Static and time-resolved structural studies of the Ca2+-ATPase of isolated sarcoplasmic reticulum

Flash photolysis of caged compounds: new tools for cellular physiology

Caged compounds are molecules or ions of physiological interest, e.g. ATP, IP3, cAMP, cGMP, GTP and Ca2+ rendered inactive by chemical modification. The modification introduces a photochemically labile bond, which on exposure to ultraviolet light cleaves rapidly, releasing the active compound. This article reviews some of the major advances and applications of the photorelease approach, and illustrates its potential in several areas of interest to cellular neuroscientists.

Electron microscopic analysis of two-dimensional crystals of the Ca2+-transport ATPase--a freeze-fracture study

Two distinct forms of Ca2+-ATPase crystals have been analysed in sarcoplasmic reticulum (SR) membranes. The E1-type crystals, induced by Ca2+ or lanthanide ions, consist of single chains of ATPase monomers, and the E2-type crystals, induced by vanadate ions, consist of dimer chains. Using improved freeze-fracture techniques we have obtained high-resolution images of complementary surface replicas of SR membranes containing these crystal forms. In E1 crystals, the concave fracture (P) faces display obliquely oriented rows of intramembrane particles (IMPs) spaced at congruent to 6-7 nm along both crystal axes, while the convex fracture (E) faces show corresponding rows of pits. In E2 crystals, regular arrays of oblique parallel ridges with spacing of congruent to 10.5-11 nm appear on the P-faces and complementary grooves or furrows on the E-faces. In many instances the ridges break up into elongated particles repeating every 5.5 nm. When the direction of the shadow is almost parallel to the axis of the ridges, these 9.5 nm particles can be resolved into two domains, which represent intramembranous contacts between the two monomers of the two adjacent dimer chains. Complementary grooves on the E-faces can also be resolved into rows of pits complementary to the particles of the ridges on the P-faces. In the control SR membranes, randomly dispersed IMPs and corresponding pits are observed on the P- and E-faces, respectively. The data suggest that transport of Ca2+ involves significant structural changes of the enzyme molecule, reflected in the ATPase-ATPase interactions both on the cytoplasmic surface and in the lipid bilayer.

Effect of chemical modification on the crystallization of Ca2+-ATPase in sarcoplasmic reticulum

The influence of chemical modification on the morphology of crystalline ATPase aggregates was analyzed in sarcoplasmic reticulum (SR) vesicles. The Ca2+-ATPase forms monomer-type (P1) type crystals in the E1 and dimer-type (P2) crystals in the E2 conformation. The P1 type crystals are induced by Ca2+ or lanthanides; P2 type crystals are observed in Ca2+-free media in the presence of vanadate or inorganic phosphate. P1- and P2-type Ca2+-ATPase crystals do not coexist in significant amounts in native sarcoplasmic reticulum membrane. The crystallization of Ca2+-ATPase in the E2 conformation is inhibited by guanidino-group reagents (2,3-butanedione and phenylglyoxal), SH-group reagents, phospholipases C or A2, and detergents, together with inhibition of ATPase activity. Amino-group reagents (fluorescein 5'-isothiocyanate, pyridoxal phosphate and fluorescamine) inhibit ATPase activity but do not interfere with the crystallization of Ca2+-ATPase induced by vanadate. In fluorescamine-treated sarcoplasmic reticulum the vanadate-induced crystals contain significant P1-type regions in addition to the dominant P2 form.

Three-dimensional reconstruction of negatively stained crystals of the Ca2+-ATPase from muscle sarcoplasmic reticulum

The structure of the Ca2+ transport ATPase from rabbit skeletal muscle sarcoplasmic reticulum has been determined to 25 A resolution by three-dimensional image reconstruction of crystalline membrane tubules induced through exposure to Na3VO4 and preserved for electron microscopy in negative stain. The crystalline arrays have projection symmetry p2 and consist of chains of Ca2+-ATPase dimers arranged in a right-handed helix. The density map shows protein features that project from the membrane surface into the cytoplasm. The luminal side of the membrane tubules is featureless, presumably because very little of the Ca2+-ATPase molecule projects into the luminal space. The cytoplasmic region of the Ca2+-ATPase molecule is pear-shaped, with a lobe oriented nearly parallel to the axis of the dimer ribbons, about 16 A above the surface of the membrane bilayer. The structure seen in the maps has a volume of 71,000 A3, corresponding to a molecular weight of 57,000. The two Ca2+-ATPase profiles that constitute a dimer are connected by a stain-excluding bridge that is oriented parallel with the axis of the tubule at a height of about 42 A above the surface of the bilayer.

