Effect of Mg2+ concentration on Ca2+ uptake kinetics and structure of the sarcoplasmic reticulum membrane

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.

Entropic drive in the sarcoplasmic reticulum ATPase interaction with Mg2+ and Pi

Thermodynamic quantities for the binding of Mg2+ (in the presence of Ca2+) and Pi (in the presence of Mg2+ and absence of Ca2+) to sarcoplasmic reticulum ATPase were determined from isothermal titration calorimetry measurements at 25 degrees C. Mg2+ and Pi are involved in reversal of the ATPase hydrolytic reaction, and their interactions with the ATPase are conveniently studied under equilibrium conditions. We found that the Mg2+ binding reaction is endothermic with a binding constant (Kb) = 142 +/- 4 M(-1), a binding enthalpy of 180 +/- 3 kJ mol(-1), and an entropy contribution (TdeltaSb) = 192 +/- 3 kJ mol(-1). Similarly, Pi binding is also an endothermic reaction with Kb = 167 +/- 17 M(-1), deltaHb = 65.3 +/- 5.4 kJ mol(-1), and TdeltaSb = 77.9 +/- 5.4 kJ mol(-1). Our measurements demonstrate that the ATPase can absorb heat from the environment upon ligand binding, and emphasize the important role of entropic mechanisms in energy transduction by this enzyme.

Changes in the profile structure of the sarcoplasmic reticulum membrane induced by phosphorylation of the Ca2+ ATPase enzyme in the presence of terbium: a time-resolved x-ray diffraction study

The design of the time-resolved x-ray diffraction experiments reported in this and an accompanying paper was based on direct measurements of enzyme phosphorylation using [gamma-32P]ATP that were employed to determine the extent to which the lanthanides La3+ and Tb3+ activate phosphorylation of the Ca2+ATPase and their effect on the kinetics of phosphoenzyme formation and decay. We found that, under the conditions of our experiments, the two lanthanides are capable of activating phosphorylation of the ATPase, resulting in substantial levels of phosphoenzyme formation and they slow the formation and dramatically extend the lifetime of the phosphorylated enzyme conformation, as compared with calcium activation. The results from the time-resolved, nonresonance x-ray diffraction work reported in this paper are consistent with the enzyme phosphorylation experiments; they indicate that the changes in the profile structure of the SR membrane induced by terbium-activated phosphorylation of the ATPase enzyme are persistent over the much longer lifetime of the phosphorylated enzyme and are qualitatively similar to the changes induced by calcium-activated phosphorylation, but smaller in magnitude. These results made possible the time-resolved, resonance x-ray diffraction studies reported in an accompanying paper utilizing the resonance x-ray scattering from terbium, replacing calcium, to determine not only the location of high-affinity metal-binding sites in the SR membrane profile, but also the redistribution of metal density among those sites upon phosphorylation of the Ca2+ATPase protein, as facilitated by the greatly extended lifetime of the phosphoenzyme.

Effect of Ca2+ binding on the profile structure of the sarcoplasmic reticulum membrane using time-resolved x-ray diffraction

A number of studies have indicated that Ca(2+)-ATPase, the integral membrane protein of the sarcoplasmic reticulum (SR) membrane, undergoes some structural change upon Ca2+ binding to its high affinity binding sites (i.e., upon conversion of the E1 to the CaxE1 form of the enzyme). We have used x-ray diffraction to study the changes in the electron density profile of the SR membrane upon high-affinity Ca2+ binding to the enzyme in the absence of enzyme phosphorylation. The photolabile Ca2+ chelator DM-nitrophen was used to rapidly release Ca2+ into the extravesicular spaces throughout an oriented SR membrane multilayer and thereby synchronously in the vicinity of the high affinity binding sites of each enzyme molecule in the multilayer. A critical control was developed to exclude possible artifacts arising from heating and non-Ca2+ photolysis products in the membrane multilayer specimens upon photolysis of the DM-nitrophen. Upon photolysis, changes in the membrane electron density profile arising from high-affinity Ca2+ binding to the enzyme are found to be localized to three different regions within the profile. These changes can be attributed to the added electron density of the Ca2+ bound at three discrete sites centered at 5, approximately 30, and approximately 67 A in the membrane profile, but they also require decreased electron density within the cylindrically averaged profile structure of the Ca(2+)-ATPase immediately adjacent (< 15 A) to these sites. The locations of these three Ca2+ binding sites in the SR membrane profile span most of the membrane profile in the absence of enzyme phosphorylation,in agreement with the locations of lanthanide (Tb3+ and La3+) binding sites in the membrane profile determined independently by using resonance x-ray diffraction.

