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

Structural characterization of the voltage-sensor domain and voltage-gated K+-channel proteins vectorially oriented within a single bilayer membrane at the solid/vapor and solid/liquid interfaces via neutron interferometry

The voltage-sensor domain (VSD) is a modular four-helix bundle component that confers voltage sensitivity to voltage-gated cation channels in biological membranes. Despite extensive biophysical studies and the recent availability of X-ray crystal structures for a few voltage-gated potassium (Kv) channels and a voltage-gate sodium (Nav) channel, a complete understanding of the cooperative mechanism of electromechanical coupling, interconverting the closed-to-open states (i.e., nonconducting to cation conducting) remains undetermined. Moreover, the function of these domains is highly dependent on the physical-chemical properties of the surrounding lipid membrane environment. The basis for this work was provided by a recent structural study of the VSD from a prokaryotic Kv-channel vectorially oriented within a single phospholipid (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)) membrane investigated by X-ray interferometry at the solid/moist He (or solid/vapor) and solid/liquid interfaces, thus achieving partial to full hydration, respectively (Gupta et al. Phys. Rev. E2011, 84, 031911-1-15). Here, we utilize neutron interferometry to characterize this system in substantially greater structural detail at the submolecular level, due to its inherent advantages arising from solvent contrast variation coupled with the deuteration of selected submolecular membrane components, especially important for the membrane at the solid/liquid interface. We demonstrate the unique vectorial orientation of the VSD and the retention of its molecular conformation manifest in the asymmetric profile structure of the protein within the profile structure of this single bilayer membrane system. We definitively characterize the asymmetric phospholipid bilayer solvating the lateral surfaces of the VSD protein within the membrane. The profile structures of both the VSD protein and phospholipid bilayer depend upon the hydration state of the membrane. We also determine the distribution of water and exchangeable hydrogen throughout the profile structure of both the VSD itself and the VSD:POPC membrane. These two experimentally determined water and exchangeable hydrogen distribution profiles are in good agreement with molecular dynamics simulations of the VSD protein vectorially oriented within a fully hydrated POPC bilayer membrane, supporting the existence of the VSD's water pore. This approach was extended to the full-length Kv-channel (KvAP) at a solid/liquid interface, providing the separate profile structures of the KvAP protein and the POPC bilayer within the reconstituted KvAP:POPC membrane.

Profile structures of the voltage-sensor domain and the voltage-gated K(+)-channel vectorially oriented in a single phospholipid bilayer membrane at the solid-vapor and solid-liquid interfaces determined by x-ray interferometry

One subunit of the prokaryotic voltage-gated potassium ion channel from Aeropyrum pernix (KvAP) is comprised of six transmembrane α helices, of which S1-S4 form the voltage-sensor domain (VSD) and S5 and S6 contribute to the pore domain (PD) of the functional homotetramer. However, the mechanism of electromechanical coupling interconverting the closed-to-open (i.e., nonconducting-to-K(+)-conducting) states remains undetermined. Here, we have vectorially oriented the detergent (OG)-solubilized VSD in single monolayers by two independent approaches, namely "directed-assembly" and "self-assembly," to achieve a high in-plane density. Both utilize Ni coordination chemistry to tether the protein to an alkylated inorganic surface via its C-terminal His_{6} tag. Subsequently, the detergent is replaced by phospholipid (POPC) via exchange, intended to reconstitute a phospholipid bilayer environment for the protein. X-ray interferometry, in which interference with a multilayer reference structure is used to both enhance and phase the specular x-ray reflectivity from the tethered single membrane, was used to determine directly the electron density profile structures of the VSD protein solvated by detergent versus phospholipid, and with either a moist He (moderate hydration) or bulk aqueous buffer (high hydration) environment to preserve a native structure conformation. Difference electron density profiles, with respect to the multilayer substrate itself, for the VSD-OG monolayer and VSD-POPC membranes at both the solid-vapor and solid-liquid interfaces, reveal the profile structures of the VSD protein dominating these profiles and further indicate a successful reconstitution of a lipid bilayer environment. The self-assembly approach was similarly extended to the intact full-length KvAP channel for comparison. The spatial extent and asymmetry in the profile structures of both proteins confirm their unidirectional vectorial orientation within the reconstituted membrane and indicate retention of the protein's folded three-dimensional tertiary structure upon completion of membrane bilayer reconstitution. Moreover, the resulting high in-plane density of vectorially oriented protein within a fully hydrated single phospholipid bilayer membrane at the solid-liquid interface will enable investigation of their conformational states as a function of the transmembrane electric potential.