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.

Dimer ribbons in the three-dimensional structure of sarcoplasmic reticulum

The three-dimensional structure of scallop sarcoplasmic reticulum membranes has been determined from electron micrographs of two classes of stain-filled tubules by helical reconstruction methods. These structures are characterized by dimer ribbons of Ca2+-ATPase molecules running diagonally around the tube wall. Deep right-handed grooves separate the ribbons. The elongated, curved units of the dimer (approximately 95 A long in the radial direction; 60 to 70 A axially, and about 30 A wide) are displaced axially by approximately 34 A and are connected at their outer ends by a bridge running nearly parallel to the tube axis. The monomers make a second contact at their inner ends. Adjacent units with the same orientation form a strong contact that is responsible for the ribbon appearance. Comparison of tubules of different diameter shows that one set of connections between the dimer ribbons is conserved: the inner ends of axially displaced dimers appear to make contact along a left-handed path almost perpendicular to the major grooves. The lipid bilayer cannot be clearly identified. The two-dimensional map obtained from flattened tubules is consistent with the three-dimensional reconstruction in showing dimer ribbons connected by a weak contact across the grooves, strongly resembling the inter-dimer bond observed in three dimensions. The two-dimensional map shows a 2-fold axis relating units of the dimer, but the three-dimensional tubes show a slight axial polarity that may arise from the presence of proteins other than the Ca2+-ATPase.

Density and disposition of Ca2+-ATPase in sarcoplasmic reticulum membrane as determined by shadowing techniques

We have studied the disposition of calcium ATPase in the native sarcoplasmic reticulum (SR) membrane of vertebrate muscles by rotary shadowing of freeze-dried isolated vesicles and of freeze-fractured in situ membranes. The predominant disposition of the ATPase molecules is disorderly, but small oligomers (dimers, tetramers, and occasionally larger aggregates) are seen. In vesicles from white hind legs of rabbits, the density of ATPase over nonjunctional SR is 31-34,000/microns2. ATPase density is always quite high, but small protein-free lipid patches may be interspersed with it.

Synthesis of pyrophosphate by chromatophores of Rhodospirillum rubrum in the light and by soluble yeast inorganic pyrophosphatase in water-organic solvent mixtures

Chromatophores of Rhodospirillum rubrum contain a membrane-bound pyrophosphatase that synthesizes pyrophosphate when an electrochemical H+ gradient is formed across the chromatophore membrane upon illumination. In this report it is shown that MgCl2 and Pi have different effects on the synthesis of pyrophosphate in the light depending on whether initial velocities or steady-state levels are examined. When the water activity of the medium is reduced by the addition of organic solvents, soluble yeast inorganic pyrophosphatase (no H+ gradient present) synthesizes pyrophosphate in amounts similar to those synthesized by the chromatophores in totally aqueous medium during illumination, (H+ gradient present). The pH, MgCl2 and Pi dependence for the synthesis of pyrophosphate by the chromatophores at steady-state is similar to that observed at equilibrium with the soluble enzyme in the presence of organic solvents. The possibility is raised that a decrease in water activity may play a role in the mechanism by which the energy derived from the electrochemical H+ gradient is used for the synthesis of pyrophosphate in chromatophores of R. rubrum.

Ordered arrays of Ca2+-ATPase on the cytoplasmic surface of isolated sarcoplasmic reticulum

Isolated sarcoplasmic reticulum (SR) vesicles with polymerized calcium pump protein were freeze-dried and rotary shadowed following uranyl acetate stabilization. This technique allows direct observation of a single side of the vesicle without requiring optical filtering. The heads of individual ATPase molecules, projecting above the cytoplasmic surface, are clearly resolved in the replicas. Ca ATPase molecules form extensive arrays in vanadate-treated, rabbit SR vesicles and in gently isolated, native SR vesicles from scallop. Gentle isolation results in limited areas of orderly structure in native SR isolated from vertebrate muscles. Special attention is given to the effect of various shadow thicknesses on the appearance of the heads. This information is essential to the interpretation of images in the accompanying paper (Franzini-Armstrong, C., and D.J. Ferguson, 1985, Biophys. J., 48:607-615).