Polarized infrared attenuated total reflectance spectroscopy of the Ca(2+)-ATPase of sarcoplasmic reticulum

The mean orientations of the transition dipole moments associated with vibrational modes of the proteins and phospholipids of sarcoplasmic reticulum were determined on dry and hydrated membrane multilayers deposited on germanium or zinc selenide crystals, using polarized infrared attenuated total reflectance spectroscopy (P-IR-ATR). For preservation of the enzymatic activity of the Ca(2+)-ATPase the films were prepared from solutions containing 0.05 M KCl, 5 mM imidazole (pH 7.4), 0.5 mM MgCl2, 1-10 mM trehalose and dithiothreitol. The anisotropy was highest in dry films containing congruent to 7.5 micrograms protein/cm2, and decreased with increasing membrane thickness or hydration. The dichroic ratio of the CH2 vibrations (2923 cm-1) of extracted sarcoplasmic reticulum phospholipids on Ge plate was 1.56, compared with a dichroic ratio of 1.68 obtained on dry films of whole sarcoplasmic reticulum. The dichroic ratios of the amide I band (1650 cm-1) of the Ca(2+)-ATPase in the Ca2-E1 state and in the EGTA and vanadate stabilized E2-V state were nearly identical (1.60 vs. 1.62). The dichroism of the amide I, amide II and lipid CH2 vibrations was not affected by changes in the concentration of KCl (25-100 mM) or Ca2+ (approximately equal to 10(-8)-10(-4) M) and by the addition of vanadate (1 mM) or Pi (5 mM) in a calcium-free medium containing 0.5 mM EGTA. The dichroic ratio of the C-C (1033 cm-1) or CO stretching band (1046 cm-1) of trehalose incorporated into SR films was 1.2 on Ge plate; this corresponds to a mean angle of approximately 70 degrees between the plane of the trehalose ring and the normal of the film plane, suggesting that the trehalose molecules are surprisingly well oriented in the polar headgroup region of the phospholipids. The orientation of the trehalose was not affected by the presence of Ca(2+)-ATPase.

Covalent labeling of the cytoplasmic or luminal domains of the sarcoplasmic reticulum Ca(2+)-ATPase with fluorescent azido dyes

Sarcoplasmic reticulum (SR) vesicles were incubated with azido derivatives of Cascade blue (ACB), Lucifer yellow (ALY), 2,7-naphthalene-disulfonic acid (ANDS), and fluorescein (AF) for 0.1-24 h at 2 degrees C. All four dyes gave intense reaction with the cytoplasmic domain of the Ca(2+)-ATPase on photoactivation after brief incubation. The penetration of the dyes into the luminal space of the SR was determined after centrifugation through Sephadex microcolumns to remove the external dye, followed by photolabeling and gel electrophoresis of the photolabeled proteins. The reaction of ACB and ANDS with the Ca(2+)-ATPase and with calsequestrin increased progressively during incubation up to 24 h indicating their slow accumulation in the luminal space, while ALY and AF did not show significant penetration into the vesicles. The distribution of the covalently attached ACB in the Ca(2+)-ATPase was tested by tryptic proteolysis after labeling exclusively from the outside (OS), from the inside (IS) or from both sides (BS). In all cases intense ACB fluorescence was seen in the A fragment with inhibition of ATPase activity. In the OS preparations the A1, while in IS the A2 fragment was more intensely labeled. There was no significant incorporation of ACB into the region of B fragment identified by FITC fluorescence. The crystallization of the Ca(2+)-ATPase by EGTA + decavanadate was completely inhibited in the BS samples after labeling either in the Ca2E1 or E2V conformation. There was no inhibition of crystallization in the OS preparations. In the IS preparations labeled in the Ca2E1 state the crystallization was impaired, while in the E2V state there was only slight disorganization of the crystals. The total amount of ACB photoincorporated into SR proteins after incubation for 24 h was 1.75 nmol/mg protein; 2/3 of this labeling occurred from the outside and 1/3 from the inside. Similar level of labeling was obtained in media that stabilize the E1 or the E2 conformation of the Ca(2+)-ATPase.