Monolayers of a model anesthetic-binding membrane protein: formation, characterization, and halothane-binding affinity

hbAP0 is a model membrane protein designed to possess an anesthetic-binding cavity in its hydrophilic domain and a cation channel in its hydrophobic domain. Grazing incidence x-ray diffraction shows that hbAP0 forms four-helix bundles that are vectorially oriented within Langmuir monolayers at the air-water interface. Single monolayers of hbAP0 on alkylated solid substrates would provide an optimal system for detailed structural and dynamical studies of anesthetic-peptide interaction via x-ray and neutron scattering and polarized spectroscopic techniques. Langmuir-Blodgett and Langmuir-Schaeffer deposition and self-assembly techniques were used to form single monolayer films of the vectorially oriented peptide hbAP0 via both chemisorption and physisorption onto suitably alkylated solid substrates. The films were characterized by ultraviolet absorption, ellipsometry, circular dichroism, and polarized Fourier transform infrared spectroscopy. The alpha-helical secondary structure of the peptide was retained in the films. Under certain conditions, the average orientation of the helical axis was inclined relative to the plane of the substrate, approaching perpendicular in some cases. The halothane-binding affinity of the vectorially oriented hbAP0 peptide in the single monolayers, with the volatile anesthetic introduced into the moist vapor environment of the monolayer, was found to be similar to that for the detergent-solubilized peptide.

A time-resolved Fourier transformed infrared difference spectroscopy study of the sarcoplasmic reticulum Ca(2+)-ATPase: kinetics of the high-affinity calcium binding at low temperature

We have used time-resolved Fourier transformed infrared difference spectroscopy to characterize the amplitude, frequency, and kinetics of the absorbance changes induced in the infrared (IR) spectrum of sarcoplasmic reticulum Ca(2+)-ATPase by calcium binding at the high-affinity transport sites. 1-(2-Nitro-4,5-dimethoxyphenyl)-N,N,N',N'-tetrakis [(oxycarbonyl)methyl]-1,2-ethanediamine (DM-nitrophen) was used as a caged-calcium compound to trigger the release of calcium in the IR samples. Calcium binding to Ca(2+)-ATPase induces the appearance of spectral bands in difference spectra that are all absent in the presence of the inhibitor thapsigargin. Spectral bands above 1700 cm-1 indicate that glutamic and/or aspartic acid side chains are deprotonated upon calcium binding, whereas other bands may be induced by reactions of asparagine, glutamine, and tyrosine residues. Some of the bands appearing in the 1690-1610 cm-1 region arise from modifications of peptide backbone carbonyl groups. The band at 1653 cm-1 is a candidate for a change in an alpha-helix, whereas other bands could arise from modifications of random, turn, or beta-sheet structures or from main-chain carbonyl groups playing the role of calcium ligands. Only a few residues are involved in secondary structure changes. The kinetic evolution of these bands was recorded at low temperature (-9 degrees C). All bands exhibited a monophasic kinetics of rate constant 0.026 s-1, which is compatible with that measured in previous study at the same temperature in a suspension of sarcoplasmic reticulum vesicles by intrinsic fluorescence of Ca(2+)-ATPase.

Structural changes of sarcoplasmic reticulum Ca(2+)-ATPase upon Ca2+ binding studied by simultaneous measurement of infrared absorbance changes and changes of intrinsic protein fluorescence

