Kinetics of the Ca(2+), H(+), and Mg(2+) interaction with the ion-binding sites of the SR Ca-ATPase

Structure and Mechanism of P-Type ATPase Ion Pumps

P-type ATPases are found in all kingdoms of life and constitute a wide range of cation transporters, primarily for H⁺, Na⁺, K⁺, Ca²⁺, and transition metal ions such as Cu(I), Zn(II), and Cd(II). They have been studied through a wide range of techniques, and research has gained very significant insight on their transport mechanism and regulation. Here, we review the structure, function, and dynamics of P2-ATPases including Ca²⁺-ATPases and Na,K-ATPase. We highlight mechanisms of functional transitions that are associated with ion exchange on either side of the membrane and how the functional cycle is regulated by interaction partners, autoregulatory domains, and off-cycle states. Finally, we discuss future perspectives based on emerging techniques and insights.

Reliable measurement of free Ca2+ concentrations in the ER lumen using Mag-Fluo-4

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Mag-Fluo-4 loaded into ER by incubation of cells with Mag-Fluo-4 AM reliably reports ER free [Ca²⁺].

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However, the Ca²⁺ affinity of Mag-Fluo-4 determined in vitro appears too high to allow measurements of ER luminal [Ca²⁺].

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We use an antibody (QAb) to quench the fluorescence of leaked indicator.

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Using QAb, we show that indicator within the ER has low affinity and sensitivity across a wide range of [Ca²⁺].

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Incomplete de-esterification of compartmentalized indicator allows it to effectively report ER free [Ca²⁺].

Synthetic Ca²⁺ indicators are widely used to report changes in free [Ca²⁺], usually in the cytosol but also within organelles. Mag-Fluo-4, loaded into the endoplasmic reticulum (ER) by incubating cells with Mag-Fluo-4 AM, has been used to measure changes in free [Ca²⁺] within the ER, where the free [Ca²⁺] is estimated to be between 100 μM and 1 mM. Many results are consistent with Mag-Fluo-4 reliably reporting changes in free [Ca²⁺] within the ER, but the results are difficult to reconcile with the affinity of Mag-Fluo-4 for Ca²⁺ measured in vitro (K_D^Ca ∼22 μM). Using an antibody to quench the fluorescence of indicator that leaked from the ER, we established that the affinity of Mag-Fluo-4 within the ER is much lower (K_D^Ca ∼1 mM) than that measured in vitro. We show that partially de-esterified Mag-Fluo-4 has reduced affinity for Ca²⁺, suggesting that incomplete de-esterification of Mag-Fluo-4 AM within the ER provides indicators with affinities for Ca²⁺ that are both appropriate for the ER lumen and capable of reporting a wide range of free [Ca²⁺].

The SarcoEndoplasmic Reticulum Calcium ATPase

The calcium pump (a.k.a. Ca²⁺-ATPase or SERCA) is a membrane transport protein ubiquitously found in the endoplasmic reticulum (ER) of all eukaryotic cells. As a calcium transporter, SERCA maintains the low cytosolic calcium level that enables a vast array of signaling pathways and physiological processes (e.g. synaptic transmission, muscle contraction, fertilization). In muscle cells, SERCA promotes relaxation by pumping calcium ions from the cytosol into the lumen of the sarcoplasmic reticulum (SR), the main storage compartment for intracellular calcium. X-ray crystallographic studies have provided an extensive understanding of the intermediate states that SERCA populates as it progresses through the calcium transport cycle. Historically, SERCA is also known to be regulated by small transmembrane peptides, phospholamban (PLN) and sarcolipin (SLN). PLN is expressed in cardiac muscle, whereas SLN predominates in skeletal and atrial muscle. These two regulatory subunits play critical roles in cardiac contractility. While our understanding of these regulatory mechanisms are still developing, SERCA and PLN are one of the best understood examples of peptide-transporter regulatory interactions. Nonetheless, SERCA appeared to have only two regulatory subunits, while the related sodium pump (a.k.a. Na⁺, K⁺-ATPase) has at least nine small transmembrane peptides that provide tissue specific regulation. The last few years have seen a renaissance in our understanding of SERCA regulatory subunits. First, structures of the SERCA-SLN and SERCA-PLN complexes revealed molecular details of their interactions. Second, an array of micropeptides concealed within long non-coding RNAs have been identified as new SERCA regulators. This chapter will describe our current understanding of SERCA structure, function, and regulation.

Activation of SK2 channels preserves ER Ca²⁺ homeostasis and protects against ER stress-induced cell death

Alteration of endoplasmic reticulum (ER) Ca(2+) homeostasis leads to excessive cytosolic Ca(2+) accumulation and delayed neuronal cell death in acute and chronic neurodegenerative disorders. While our recent studies established a protective role for SK channels against excessive intracellular Ca(2+) accumulation, their functional role in the ER has not been elucidated yet. We show here that SK2 channels are present in ER membranes of neuronal HT-22 cells, and that positive pharmacological modulation of SK2 channels with CyPPA protects against cell death induced by the ER stressors brefeldin A and tunicamycin. Calcium imaging of HT-22 neurons revealed that elevated cytosolic Ca(2+) levels and decreased ER Ca(2+) load during sustained ER stress could be largely prevented by SK2 channel activation. Interestingly, SK2 channel activation reduced the amount of the unfolded protein response transcription factor ATF4, but further enhanced the induction of CHOP. Using siRNA approaches we confirmed a detrimental role for ATF4 in ER stress, whereas CHOP regulation was dispensable for both, brefeldin A toxicity and CyPPA-mediated protection. Cell death induced by blocking Ca(2+) influx into the ER with the SERCA inhibitor thapsigargin was not prevented by CyPPA. Blocking the K(+) efflux via K(+)/H(+) exchangers with quinine inhibited CyPPA-mediated neuroprotection, suggesting an essential role of proton uptake and K(+) release in the SK channel-mediated neuroprotection. Our data demonstrate that ER SK2 channel activation preserves ER Ca(2+) uptake and retention which determines cell survival in conditions where sustained ER stress contributes to progressive neuronal death.

