muscle. Saturation-transfer EPR measurements of maleimide spin-labeled Ca-ATPase and parallel measurements of fatty acyl chain dynamics demonstrate that, accompanying heat inactivation, the Ca-ATPase from aged skeletal muscle more readily undergoes self-association to form inactive oligomeric species without initial age-related differences in association state of the protein. Neither age nor heat inactivation results in differences in acyl chain dynamics of the bilayer including those lipids at the lipid-protein interface. Initial rates of tryptic digestion associated with the Ca-ATPase in SR isolated from aged muscle are 16(+/- 2)% higher relative to that from young muscle. indicating more solvent exposure of a portion of the cytoplasmic domain. During heat inactivation these structural differences are amplified as a result of immediate and rapid further unfolding of the Ca-ATPase isolated from aged muscle relative to the delayed unfolding of the Ca-ATPase isolated from young muscle. Thus age-related alterations in the solvent exposure of cytoplasmic peptides of the Ca-ATPase are likely to be critical to the loss of conformational and functional stability.

Separate effects of long-chain phosphatidylcholines on dephosphorylation of the Ca(2+)-ATPase and on Ca2+ binding

The steady-state activity of the Ca(2+)-ATPase of skeletal muscle sarcoplasmic reticulum (SR) is low when reconstituted into bilayers of the long-chain phosphatidylcholines dierucyl phosphatidylcholine [di(C22:1)PC] or dinervonyl phosphatidylcholine [di(C24:1)PC]. In di(C24:1)PC the ATPase binds a single Ca2+ ion, whereas in di(C22:1)PC it binds two, as in the native SR [Starling, East and Lee (1993) Biochemistry 32, 1593-1600]. In di(C22:1)PC, rates of phosphorylation of the ATPase by ATP and the rate of ATP-induced Ca2+ dissociation are slightly lower than in the native ATPase. However, a much more marked decrease is observed in di(C22:1)PC in the rate of dephosphorylation of the phosphorylated ATPase, which explains the low steady-state ATPase activity. The level of phosphorylation of the ATPase by Pi was little affected by reconstitution in di(C22:1)PC, suggesting that the rate of phosphorylation by Pi is also decreased. The very similar effects of di(C22:1)PC and di(C24:1)PC (Starling, East and Lee (1995) Biochem. J. 310, 875-879) on phosphorylation and dephosphorylation suggest that changes in these steps and the change in Ca2+ binding stoichiometry observed in di(C24:1)PC represent independent changes on the ATPase.