Ca2+ binding to sarcoplasmic reticulum Ca(2+)-ATPase was investigated by Fourier transform infrared (FTIR) spectroscopy using the photolytic release of Ca2+ from the photolabile Ca2+ chelator 1-(2-nitro-4,5-dimethoxy)-N,N,N',N',- tetrakis[(oxycarbonyl)]methyl-1,2-ethandiamine (DM-nitrophen). IR absorbance changes in 1H2O and 2H2O were detected in the spectral region from 1800 cm-1 to 1200 cm-1, reflecting photolysis of DM-nitrophen and Ca2+ binding to the Ca(2+)-ATPase. As an independent probe for protein conformational changes, intrinsic fluorescence changes upon Ca2+ release were monitored simultaneously to the FTIR measurements. Both the IR absorbance changes and the fluorescence intensity changes correlated well with the Ca2+ binding activity of the ATPase in this specific step. Ca2+ binding caused IR difference bands mainly in the region of amide I absorption of the polypeptide backbone, reflecting conformational changes of the protein. The small amplitude of the signals indicates that only a few residues perform local structural changes such as changes of bond angles or hydrogen bonding. Other absorbance changes appearing above 1700 cm-1 can be assigned to Ca2+ binding to Glu or Asp side chain carboxyl groups and concomitant deprotonation of these residues. This assignment is strengthened by downshifts of these bands by 4 cm-1 to 6 cm-1 upon 1H2O/2H2O exchange. This is in line with results of mutagenesis studies where such residues containing carboxyl groups were associated with the high affinity Ca2+ binding site (Clarke, D.M., Loo, T.W. and MacLennan, D.H. (1990) J. Biol. Chem. 265, 6262-6267).

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.

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

Direct measurements of phosphorylation of the Ca2+ ATPase of the sarcoplasmic reticulum (SR) have shown that the lifetime of the first phosphorylated intermediate in the Ca2+ transport cycle, E1 approximately P, increases with decreasing [Mg2+] (Dupont, Y. 1980. Eur. J. Biochem. 109:231-238). Previous x-ray diffraction work (Pascolini, D., and J.K. Blasie. 1988. Biophys. J. 54:669-678) under high [Mg2+] conditions (25 mM) indicated that changes in the profile structure of the SR membrane could be responsible for the low-temperature transient trapping of E1 approximately P that occurs at temperatures below 2-3 degrees C, the upper characteristic temperature th for lipid lateral phase separation in the membrane. We now present results of our study of the Ca2+ uptake kinetics and of the structure of the SR membrane at low [Mg2+] (less than or equal to 100 microM). Our results show a slowing in the kinetics of both phases of the Ca2+ uptake process and an increase in the duration of the plateau of the fast phase before the onset of the slow phase, indicating an increase in the lifetime (transient trapping) of E1 approximately P. Calcium uptake kinetics at low [Mg2+] and moderately low temperature (approximately 0 degree C) are similar to those observed at much lower temperatures (approximately -10 degrees C) at high [Mg2+]. The temperature-induced structural changes that we observed at low [Mg2+] are much more pronounced than those found to occur at higher [Mg2+]. Also, at the lower [Mg2+] the upper characteristic temperature th for lipid lateral phase separation was found to be higher, at approximately 8-10 degrees C. Our studies indicate that both temperature and [Mg2+] affect the structure and the functionality (as measured by changes in the kinetics of Ca2+ uptake) of the SR membrane. Membrane lipid phase behavior and changes in the Ca2+ ATPase profile structure seem to be related, and we have found that structural changes are responsible for the slowing of the kinetics of the fast phase of Ca2+ uptake, and could also mediate the effect that [Mg2+] has on E1 approximately P lifetime.

Moderate resolution profile structure of the sarcoplasmic reticulum membrane under low temperature conditions for the transient trapping of E1 approximately P

The calcium uptake reaction kinetics of isolated sarcoplasmic reticulum (SR) vesicles have previously been shown to be at least biphasic over a range of temperatures (26 to 10 degrees C) with a fast phase identified with the formation of E1 approximately P and calcium occlusion and a slow phase with Ca2+ translocation across the membrane and turnover of the Ca2+ ATPase ensemble. At "low" temperatures, namely 0 degrees C or lower, E1 approximately P formation is slowed and E1 approximately P is transiently trapped for at least several seconds, as indicated by the absence of the slow phase for 6 s or more. We now report that a reversible, temperature-induced structural transition occurs at about 2-3 degrees C for the isolated SR membrane. We have investigated the nature of this structural transition utilizing meridional and equatorial x-ray diffraction studies of the oriented SR membrane multilayers in the range of temperatures between 7.5 and -2 degrees C. The phase meridional (lamellar) diffraction has provided the profile structure for the SR membrane at the highest vs. lowest temperature at the same moderate resolution of 16-17 A while the equatorial diffraction has provided information on the average lipid chain packing in the SR membrane plane in the two cases. To identify the contribution of each membrane component in producing the differences between the profile structures at 7.5 and -2 degrees C, step-function models have been fitted to the moderate resolution electron density profiles. Lipid lateral phase separation may be responsible for inducing the structural change in the Ca2+ ATPase, thereby resulting in the slowing of E1 approximately P formation and the transient trapping of E1 approximately P at the "lower" temperatures.