Time-resolved x-ray diffraction studies of the sarcoplasmic reticulum membrane during active transport

X-ray and neutron diffraction studies of oriented multilayers of a highly purified fraction of isolated sarcoplasmic reticulum (SR) have previously provided the separate profile structures of the lipid bilayer and the Ca2+-ATPase molecule within the membrane profile to approximately 10-A resolution. These studies used biosynthetically deuterated SR phospholipids incorporated isomorphously into the isolated SR membranes via phospholipid transfer proteins. Time-resolved x-ray diffraction studies of these oriented SR membrane multilayers have detected significant changes in the membrane profile structure associated with phosphorylation of the Ca2+-ATPase within a single turnover of the Ca2+-transport cycle. These studies used the flash photolysis of caged ATP to effectively synchronize the ensemble of Ca2+-ATPase molecules in the multilayer, synchrotron x-radiation to provide 100-500-ms data collection times, and double-beam spectrophotometry to monitor the Ca2+-transport process directly in the oriented SR membrane multilayer.

Structure-function studies of canine cardiac sarcolemmal membranes. II. Structural organization of the sarcolemmal membrane as determined by electron microscopy and lamellar X-ray diffraction

The morphological and ultrastructural properties of highly purified canine cardiac sarcolemmal vesicles, prepared by a modification (Colvin, R.A., Ashavaid, T.F. and Herbette, L.G. (1985) Biochim. Biophys. Acta 812, 601-608) of the method of Jones et al. (Jones L.R., Madlock, S.W. and Besch, H.R. (1980) J. Biol. Chem. 255, 9971-9980), were examined by several techniques. Thin-section electron microscopy showed predominantly intact unilamellar vesicles with little staining beyond the lipid bilayer boundaries. Freeze-fracture electron microscopy demonstrated that the majority of particles are approx. 90 A diameter and present at a density of 780 +/- 190 micrometers-2 (+/- S.D.). If it is assumed that some of these particles represent the (Na+ + K+)-ATPase, the finding that they are largely confined to the convex fracture face suggests a predominant right-side-out orientation of these sarcolemmal vesicles that is consistent with biochemical assays. The sarcolemmal membrane width measured by electron microscopy (unhydrated membrane width of 50-70 A) is consistent with the unit cell dimensions of 56-77 A determined by lamellar X-ray diffraction (hydrated membrane width). A unit cell dimension of 56-62 A was also found by X-ray diffraction for sarcolemmal lipids extracted from these preparations, indicating that the isolated sarcolemmal preparations do not contain a significant surface coat (glycocalyx). As both cardiac and skeletal sarcoplasmic reticulum membranes have a 80-100 A membrane width, these findings demonstrate that the purified sarcolemmal membrane is structurally distinct from both cardiac and skeletal sarcoplasmic reticulum. In contrast to the protein-rich skeletal sarcoplasmic reticulum membrane, which contains a single essential protein responsible for the regulation of cytosolic Ca2+ concentration, the sarcolemma is a lipid-rich membrane that contains a variety of proteins associated with many regulatory functions served by this membrane in cardiac muscle.

Decavanadate is responsible for vanadate-induced two-dimensional crystals in sarcoplasmic reticulum

Two-dimensional protein crystals of the calcium pump protein of sarcoplasmic reticulum (SR) from fast skeletal muscle were induced using Na3VO3 as first described by Dux and Martonosi. These crystals exhibit repeat rows approximately 11 nm apart which contain discrete units with approximately 7 nm repeats. Four different methods of sample preparation for electron microscopy, i.e., negative staining, freeze-drying, freeze-fracturing, and thin-sectioning electron microscopy, each give complimentary repeat units. The SR-membrane crystals exhibit surface structure by the freeze-drying technique and row-like structures on the normally smooth outer face of normal SR. The formation of the membrane crystals is dependent on the pH and concentration of the vanadate. Only conditions favoring the presence of decavanadate yield crystals. At low concentrations and neutral pH, decavanadate is unstable and with time converts to smaller oligomers and the monomer. The presence of membrane crystals was correlated with the life span of the decavanadate. Membrane crystals were obtained in the SR membrane from fast twitch muscle from light and heavy SR, referable to longitudinal and terminal cisternae as well as from reconstituted SR. Canine cardiac SR did not crystallize under these conditions.