CalDyx: an interactive self-study teaching program on calcium dynamics during excitation-contraction coupling in skeletal muscle cells

Skeletal muscle cells release and resequester stored calcium ions to contract and subsequently relax. CalDyx (short for "Calcium Dynamics") is a self-contained software program that teaches the student about the major events that underlie the excitation-contraction (EC) coupling process in skeletal muscle. The program simulates changes in the concentrations of myoplasmic calcium (Ca(2+)) in three types of adult vertebrate muscle fibers (slow-twitch, fast-twitch, and superfast-twitch) in response to action potentials. The simulations are closely tied to experimental measurements of changes in the myoplasmic free Ca(2+) concentration performed with calcium indicator dyes introduced by microinjection into these fiber types. The program includes 10 exercises that guide the student through the major signaling events of EC coupling, including the release of Ca(2+) from the sarcoplasmic reticulum (SR), the binding of Ca(2+) to the major myoplasmic Ca(2+) binding sites, and the return of Ca(2+) to the SR by the Ca "pump" proteins of the SR. The program is quantitatively based and should increase the student's understanding of kinetic processes within cells.

Potassium fluxes across the endoplasmic reticulum and their role in endoplasmic reticulum calcium homeostasis

There are a number of known and suspected channels and exchangers in the endoplasmic reticulum that may participate in potassium flux across its membrane. They include trimeric intracellular cation channels permeable for potassium, ATP-sensitive potassium channels, calcium-activated potassium channels and the potassium-hydrogen exchanger. Apart from trimeric intracellular cation channels, which are specific to the endoplasmic reticulum, other potassium channels are also expressed in the plasma membrane and/or mitochondria, and their specific role in the endoplasmic reticulum has not yet been fully established. In addition to these potassium-selective channels, the ryanodine receptor and, potentially, the inositol 1,4,5-trisphosphate receptor are permeable to potassium ions. Also, the role of potassium fluxes across the endoplasmic reticulum membrane has remained elusive. It has been proposed that their main role is to balance the charge movement that occurs during calcium release and uptake from or to the endoplasmic reticulum. This review aims to summarize current knowledge on endoplasmic reticulum potassium channels and fluxes and their potential role in endoplasmic reticulum calcium uptake and release.

Intracellular calcium movements during relaxation and recovery of superfast muscle fibers of the toadfish swimbladder

Ca²⁺ movements during intercall intervals in superfast toadfish swimbladder muscle fibers depend on the accumulation of Ca²⁺ on parvalbumin and a consequent slow rate of Ca²⁺ pumping.

The mating call of the Atlantic toadfish is generated by bursts of high-frequency twitches of the superfast twitch fibers that surround the swimbladder. At 16°C, a calling period can last several hours, with individual 80–100-Hz calls lasting ∼500 ms interleaved with silent periods (intercall intervals) lasting ∼10 s. To understand the intracellular movements of Ca²⁺ during the intercall intervals, superfast fibers were microinjected with fluo-4, a high-affinity fluorescent Ca²⁺ indicator, and stimulated by trains of 40 action potentials at 83 Hz, which mimics fiber activity during calling. The fluo-4 fluorescence signal was measured during and after the stimulus trains; the signal was also simulated with a kinetic model of the underlying myoplasmic Ca²⁺ movements, including the binding and transport of Ca²⁺ by the sarcoplasmic reticulum (SR) Ca²⁺ pumps. The estimated total amount of Ca²⁺ released from the SR during a first stimulus train is ∼6.5 mM (concentration referred to the myoplasmic water volume). At 40 ms after cessation of stimulation, the myoplasmic free Ca²⁺ concentration ([Ca²⁺]) is below the threshold for force generation (∼3 µM), yet the estimated concentration of released Ca²⁺ remaining in the myoplasm (Δ[Ca_M]) is large, ∼5 mM, with ∼80% bound to parvalbumin. At 10 s after stimulation, [Ca²⁺] is ∼90 nM (three times the assumed resting level) and Δ[Ca_M] is ∼1.3 mM, with 97% bound to parvalbumin. Ca²⁺ movements during the intercall interval thus appear to be strongly influenced by (a) the accumulation of Ca²⁺ on parvalbumin and (b) the slow rate of Ca²⁺ pumping that ensues when parvalbumin lowers [Ca²⁺] near the resting level. With repetitive stimulus trains initiated at 10-s intervals, Ca²⁺ release and pumping come quickly into balance as a result of the stability (negative feedback) supplied by the increased rate of Ca²⁺ pumping at higher [Ca²⁺].

Endoplasmic reticulum potassium–hydrogen exchanger and small conductance calcium-activated potassium channel activities are essential for ER calcium uptake in neurons and cardiomyocytes

Calcium pumping into the endoplasmic reticulum (ER) lumen is thought to be coupled to a countertransport of protons through sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA) and the members of the ClC family of chloride channels. However, pH in the ER lumen remains neutral, which suggests a mechanism responsible for proton re-entry. We studied whether cation–proton exchangers could act as routes for such a re-entry. ER Ca2+ uptake was measured in permeabilized immortalized hypothalamic neurons, primary rat cortical neurons and mouse cardiac fibers. Replacement of K+ in the uptake solution with Na+ or tetraethylammonium led to a strong inhibition of Ca2+ uptake in neurons and cardiomyocytes. Furthermore, inhibitors of the potassium–proton exchanger (quinine or propranolol) but not of the sodium–proton exchanger reduced ER Ca2+ uptake by 56–82%. Externally added nigericin, a potassium–proton exchanger, attenuated the inhibitory effect of propranolol. Inhibitors of small conductance calcium-sensitive K+ (SKCa) channels (UCL 1684, dequalinium) blocked the uptake of Ca2+ by the ER in all preparations by 48–94%, whereas inhibitors of other K+ channels (IKCa, BKCa and KATP) had no effect. Fluorescence microscopy and western blot analysis revealed the presence of both SKCa channels and the potassium–proton exchanger leucine zipper-EF-hand-containing transmembrane protein 1 (LETM1) in ER in situ and in the purified ER fraction. The data obtained demonstrate that SKCa channels and LETM1 reside in the ER membrane and that their activity is essential for ER Ca2+ uptake.