Location of high-affinity metal binding sites in the profile structure of the Ca+2-ATPase in the sarcoplasmic reticulum by resonance x-ray diffraction

Resonance x-ray diffraction measurements on the lamellar diffraction from oriented multilayers of isolated sarcoplasmic reticulum (SR) membranes containing a small concentration of lanthanide (III) ions (lanthanide/protein molar ratio approximately 4) have allowed us to calculate both the electron density profile of the SR membrane and the separate electron density profile of the resonant lanthanide atoms bound to the membrane to a relatively low spatial resolution of approximately 40 A. Analysis of the membrane electron density profile and modeling of the separate low resolution lanthanide atom profile, using step-function electron density models based on the assumption that metal binding sites in the membrane profile are discrete and localized, resulted in the identification of a minimum of three such binding sites in the membrane profile. Two of these sites are low-affinity, low-occupancy sites identified with the two phospholipid polar headgroup regions of the lipid bilayer within the membrane profile. Up to 20% of the total lanthanide (III) ions bind to these low-affinity sites. The third site has relatively high affinity for lanthanide ion binding; its Ka is roughly an order of magnitude larger than that for the lower affinity polar headgroup sites. Approximately 80% of the total lanthanide ions present in the sample are bound to this high-affinity site, which is located in the "stalk" portion of the "headpiece" within the profile structure of the Ca+2 ATPase protein, approximately 12 A outside of the phospholipid polar headgroups on the extravesicular side of the membrane profile. Based on the nature of our results and on previous reports in the literature concerning the ability of lanthanide (III) ions to function as Ca+2 analogues for the Ca+2 ATPase we suggest that we have located a high-affinity metal binding site in the membrane profile which is involved in the active transport of Ca+2 ions across the SR membrane by the Ca+2 ATPase.

Effects of divalent cations on the time course of post-tetanic decay of miniature endplate potential frequency in frogs

Relatively high external Mg2+ specifically induces a shoulder in the post-tetanic decay of miniature endplate potential frequency at frog neuromuscular junctions. This effect is antagonized by Ca2+, but not by Sr2+ and Mn2+. This shoulder formation is not caused by Mn2+ or dinitrophenol in the medium. It is suggested that Mg2+ enters nerve terminals and displaces Ca2+ from internal stores; Mg2+ subsequently interferes with Ca2+ removal until the Mg2+ itself has been removed. The dinitrophenol result suggests that ATP-dependent Ca2+ extrusion is not permanently slowed. An equally likely and simpler explanation is that Mg2+ enters nerve terminals and the Mg2+ itself causes an increase in miniature endplate potential frequency. External Ca2+ competes with Mg2+ for channel entry and prevents this effect. The time course of the decay of miniature endplate potential frequency reflects the processes involved in Mg2+ extrusion and/or uptake.

A new method for monitoring the kinetics of calcium binding to the sarcoplasmic reticulum Ca(2+)-ATPase employing the flash-photolysis of caged-calcium

The kinetics of Ca2+ binding to the high-affinity sites of the sarcoplasmic reticulum (SR) Ca2(+)-ATPase were directly investigated by continuously monitoring the extravesicular calcium concentration via the metallochromic indicator Arsenazo III following the release of Ca2+ from a photolabile caged-calcium molecule, 1-(2-nitro-4,5-dimethoxyphenyl)-N,N,N',N'-tetrakis [(oxycarbony)methyl]-1,2-ethanediamine (DM-nitrophen), utilizing a pulsed Nd:YAG laser for photolysis. The nature of the binding kinetics is at least biphasic over the first 400 ms for vesicular dispersions of SR. The stoichiometry for calcium binding expressed as Ca:E1 approximately P has been calculated to be approximately 1.4:1 for the pure SR preparation under the reaction conditions employed.

Structural dynamics of the Ca2(+)-ATPase of sarcoplasmic reticulum. Temperature profiles of fluorescence polarization and intramolecular energy transfer

The temperature dependence of fluorescence polarization and Förster-type resonance energy transfer (FRET) was analyzed in the Ca2(+)-ATPase of sarcoplasmic reticulum using protein tryptophan and site-specific fluorescence indicators such as 5-[2-[iodoacetyl)amino)ethyl]aminonaphthalene-1-sulfonic acid (IAEDANS), fluorescein 5'-isothiocyanate (FITC), 2',3'-O-(2,4,3-trinitrophenyl)adenosine monophosphate (TNP-AMP) or lanthanides (Pr3+, Nd3+) as probes. The normalized energy transfer efficiency between AEDANS bound at cysteine-670 and -674 and FITC bound at lysine-515 increases with increasing temperature in the range of 10-37 degrees C, indicating the existence of a relatively flexible structure in the region of the ATPase molecule that links the AEDANS to the FITC site. These observations are consistent with the theory of Somogyi, Matko, Papp, Hevessy, Welch and Damjanovich (Biochemistry 23 (1984) 3403-3411) that thermally induced structural fluctuations increase the energy transfer. Structural fluctuations were also evident in the energy transfer between FITC linked to the nucleotide-binding domain and Nd3+ bound at the putative Ca2+ sites. By contrast the normalized energy transfer efficiency between AEDANS and Pr3+ was relatively insensitive to temperature, suggesting that the region between cysteine-670 and the putative Ca2+ site monitored by the AEDANS-Pr3+ pair is relatively rigid. A combination of the energy transfer data with the structural information derived from analysis of Ca2(+)-ATPase crystals yields a structural model, in which the location of the AEDANS-, FITC- and Ca2+ sites are tentatively identified.