Phospholipid asymmetry in the isolated sarcoplasmic reticulum membrane

The total phospholipid content and distribution of phospholipid species between the outer and inner monolayers of the isolated sarcoplasmic reticulum membrane was measured by phospholipase A2 activities and neutron diffraction. Phospholipase measurements showed that specific phospholipid species were asymmetric in their distribution between the outer and inner monolayers of the sarcoplasmic reticulum lipid bilayer; phosphatidylcholine (PC) was distributed 48/52 +/- 2% between the outer and inner monolayer of the sarcoplasmic reticulum bilayer, 69% of the phosphatidyl-ethanolamine (PE) resided mainly in the outer monolayer of the bilayer, 85% of the phosphatidylserine (PS) and 88% of the phosphatidylinositol (PI) were localized predominantly in the inner monolayer. The total phospholipid distribution determined by these measurements was 48/52 +/- 2% for the outer/inner monolayer of the sarcoplasmic reticulum lipid bilayer. Sarcoplasmic reticulum phospholipids were biosynthetically deuterated and exchanged into isolated vesicles with both a specific lecithin and a general exchange protein. Neutron diffraction measurements directly provided lipid distribution profiles for both PC and the total lipid content in the intact sarcoplasmic reticulum membrane. The outer/inner monolayer distribution for PC was 47/53 +/- 1%, in agreement with phospholipase measurements, while that for the total lipid was 46/54 +/- 1%, similar to the phospholipase measurements. These neutron diffraction results regarding the sarcoplasmic reticulum membrane bilayer were used in model calculations for decomposing the electron-density profile structure (10 A resolution) of isolated sarcoplasmic reticulum previously determined by X-ray diffraction into structures for the separate membrane components. These structure studies showed that the protein profile structure within the membrane lipid bilayer was asymmetric, complementary to the asymmetric lipid structure. Thus, the total phospholipid asymmetry obtained by two independent methods was small but consistent with a complementary asymmetric protein structure, and may be related to the highly vectorial functional properties of the calcium pump ATPase protein in the sarcoplasmic reticulum membrane.

Crystallization of intramembrane particles in rabbit sarcoplasmic reticulum vesicles by vanadate

Sarcoplasmic reticulum (SR) membranes isolated from rabbit skeletal muscle appear in freeze-fracture as 0.15-0.2 micron vesicles. The concave fracture surface (P-face) contains a dense population of 8.5 nm particles that were previously identified as the Ca2+-transport ATPase. The convex surface (E-face) is mostly smooth, displaying an occasional particle but no complementary arrays of pits. Incubation of the vesicles at 4 degrees C in calcium-free solutions containing 5 mM Na3VO4 induces the formation of two-dimensional crystalline arrays of the Ca2+, Mg2+-ATPase, accompanied by structural changes visible by freeze-etch electron microscopy. Most vesicles elongate into tubules 60-80 nm in diameter and the 8.5 nm intramembrane particles of the P-face become regularly organized into parallel ridges. The ridges are coiled around the tubules in right-handed helices, oriented at 50-60 degrees angle to the long axis of the tubules. The particles repeat along the rows at about 5.5 nm and the rows repeat at 10.5-11.0. Occasionally the ridges seem to break up into 8.5 nm particles. Parallel furrows are visible on the (convex) E-face of the tubules. In high resolution replicas, the furrows are resolved into rows of pits that are complementary images of the ridges. Deep etching and rotary shadowing reveal oblique crests on the protoplasmic surface, consisting of dimeric particles close to 8.5 X 5.5 nm in size, in which each monomer can frequently be resolved into two structural domains. These data suggest that vanadate induces a conformational change in the Ca2+-transport ATPase, with crystallization of the intramembrane particles.

Structure of the vanadate-induced crystals of sarcoplasmic reticulum Ca2+-ATPase

The projected structure of the vanadate-induced crystalline aggregates of Ca2+-ATPase molecules in isolated sarcoplasmic reticulum membranes has been determined. The molecules form tubular crystals with an oblique surface lattice having cell dimensions a = 65.9 A, b = 114.4 A and gamma = 77.9 degrees. The space group is P2. The crystalline tubules are formed through lateral aggregation of chains made up of dimers of Ca2+-ATPase molecules.