Kinetics of luminal proton binding to the SR Ca-ATPase

An open membrane preparation containing SR Ca-ATPase was prepared from sarcoplasmic-reticulum vesicles to study the ion binding kinetics in the P-E(2) conformation. Because Ca(2+) and H(+) binding are electrogenic reactions, fluorescent styryl dyes could be used to determine changes in the binding site occupation in equilibrium titration experiments and time-resolved relaxation processes triggered by a pH jump. By photo release from caged proton the pH of the electrolyte could be decreased in a step of 0.1 pH units by a single ultraviolet-laser flash. Analysis of the pH-jump induced relaxation process in the P-E(2) conformation showed that three Ca-ATPase-specific processes could be identified, fast H(+) binding (τ < 100 μs) and pH-insensitive conformational relaxations after the release of the Ca(2+) ion (τ ∼160 ms), and a slow process (τ ∼3.4 s) whose origin could not be unambiguously revealed. The Ca(2+)-binding affinity in the P-E(2) conformation was reduced with increasing pH, a behavior that can be explained by a reversible transition of the empty P-E(2) state to an inactivated state of the ion pump. All findings are interpreted in the framework of the Post-Albers pump cycle introduced previously, supplemented by an additional transition to an inhibited state of the ion pump.

Modeling Ca2+ dynamics of mouse cardiac cells points to a critical role of SERCA's affinity for Ca2+

The SERCA2a isoform of the sarco/endoplasmic reticulum Ca(2+) pumps is specifically expressed in the heart, whereas SERCA2b is the ubiquitously expressed variant. It has been shown previously that replacement of SERCA2a by SERCA2b in mice (SERCA2(b/b) mice) results in only a moderate functional impairment, whereas SERCA activity is decreased by a 40% lower SERCA protein expression and by increased inhibition by phospholamban. To find out whether the documented kinetic differences in SERCA2b relative to SERCA2a (i.e., a twofold higher apparent Ca(2+) affinity, but twofold lower maximal turnover rate) can explain these compensatory changes, we simulated Ca(2+) dynamics in mouse ventricular myocytes. The model shows that the relative Ca(2+) transport capacity of SERCA2a and SERCA2b depends on the SERCA concentration. The simulations point to a dominant effect of SERCA2b's higher Ca(2+) affinity over its lower maximal turnover rate. The results suggest that increased systolic and decreased diastolic Ca(2+) levels in unstimulated conditions could contribute to the downregulation of SERCA in SERCA2(b/b) mice. In stress conditions, Ca(2+) handling is less efficient by SERCA2b than by SERCA2a, which might contribute to the observed hypertrophy in SERCA2(b/b) mice. Altogether, SERCA2a might be a better compromise between performance in basal conditions and performance during β-adrenergic stress.

Calcium indicators and calcium signalling in skeletal muscle fibres during excitation-contraction coupling

During excitation-contraction coupling in skeletal muscle, calcium ions are released into the myoplasm by the sarcoplasmic reticulum (SR) in response to depolarization of the fibre's exterior membranes. Ca(2+) then diffuses to the thin filaments, where Ca(2+) binds to the Ca(2+) regulatory sites on troponin to activate muscle contraction. Quantitative studies of these events in intact muscle preparations have relied heavily on Ca(2+)-indicator dyes to measure the change in the spatially-averaged myoplasmic free Ca(2+) concentration (Δ[Ca(2+)]) that results from the release of SR Ca(2+). In normal fibres stimulated by an action potential, Δ[Ca(2+)] is large and brief, requiring that an accurate measurement of Δ[Ca(2+)] be made with a low-affinity rapidly-responding indicator. Some low-affinity Ca(2+) indicators monitor Δ[Ca(2+)] much more accurately than others, however, as reviewed here in measurements in frog twitch fibres with sixteen low-affinity indicators. This article also examines measurements and simulations of Δ[Ca(2+)] in mouse fast-twitch fibres. The simulations use a multi-compartment model of the sarcomere that takes into account Ca(2+)'s release from the SR, its diffusion and binding within the myoplasm, and its re-sequestration by the SR Ca(2+) pump. The simulations are quantitatively consistent with the measurements and appear to provide a satisfactory picture of the underlying Ca(2+) movements.

Sarcoplasmic reticulum function in smooth muscle

The sarcoplasmic reticulum (SR) of smooth muscles presents many intriguing facets and questions concerning its roles, especially as these change with development, disease, and modulation of physiological activity. The SR's function was originally perceived to be synthetic and then that of a Ca store for the contractile proteins, acting as a Ca amplification mechanism as it does in striated muscles. Gradually, as investigators have struggled to find a convincing role for Ca-induced Ca release in many smooth muscles, a role in controlling excitability has emerged. This is the Ca spark/spontaneous transient outward current coupling mechanism which reduces excitability and limits contraction. Release of SR Ca occurs in response to inositol 1,4,5-trisphosphate, Ca, and nicotinic acid adenine dinucleotide phosphate, and depletion of SR Ca can initiate Ca entry, the mechanism of which is being investigated but seems to involve Stim and Orai as found in nonexcitable cells. The contribution of the elemental Ca signals from the SR, sparks and puffs, to global Ca signals, i.e., Ca waves and oscillations, is becoming clearer but is far from established. The dynamics of SR Ca release and uptake mechanisms are reviewed along with the control of luminal Ca. We review the growing list of the SR's functions that still includes Ca storage, contraction, and relaxation but has been expanded to encompass Ca homeostasis, generating local and global Ca signals, and contributing to cellular microdomains and signaling in other organelles, including mitochondria, lysosomes, and the nucleus. For an integrated approach, a review of aspects of the SR in health and disease and during development and aging are also included. While the sheer versatility of smooth muscle makes it foolish to have a "one model fits all" approach to this subject, we have tried to synthesize conclusions wherever possible.