Large-scale structural changes in the sarcoplasmic reticulum ATPase appear essential for calcium transport

Model refinement calculations utilizing the results from time-resolved x-ray diffraction studies indicate that specific, large-scale changes (i.e., structural changes over a large length scale or long range) occur throughout the cylindrically averaged profile structure of the sarcoplasmic reticulum ATPase upon its phosphorylation during calcium active transport. Several physical-chemical factors, all of which slow the kinetics of phosphoenzyme formation, induce specific, large-scale changes throughout the profile structure of the unphosphorylated enzyme that in general are opposite to those observed upon phosphorylation. These results suggest that such large-scale structural changes in the ATPase occurring upon its phosphorylation are required for its calcium transport function.

Evidence that lipid lateral phase separation induces functionally significant structural changes in the Ca+2ATPase of the sarcoplasmic reticulum

We have studied lipid lateral phase separation (LPS) in the intact sarcoplasmic reticulum (SR) membrane and in bilayers of isolated SR membrane lipids as a function of temperature, [Mg+2], and degree of hydration. Lipid LPS was observed in both the intact membrane and in the bilayers of isolated SR lipids, and the LPS behavior of both systems was found to be qualitatively similar. Namely, lipid LPS occurs only at relatively low temperature and water content, independently of the [Mg+2], and the upper characteristic temperature (th) for lipid LPS for both the membrane and bilayers of its isolated lipids coincide to within a few degrees. However, at similar temperatures, isolated lipids show more LPS than the lipids in the intact membrane. Lipid LPS in the intact membrane and in bilayers of the isolated lipids is fully reversible, and more extensive for samples partially dehydrated at temperatures below th. Our previous x-ray diffraction studies established the existence of a temperature-induced transition in the profile structure of the sarcoplasmic reticulum Ca+2ATPase which occurs at a temperature corresponding to the [Mg+2]-dependent upper characteristic temperature for lipid LPS in the SR membrane. Furthermore, the functionality of the ATPase, and in particular the lifetime of the first phosphorylated enzyme conformation (E1 approximately P) in the Ca+2 transport cycle, were also found to be linked to the occurrence of this structural transition. The hysterisis observed in lipid LPS behavior as a function of temperature and water content provides a possible explanation for the more efficient transient trapping of the enzyme in the E1 approximately P conformation observed in SR membranes partially dehydrated at temperatures below th. The observation that LPS behavior for the intact SR membrane and bilayers of isolated SR lipids (no protein present) are qualitatively similar strongly suggests that the LPS behavior of the SR membrane lipids is responsible for the observed structural change in the Ca+2ATPase and the resulting significant increase in E1 approximately P lifetime for temperatures below th.

The effects of Mg2+ on rat liver microsomal Ca2+ sequestration

The effects of Mg2+ on the hepatic microsomal Ca2(+)-sequestering system was tested. Ca2(+)-ATPase activity and Ca2+ uptake were both dependent on the concentration of free Mg2+, reaching maximum levels at 2 mM. The effects of Mg-ATP were also influenced by the concentration of free Mg2+, being maximally effective at a ratio of 1:1. The results suggest that Mg2+ influences Ca2+ sequestration at various steps, namely in addition to forming the substrate of the Ca2(+)-ATPase reaction, Mg-ATP, Mg2+ stimulates the reaction at an additional step, as indicated by its stimulatory effect on the Ca2(+)-ATPase reaction and on Ca2+ uptake, even at optimal Mg-ATP levels. The stimulatory effect of Mg2+ was evident at various pH levels tested, and it was nucleotide specific. The stimulatory effect of Mg2+ might be exerted at the dephosphorylation step of the enzymatic reaction or at an other, yet undefined, site. The results demonstrate a plural effect of Mg2+ on the hepatic microsomal sequestration system. This indicates that, depending on its magnitude, changes in Mg2+ distribution might influence cytosolic Ca2+ levels.