Comparison of the kinetics of calcium transport in vesicular dispersions and oriented multilayers of isolated sarcoplasmic reticulum membranes

Knowledge of the functional properties of the protein in oriented multilayers, in addition to vesicular dispersions, of membranes such as the isolated sarcoplasmic reticulum (SR), extends the variety of techniques that can be effectively used in studies of the membrane protein's structure or structural changes associated with its function. One technique requiring the use of oriented multilayers to provide more direct time-averaged and time-resolved structural investigations of the SR membrane is x-ray diffraction. Therefore, the kinetics of ATP-induced calcium uptake by isolated SR vesicles in dispersions and hydrated, oriented multilayers were compared. Ca2+ uptake was necessarily initiated by the addition of ATP through flash photolysis of caged ATP, P3-1-(2-nitro)phenylethyl adenosine 5'-triphosphate, with either a frequency-doubled ruby laser or a 200 W Hg arc lamp, and measured with two different detector systems that followed the absorbance changes of the metallochromic indicator arsenazo III, which is sensitive to changes in the extravesicular [Ca2+]. The temperature range investigated was -2 degrees to 26 degrees C. The Ca2+ uptake kinetics of SR membranes in both the vesicular dispersions and oriented multilayers consist of at least two phases, an initial fast phase and a subsequent slow phase. The fast phase, generally believed to be associated with the formation of the phosphorylated enzyme, E approximately P, is kinetically comparable in both SR dispersions and multilayers. The slow phase mathematically follows first-order kinetics with specific rate constants of approximately 0.6 s-1 and approximately 1.2 s-1 for the dispersions at 26 degrees C and multilayers at 21 degrees C, respectively, with the given experimental conditions. The slow phase, generally believed to be associated with the translocation of Ca+2, across the membrane profile, appears to be the same process in SR dispersions and multilayers through their virtually identical rate constants and their identical activation energies of 22 +/-1 kcal mol -1. The stoichiometry of ~2 mol Ca2+/mol ATP hydrolyzed was measured in dispersions for the slow phase of Ca2+ uptake. Photolysis of caged ATP with the lamp and the laser provides comparable results for the Ca2+ uptake kinetics in SR dispersions and multilayers. Laser flash photolysis, however, has the advantages of optimal time resolution and effective synchronization of the ensemble of Ca2+-ATPase molecules in the ATP initiated Ca2+ transport process.

The structure of the Ca2+ ATPase as revealed by electron microscopy and image processing of ordered arrays

Two-dimensional ordered arrays of the membrane-bound Ca2+ ATPase, were formed over a wide range of conditions (i.e., pH, ionic strength, temperature) in the presence of vanadate, and studied by electron microscopy and image processing. These ordered tubular and spherical membrane vesicles of Ca2+ ATPase could also be formed with approximately one bound ATP and between one and two nonchelatable Ca2+ bound. The tubular arrays ranged between 1 and 10 microns in length and had an average flattened diameter of 90 nm, as observed in negatively stained preparations. The basic building blocks of these ordered arrays appear to be linear ribbons of Ca2+ ATPase dimers. Fourier analysis of electron micrographs of these flattened tubes revealed a near-rectangular lattice (lattice angle 73.3 +/- 4.6 degrees with average lattice constants of a = 6.2 +/- 0.25 nm, and b = 11.5 +/- 0.30 nm). The double-stranded ribbons (i.e., parallel to a) are inclined by 56 +/- 3.7 degrees relative to the tube axis in a right-handed sense, as determined from freeze-dried metal-shadowed specimens. Computer averaging of negatively stained arrays reveals a crystallographic dimer of stain-excluding matter. The dimensions of each monomer within this dimer are consistent with established structural parameters, leading us to believe a form of the Ca2+ ATPase, capable of binding at least one ATP and of binding Ca2+ ions, may exist as a dimer in the sarcoplasmic reticulum.

The determination of the separate Ca2+ pump protein and phospholipid profile structures within reconstituted sarcoplasmic reticulum membranes via X-ray and neutron diffraction

We have previously compared the electron density profiles for several highly-functional reconstituted sarcoplasmic reticulum membranes with that for the isolated sarcoplasmic reticulum membrane (Herbette, L., Scarpa, A., Blasie, J.K., Wang, C.T., Saito, A. and Fleischer, S. (1981) Biophys. J. 36, 47-72). In this paper, we compare the separate calcium pump protein profile within these reconstituted sarcoplasmic reticulum membranes, as derived by X-ray and neutron diffraction methods, with that within isolated sarcoplasmic reticulum membranes. In addition, the time-average perturbation of the lipid bilayer by the incorporated calcium pump protein within these reconstituted sarcoplasmic reticulum membranes has been determined in some detail.