A Lorentz model for weak magnetic field bioeffects: part II--secondary transduction mechanisms and measures of reactivity

In Part I it was shown that the thermal component of the motion of a charged particle in an oscillator potential, that is, within a molecular binding site, rotates at the Larmor frequency in an applied magnetic field. It was also shown that the Larmor angular frequency is independent of the thermal noise strength and thus offers a mechanism for the biological detection of weak (microT-range) magnetic fields. Part II addresses the question of how the Larmor trajectory could affect biological reactivity. The projection of the motion onto a Cartesian axis measures the nonuniformity of the Larmor trajectory in AC and combined AC/DC magnetic fields, suggesting a means of assessing resonances. A physically meaningful measure of reactivity based upon the classical oscillator trajectory is suggested, and the problem of initial conditions is addressed through averaging over AC phases. AC resonance frequencies occur at the Larmor frequency and at other frequencies, and are dependent upon the ratio of AC/DC amplitudes and target kinetics via binding lifetime. The model is compared with experimental data reported for a test of the ion parametric resonance (IPR) model on data from Ca2+ flux in membrane vesicles, neurite outgrowth from PC-12 cells and a cell-free calmodulin-dependent myosin phosphorylation system, and suggests Mg2+ is the target for these systems. The results do not require multiple-ion targets, selection of isotopes, or additional curve fitting. The sole fitting parameter is the binding lifetime of the target system and the results shown are consistent with the literature on binding kinetics.

High-yield heterologous expression of wild type and mutant Ca(2+) ATPase: Characterization of Ca(2+) binding sites by charge transfer

High-yield heterologous SERCA1 (Ca(2+) ATPase) expression was obtained in COS-1 cells infected with recombinant adenovirus vector (rAdSERCA). Higher transcription and expression were obtained in the presence of a His(6) tag at the amino terminus, as compared with a His(6) tag at the carboxyl SERCA terminus, or no tag. The expressed protein was targeted extensively to intracellular membranes. Optimal yield of functional Ca(2+) ATPase corresponded to 10% of total protein, with phosphoenzyme levels, catalytic turnover and Ca(2+) transport identical with those of native SERCA1. This recombinant membrane-bound (detergent-free) enzyme was used for characterization of Ca(2+) binding at the two specific transmembrane sites (ATP-free) by measurements of net charge transfer upon Ca(2+) binding to the protein, yielding cooperative isotherms (K(1)=5.9+/-0.5x10(5) M(-1) and K(2)=5.7+/-0.3x10(6) M(-1)). Non-cooperative binding of only one Ca(2+), and loss of ATPase activation, were observed following E309 mutation at site II. On the other hand, as a consequence of the site II mutation, the affinity of site I for Ca(2+) was increased (K=4.4+/-0.2x10(6) M(-1)). This change was due to a pK(a) shift of site I acidic residues, and to contributions of oxygen functions from empty site II to Ca(2+) binding at site I. No charge movement was observed following E771Q mutation at site I, indicating no Ca(2+) binding to either site. Therefore, calcium occupancy of site I is required to trigger cooperative binding to site II and catalytic activation. In the presence of millimolar Mg(2+), the charge movement upon addition of Ca(2+) to WT ATPase was reduced by 50%, while it was reduced by 90% when Ca(2+) was added to the E309Q/A mutants, demonstrating that competitive Mg(2+) binding can occur at site I but not at site II.

Properties of the SR Ca-ATPase in an Open Microsomal Membrane Preparation

SR vesicles isolated from rabbit muscle were treated by a SDS incubation and subsequent dialysis to obtain open membrane fragments that allow a direct access to the luminal membrane surface and especially to the ion-binding sites in the P-E(2) conformation of the Ca-ATPase. The open membrane fragments showed about 80% of the enzyme activity in the untreated membranes. Pump function was investigated by using electrochromic styryl dyes. The kinetic properties of cytoplasmic ion binding showed no significant differences between the Ca-ATPases in SR vesicles and in membrane fragments. From pH-dependent Ca(2+) binding it could be deduced that due to the SDS treatment the density of negatively charged lipid was increased by one elementary charge per 12 lipid molecules. Major differences between Ca-ATPase from SR vesicles and membrane fragments were the respective fluorescence amplitudes. This effect is, however, produced by dye-lipid interaction and not by pump function. It was demonstrated that time-resolved kinetics may be study by the use of caged compounds such as caged ATP or caged calcium also in the case of the membrane fragments.

Charge transfer in P-type ATPases investigated on planar membranes

Planar lipid bilayers, e.g., black lipid membranes (BLM) and solid supported membranes (SSM), have been employed to investigate charge movements during the reaction cycle of P-type ATPases. The BLM/SSM method allows a direct measurement of the electrical currents generated by the cation transporter following chemical activation by a substrate concentration jump. The electrical current transients provides information about the reaction mechanism of the enzyme. In particular, the BLM/SSM technique allows identification of electrogenic steps which in turn may be used to localize ion translocation during the reaction cycle of the pump. In addition, using the high time resolution of the technique, especially when rapid activation via caged ATP is employed, rate constants of electrogenic and electroneutral steps can be determined. In the present review, we will discuss the main results obtained by the BLM and SSM methods and how they have contributed to unravel the transport mechanism of P-type ATPases.

Side-chain protonation and mobility in the sarcoplasmic reticulum Ca2+-ATPase: implications for proton countertransport and Ca2+ release

Protonation of acidic residues in the sarcoplasmic reticulum Ca(2+)-ATPase (SERCA 1a) was studied by multiconformation continuum electrostatic calculations in the Ca(2+)-bound state Ca(2)E1, in the Ca(2+)-free state E2(TG) with bound thapsigargin, and in the E2P (ADP-insensitive phosphoenzyme) analog state with MgF(4)(2-) E2(TG+MgF(4)(2-)). Around physiological pH, all acidic Ca(2+) ligands (Glu(309), Glu(771), Asp(800), and Glu(908)) were unprotonated in Ca(2)E1; in E2(TG) and E2(TG+MgF(4)(2-)) Glu(771), Asp(800), and Glu(908) were protonated. Glu(771) and Glu(908) had calculated pK(a) values larger than 14 in E2(TG) and E2(TG+MgF(4)(2-)), whereas Asp(800) titrated with calculated pK(a) values near 7.5. Glu(309) had very different pK(a) values in the Ca(2+)-free states: 8.4 in E2(TG+MgF(4)(2-)) and 4.7 in E2(TG) because of a different local backbone conformation. This indicates that Glu(309) can switch between a high and a low pK(a) mode, depending on the local backbone conformation. Protonated Glu(309) occupied predominantly two main, very differently orientated side-chain conformations in E2(TG+MgF(4)(2-)): one oriented inward toward the other Ca(2+) ligands and one oriented outward toward a protein channel that seems to be in contact with the cytoplasm. Upon deprotonation, Glu(309) adopted completely the outwardly orientated side-chain conformation. The contact of Glu(309) with the cytoplasm in E2(TG+MgF(4)(2-)) makes this residue unlikely to bind lumenal protons. Instead it might serve as a proton shuttle between Ca(2+)-binding site I and the cytoplasm. Glu(771), Asp(800), and Glu(908) are proposed to take part in proton countertransport.