Changes in the sarcoplasmic reticulum membrane profile induced by enzyme phosphorylation to E1 approximately P at 16 A resolution via time-resolved x-ray diffraction

Time-resolved x-ray diffraction studies of the isolated sarcoplasmic reticulum (SR) membrane have provided the difference electron density profile for the SR membrane for which the Ca2+ ATPase is transiently trapped exclusively in the first phosphorylated intermediate state, E1 approximately P, in absence of detectable enzyme turnover vs. that before ATP-initiated phosphorylation of the enzyme. These diffraction studies, which utilized the flash-photolysis of caged ATP, were performed at temperatures between 0 and -2 degrees C and with a time-resolution of 2-5 s. Analogous time-resolved x-ray diffraction studies of the SR membrane at 7-8 degrees C with a time resolution of 0.2-0.5 s have previously provided the difference electron density profile for the SR membrane for which the Ca2+ ATPase is only predominately in the first phosphorylated intermediate state under conditions of enzyme turnover vs. that before enzyme phosphorylation. The two difference profiles, compared at the same low resolution (approximately 40 A), are qualitatively similar but nevertheless contain some distinctly different features and have therefore been analyzed via a step-function model analysis. This analysis was based on the refined step-function models for the two different electron density profiles obtained independently from x-ray diffraction studies at higher resolution (16-17 A) of the SR membrane before enzyme phosphorylation at 7.5 and -2 degrees C. The step-function model analysis indicated that the low resolution difference profiles derived from both time-resolved x-ray diffraction experiments arise from a net movement of Ca2+ ATPase protein mass from the outer monolayer to the inner monolayer of the SR membrane lipid bilayer. The conserved redistribution of this protein mass is however somewhat different for the two cases, especially at the extravesicular membrane surface containing the Ca2+ATPase "headpiece." However, the conserved redistribution of protein mass within the SR membrane lipid bilayer common to both cases is clearly due to E1~P formation.

The separate profile structures of the functional calcium pump protein and the phospholipid bilayer within isolated sarcoplasmic reticulum membranes determined by X-ray and neutron diffraction

The detailed profile structure of the isolated sarcoplasmic reticulum membrane was studied utilizing a combination of X-ray and neutron diffraction. The water and lipid profile structures within the sarcoplasmic reticulum membrane were determined at 28 A resolution directly by neutron diffraction and selective deuteration of the water and lipid components. The previously determined electron density profile structure of the sarcoplasmic reticulum membrane at 12 A resolution was subjected to model refinement analysis constrained by the neutron diffraction results, thereby providing unique higher resolution calculated lipid and protein profile structures. It was found that the lipid bilayer profile structure of the isolated sarcoplasmic reticulum membrane is asymmetric, primarily the result of more lipid residing in the inner versus the outer monolayer of the sarcoplasmic reticulum lipid bilayer. The asymmetry in the lipid composition was necessarily coincident with a complimentary asymmetry in the protein mass distribution between the two monolayers in order to preserve the overall cross-sectional area of lipid and protein throughout the lipid bilayer region of the sarcoplasmic reticulum membrane profile structure. Approximately 50% of the mass of the total protein was found to be localized externally to the sarcoplasmic reticulum membrane lipid bilayer protruding from the outer lipid monolayer into the extravesicular medium. The structural features of the protein protrusion appear to be rather variable depending upon the environment of the sarcoplasmic reticulum membrane. This highly asymmetric structural organization of the sarcoplasmic reticulum membrane profile is consistent with its primary function of unidirectional calcium transport.

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.