Conformational fluctuations of the Ca2+-ATPase in the native membrane environment. Effects of pH, temperature, catalytic substrates, and thapsigargin

Digestion with proteinase K or trypsin yields complementary information on conformational transitions of the Ca(2+)-ATPase (SERCA) in the native membrane environment. Distinct digestion patterns are obtained with proteinase K, revealing interconversion of E1 and E2 or E1 approximately P and E2-P states. The pH dependence of digestion patterns shows that, in the presence of Mg(2+), conversion of E2 to E1 pattern occurs (even when Ca(2+) is absent) as H(+) dissociates from acidic residues. Mutational analysis demonstrates that the Glu(309) and Glu(771) acidic residues (empty Ca(2+)-binding sites I and II) are required for stabilization of E2. Glu(309) ionization is most important to yield E1. However, a further transition produced by Ca(2+) binding to E1 (i.e. E1.2Ca(2+)) is still needed for catalytic activation. Following ATP utilization, H(+)/Ca(2+) exchange is involved in the transition from the E1 approximately P.2Ca(2+) to the E2-P pattern, whereby alkaline pH will limit this conformational transition. Complementary experiments on digestion with trypsin exhibit high temperature dependence, indicating that, in the E1 and E2 ground states, the ATPase conformation undergoes strong fluctuations related to internal protein dynamics. The fluctuations are tightly constrained by ATP binding and phosphoenzyme formation, and this constraint must be overcome by thermal activation and substrate-free energy to allow enzyme turnover. In fact, a substantial portion of ATP free energy is utilized for conformational work related to the E1 approximately P.2Ca(2+) to E2-P transition, thereby disrupting high affinity binding and allowing luminal diffusion of Ca(2+). The E2 state and luminal path closure follow removal of conformational constraint by phosphate.

Simulation of Ca2+ movements within the sarcomere of fast-twitch mouse fibers stimulated by action potentials

Ca²⁺ release from the sarcoplasmic reticulum (SR) of skeletal muscle takes place at the triadic junctions; following release, Ca²⁺ spreads within the sarcomere by diffusion. Here, we report multicompartment simulations of changes in sarcomeric Ca²⁺ evoked by action potentials (APs) in fast-twitch fibers of adult mice. The simulations include Ca²⁺ complexation reactions with ATP, troponin, parvalbumin, and the SR Ca²⁺ pump, as well as Ca²⁺ transport by the pump. Results are compared with spatially averaged Ca²⁺ transients measured in mouse fibers with furaptra, a low-affinity, rapidly responding Ca²⁺ indicator. The furaptra Δf_CaD signal (change in the fraction of the indicator in the Ca²⁺-bound form) evoked by one AP is well simulated under the assumption that SR Ca²⁺ release has a peak of 200–225 μM/ms and a FDHM of ∼1.6 ms (16°C). Δf_CaD elicited by a five-shock, 67-Hz train of APs is well simulated under the assumption that in response to APs 2–5, Ca²⁺ release decreases progressively from 0.25 to 0.15 times that elicited by the first AP, a reduction likely due to Ca²⁺ inactivation of Ca²⁺ release. Recovery from inactivation was studied with a two-AP protocol; the amplitude of the second release recovered to >0.9 times that of the first with a rate constant of 7 s⁻¹. An obvious feature of Δf_CaD during a five-shock train is a progressive decline in the rate of decay from the individual peaks of Δf_CaD. According to the simulations, this decline is due to a reduction in available Ca²⁺ binding sites on troponin and parvalbumin. The effects of sarcomere length, the location of the triadic junctions, resting [Ca²⁺], the parvalbumin concentration, and possible uptake of Ca²⁺ by mitochondria were also investigated. Overall, the simulations indicate that this reaction-diffusion model, which was originally developed for Ca²⁺ sparks in frog fibers, works well when adapted to mouse fast-twitch fibers stimulated by APs.

Electrogenic steps of the SR Ca-ATPase enzymatic cycle and the effect of curcumin

Sarcoplasmic reticulum (SR) vesicles were adsorbed on an octadecanethiol/phosphatidylcholine mixed bilayer anchored to a gold electrode, and the Ca-ATPase contained in the vesicles was activated by ATP concentration jumps in the presence of calcium ions. The resulting capacitive current transients are compared with those calculated on the basis of the enzymatic cycle of the calcium pump. This comparison provides information on the kinetics of the E(2)-E(1) conformational change and on its pH dependence. The alteration in the current transients following ATP concentration jumps in the presence of curcumin is examined. In particular, curcumin decreases the rate of slippage of the Ca-ATPase, and at concentrations above 10 microM reduces calcium transport by this pump.

Protonation and hydrogen bonding of Ca2+ site residues in the E2P phosphoenzyme intermediate of sarcoplasmic reticulum Ca2+-ATPase studied by a combination of infrared spectroscopy and electrostatic calculations

Protonation of the Ca(2+) ligands of the SR Ca(2+)-ATPase (SERCA1a) was studied by a combination of rapid scan FTIR spectroscopy and electrostatic calculations. With FTIR spectroscopy, we investigated the pH dependence of C=O bands of the Ca(2+)-free phosphoenzyme (E2P) and obtained direct experimental evidence for the protonation of carboxyl groups upon Ca(2+) release. At least three of the infrared signals from protonated carboxyl groups of E2P are pH dependent with pK(a) values near 8.3: a band at 1758 cm(-1) characteristic of nonhydrogen-bonded carbonyl groups, a shoulder at 1720 cm(-1), and part of a band at 1710 cm(-1), both characteristic of hydrogen-bonded carbonyl groups. The bands are thus assigned to H(+) binding residues, some of which are involved in H(+) countertransport. At pH 9, bands at 1743 and 1710 cm(-1) remain which we do not attribute to Ca(2+)/H(+) exchange. We also obtained evidence for a pH-dependent conformational change in beta-sheet or turn structures of the ATPase. With MCCE on the E2P analog E2(TG+MgF(4)(2-)), we assigned infrared bands to specific residues and analyzed whether or not the carbonyl groups of the acidic Ca(2+) ligands are hydrogen bonded. The carbonyl groups of Glu(771), Asp(800), and Glu(908) were found to be hydrogen bonded and will thus contribute to the lower wave number bands. The carbonyl group of some side-chain conformations of Asp(800) is left without a hydrogen-bonding partner; they will therefore contribute to the higher wave number band.

Proton paths in the sarcoplasmic reticulum Ca(2+) -ATPase

The sarcoplasmic reticulum Ca(2+)-ATPase (SERCA1a) pumps Ca(2+) and countertransport protons. Proton pathways in the Ca(2+) bound and Ca(2+)-free states are suggested based on an analysis of crystal structures to which water molecules were added. The pathways are indicated by chains of water molecules that interact favorably with the protein. In the Ca(2+) bound state Ca(2)E1, one of the proposed Ca(2+) entry paths is suggested to operate additionally or alternatively as proton pathway. In analogs of the ADP-insensitive phosphoenzyme E2P and in the Ca(2+)-free state E2, the proton path leads between transmembrane helices M5 to M8 from the lumenal side of the protein to the Ca(2+) binding residues Glu-771, Asp-800 and Glu-908. The proton path is different from suggested Ca(2+) dissociation pathways. We suggest that separate proton and Ca(2+) pathways enable rapid (partial) neutralization of the empty cation binding sites. For this reason, transient protonation of empty cation binding sites and separate pathways for different ions are advantageous for P-type ATPases in general.

Kinetics of proton binding to the sarcoplasmic reticulum Ca-ATPase in the E1 state

A new caged proton, 2-methoxy-5-nitrophenyl sulfate, was synthesized and used in time-resolved pH jump experiments to study proton binding in the sarcoplasmic reticulum Ca-ATPase. The major advantage of this compound is that it does not produce significant artifacts in experiments in which the fluorescent styryl dye 2BITC is used to monitor ion movements in the Ca pump. Two rate-limiting processes were resolved and their dependence on pH, Ca(2+) concentration, and temperature investigated. The faster process showed a relaxation time between 4 and 8 ms independent on pH and Ca(2+) concentration, and the time constant of the slower process varied between 31 ms (0 Ca(2+)) and 100 ms (100 microM Ca(2+)). A consistent mechanism to explain the results was derived in agreement with previous studies and the generally accepted Post-Albers scheme of the pump cycle. This mechanism requires that under physiological conditions the ion-binding sites are always occupied and two protons and a Ca(2+) ion replace each other. In the absence of ATP at low pH a nonphysiological state can be induced in which up to four protons bind to the Ca pump in the E(1) conformation. So far it could not be verified whether these additional protons bind to amino acid side chains or are coordinated as hydronium ions.

Removing coordinated metal ions from proteins: a fast and mild method in aqueous solution

Thermodynamic and kinetic studies of metal binding to proteins require the investigation of metal-free proteins, which are often difficult to obtain. We have developed a very fast and mild method to eliminate metal ions from proteins by column chromatography using a commercially available Ni-NTA-type stationary phase. This material, initially designed for protein purification purposes in biotechnology, acts as a strong cation chelator when Ni2+ ions are removed. We have tested this new method with Ca-ATPase, an integral membrane protein exhibiting a strong affinity for Ca2+. By eluting the protein over the Ni2+-free NTA gel, we could remove 95% of the total Ca2+ and obtain an essentially Ca2+-free protein. This method is efficient with only a small amount of NTA gel, and we suggest that it can be applied in general for removal of metal ions from proteins. Moreover, as this procedure can be carried out under mild conditions, the chosen protein kept its enzymatic activity.

Stoichiometry of ATP and metal cofactor interaction with the sarcoplasmic reticulum Ca(2+)-ATPase: a binding model accounting for radioisotopic and fluorescence results

Sarcoplasmic reticulum Ca-ATPase belongs to the P-type ATPases family and transports calcium at the expense of ATP hydrolysis. For years, a complex pattern of activity has been observed as a function of ATP and metal cofactor concentrations, leaving the stoichiometry of both metal and ATP in the active site as an open question. In agreement with recent structural studies we present here-using Mn as analogue of Mg-radioisotopic and fluorescence results showing that two metal ions bind to the Ca-ATPase favoring ATP binding. We further show that low ATP concentration favors the binding of these ions, whereas high ATP concentration is inhibitory. We propose a binding model for ATP and metal ions, which permits simulation of our data. Finally, we suggest that (i) the contribution of two metal ions as cofactors of ATP is essential to get maximal activity; (ii) the contribution of two ATP molecules can activate or inhibit the Ca-ATPase depending on metal concentration.

Effects of tetracaine on voltage-activated calcium sparks in frog intact skeletal muscle fibers

The properties of Ca²⁺ sparks in frog intact skeletal muscle fibers depolarized with 13 mM [K⁺] Ringer's are well described by a computational model with a Ca²⁺ source flux of amplitude 2.5 pA (units of current) and duration 4.6 ms (18 °C; Model 2 of Baylor et al., 2002). This result, in combination with the values of single-channel Ca²⁺ current reported for ryanodine receptors (RyRs) in bilayers under physiological ion conditions, 0.5 pA (Kettlun et al., 2003) to 2 pA (Tinker et al., 1993), suggests that 1–5 RyR Ca²⁺ release channels open during a voltage-activated Ca²⁺ spark in an intact fiber. To distinguish between one and greater than one channel per spark, sparks were measured in 8 mM [K⁺] Ringer's in the absence and presence of tetracaine, an inhibitor of RyR channel openings in bilayers. The most prominent effect of 75–100 μM tetracaine was an approximately sixfold reduction in spark frequency. The remaining sparks showed significant reductions in the mean values of peak amplitude, decay time constant, full duration at half maximum (FDHM), full width at half maximum (FWHM), and mass, but not in the mean value of rise time. Spark properties in tetracaine were simulated with an updated spark model that differed in minor ways from our previous model. The simulations show that (a) the properties of sparks in tetracaine are those expected if tetracaine reduces the number of active RyR Ca²⁺ channels per spark, and (b) the single-channel Ca²⁺ current of an RyR channel is ≤1.2 pA under physiological conditions. The results support the conclusion that some normal voltage-activated sparks (i.e., in the absence of tetracaine) are produced by two or more active RyR Ca²⁺ channels. The question of how the activation of multiple RyRs is coordinated is discussed.

Electrostatic calculations for assignment of infrared difference bands to carboxyl groups getting protonated during protein reactions

Fourier transform infrared (FTIR) difference spectroscopy is predestined to monitor the protonation of carboxyl groups during protein reactions, making glutamic and aspartic amino acids unique to follow proton pathways. The absorption of the corresponding vibrations are clearly distinguishable from the absorption of other amino acids. However, the assignment to specific groups within the protein needs additional information, e.g., from induced spectral changes due to isotopic labeling or mutation. Here, the capability of electrostatic calculations to assign IR difference bands to specific carboxyl groups getting protonated is demonstrated by the ion pump mechanism of the sarcoplasmic reticulum Ca(2+)-ATPase. Active Ca(2+) transport is coupled to the hydrolysis of ATP. Two Ca(2+) ions are transported per ATP hydrolysed and two or three H(+) ions are countertransported. FTIR difference spectra show that during the Ca(2+) release step, carboxyl groups become protonated. Multiconformation continuum electrostatic calculations (MCCE) have been carried out to determine the equilibrium distribution of residue ionization and side chain conformation in dependence of pH. Available structural X-ray data from the calcium-bound and the calcium-free state allows us to simulate the transition between the two states monitored in the IR difference spectra. Exemplarily for Asp 800, ligand of both calcium ions, it is shown that MCCE calculations can identify this specific Asp to contribute to the IR bands and therefore to take part in the proton countertransport of the Ca(2+)-ATPase. In addition, an energy analysis can be performed to understand what interactions shift the pK(a).

Structural role of countertransport revealed in Ca(2+) pump crystal structure in the absence of Ca(2+)

Ca(2+)-ATPase of sarcoplasmic reticulum is an ATP-powered Ca(2+) pump but also a H(+) pump in the opposite direction with no demonstrated functional role. Here, we report a 2.4-A-resolution crystal structure of the Ca(2+)-ATPase in the absence of Ca(2+) stabilized by two inhibitors, dibutyldihydroxybenzene, which bridges two transmembrane helices, and thapsigargin, also bound in the membrane region. Now visualized are water and several phospholipid molecules, one of which occupies a cleft between two transmembrane helices. Atomic models of the Ca(2+) binding sites with explicit hydrogens derived by continuum electrostatic calculations show how water and protons fill the space and compensate charge imbalance created by Ca(2+)-release. They suggest that H(+) countertransport is a consequence of a requirement for maintaining structural integrity of the empty Ca(2+)-binding sites. For this reason, cation countertransport is probably mandatory for all P-type ATPases and possibly accompanies transport of water as well.

Kinetics of Ca2+ binding to the SR Ca-ATPase in the E1 state

The time-resolved kinetics of Ca2+ binding to the SR Ca-ATPase in the E1 state was investigated by Ca(2+)-concentration jump experiments. Ca2+ was released by an ultraviolet-light flash from caged calcium, and charge movements in the membrane domain of the ion pumps were detected by the fluorescent styryl dye 2BITC. The partial reaction (H3E1 <-->) E1 <--> CaE1 <--> Ca2E1 can be characterized by two time constants, tau1 and tau2, both of which are not significantly Ca(2+)-concentration-dependent and only weakly pH-dependent at pH < 7.5. Both time constants differ by a factor of approximately 50 (4.7 vs. 200 ms). The weak substrate-dependence indicates that the rate-limiting process is not related to Ca2+ migration through the access channel and ion binding to the binding sites but to conformational rearrangements preceding the ion movements. The high activation energy obtained for both processes, 42.3 kJ mol(-1) and 60.3 kJ mol(-1) at pH 7.2, support this concept. Transient binding of Ca ions to the loop L67 and a movement of the Ca-loaded loop are discussed as a mechanism that facilitates the entrance of both Ca ions into the access channel to the ion-binding sites.

Electrogenic partial reactions of the gastric H,K-ATPase

The fluorescent styryl dye RH421 was used to identify and investigate electrogenic reaction steps of the H,K-ATPase pump cycle. Equilibrium titration experiments were performed with membrane vesicles isolated from hog gastric mucosa, and cytoplasmic and luminal binding of K(+) and H(+) ions was studied. It was found that the binding and release steps of both ion species in both principal conformations of the ion pump, E(1) and P-E(2), are electrogenic, whereas the conformation transitions do not contribute significantly to a charge movement within the membrane dielectric. This behavior is in agreement with the transport mechanism found for the Na,K-ATPase and the sarcoplasmic reticulum Ca-ATPase. The data were analyzed on the basis of the Post-Albers reaction cycle. For proton binding, two pK values were found in both conformations: 6.7 and </=4.5 in the E(1) conformation; 6.7 and </=2 in the P-E(2) conformation. The equilibrium dissociation constants for K(+) binding on the cytoplasmic side were 11 and 16 mM. The respective equilibrium dissociation constants on the luminal side were obtained via K(+) concentration dependence of the enzyme activity and determined to be 0.11 mM for both luminal binding sites.

Photodynamic inactivation of the Na,K-ATPase occurs via different pathways

The photodynamic, i.e., the light-induced, inactivation of the Na,K-ATPase in the presence of the sensitizer rose bengal was studied under different conditions. The shape of inactivation curves of the enzyme activity was analyzed as well as partial reactions of the pump cycle. Both experimental approaches showed the existence of two different time constants of inactivation of the ion pump, which reflect two pathways of a photodynamic modification. This is supported by the following observations: (1) The amplitude of the initial fast decay of enzyme activity was enhanced in the presence of D2O and reduced in the presence of the singlet oxygen scavenger imidazole. (Similar results were found for the SR Ca-ATPase.) (2) Contrary to the fast enzyme inactivation the slow process shows an inverse dose-rate behavior. (3) Inactivation of the partial reactions of Na+ -binding and of K+-binding to the membrane domain of the Na,K-ATPase showed only a single time constant, which corresponded to the slower time constant of enzyme inactivation. In the presence of high concentrations of singlet oxygen the fast time constant dominated the inactivation of the ATP-induced partial reaction for which the cytoplasmic domains of the enzyme play an important role. The data support the conclusion that fast inactivation is due to modification of the cytoplasmic domains and slow inactivation due to modifications of the membrane domain of the ion pumps.

Time-resolved charge movements in the sarcoplasmatic reticulum Ca-ATPase

The time-resolved kinetics of the Ca(2+)-translocating partial reaction of the sarcoplasmatic reticulum Ca-ATPase was investigated by ATP-concentration jump experiments. ATP was released by an ultraviolet light flash from its inactive precursor and charge movements in the membrane domain of the ion pumps were detected by the fluorescent styryl dye 2BITC. Two oppositely directed cation movements were found, which were assigned to Ca(2+) release and H(+) binding. The faster process with a typical time constant of 30 ms reports the rate-limiting process before Ca(2+) release, probably the conformation transition E(1) --> E(2). The following, slow uptake of positive charge had a pH-dependent time constant, which was 1 s at low pH and approximately 3 s at pH > 8. This process is assigned to an electrically silent conformational relaxation of the state P-E(2) preceding H(+) binding. This interpretation is in agreement with the observation that the fast process was independent of the substrate concentrations (i.e., when [Ca(2+)] > 200 nM, and [ATP] > 20 micro M). The slow process was independent of the Ca(2+) concentration. The activation energy of the resolved processes was between 80 kJ/mol and 90 kJ/mol, which is comparable to the activation energy of the enzymatic activity (92 kJ/mol) and these high values point to conformational changes underlying rate-limiting steps of the pump cycle.

Transconformations of the SERCA1 Ca-ATPase: a normal mode study

The transport of Ca(2+) by Ca-ATPase across the sarcoplasmic reticulum membrane is accompanied by several transconformations of the protein. Relying on the already established functional importance of low-frequency modes in dynamics of proteins, we report here a normal mode analysis of the Ca(2+)-ATPase based on the crystallographic structures of the E1Ca(2) and E2TG forms. The lowest-frequency modes reveal that the N and A(+Nter) domains undergo the largest amplitude movements. The dynamical domain analysis performed with the DomainFinder program suggests that they behave as rigid bodies, unlike the highly flexible P domain. We highlight two types of movements of the transmembrane helices: i), a concerted movement around an axis perpendicular to the membrane which "twists open" the lumenal side of the protein and ii), an individual translational and rotational mobility which is of lower amplitude for the helices hosting the calcium binding sites. Among all modes calculated for E1Ca, only three are enough to describe the transition to E2TG; the associated movements involve almost exclusively the A and N domains, reflecting the closure of the cytoplasmic headpiece and high displacement of the L7-8 lumenal loop. Subsequently, we discuss the potential contribution of the remaining low-frequency normal modes to the transconformations occurring within the overall calcium transport cycle.

How do P-Type ATPases transport ions?

P-type ATPases are a large family of membrane proteins that perform active ion transport across biological membranes. In these proteins, the energy-providing ATP hydrolysis is coupled to ion transport of one or two ion species across the respective membrane. The pump function of the investigated pumps is described by a so-called Post-Albers cycle. Main features of the pumping process are (1) a Ping-Pong mechanism, i.e. both transported ion species are transferred successively and in opposite direction across the membrane, (2) the transport process for each ion species consists of a sequence of reaction steps, which are ion binding, ion occlusion, conformational transition of the protein, successive deocclusion of the ions and release to the other side of the membrane. (3) Recent experimental evidence shows that the ion-binding sites are placed in the transmembrane section of the proteins and that ion movements occur preferentially during the ion binding and release processes. The main features of the mechanism include narrow access channels from both sides, one gate per access channel, and an ion-binding moiety that is adapted specifically to the ions that are transported, and differently in both principal conformations.

Time-resolved charge translocation by sarcoplasmic reticulum Ca-ATPase measured on a solid supported membrane

Sarcoplasmic reticulum vesicles were adsorbed on an octadecanethiol/phosphatidylcholine mixed bilayer anchored to a gold electrode, and the Ca-ATPase contained in the vesicles was activated by ATP concentration jumps both in the absence and in the presence of K(+) ions and at different pH values. Ca(2+) concentration jumps in the absence of ATP were also carried out. The resulting capacitive current transients were analyzed together with the charge under the transients. The relaxation time constants of the current transients were interpreted on the basis of an equivalent circuit. The current transient after ATP concentration jumps and the charge after Ca(2+) concentration jumps in the absence of ATP exhibit almost the same dependence upon the Ca(2+) concentration, with a half-saturating value of approximately 1.5 microM. The pH dependence of the charge after Ca(2+) translocation demonstrates the occurrence of one H(+) per one Ca(2+) countertransport at pH 7 by direct charge-transfer measurements. The presence of K(+) decreases the magnitude of the current transients without altering their shape; this decrease is explained by K(+) binding to the cytoplasmic side of the pump in the E(1) conformation and being released to the same side during the E(1)-E(2) transition.

Structure-function relationship in P-type ATPases--a biophysical approach

P-type ATPases are a large family of membrane proteins that perform active ion transport across biological membranes. In these proteins the energy-providing ATP hydrolysis is coupled to ion-transport that builds up or maintains the electrochemical potential gradients of one or two ion species across the membrane. P-type ATPases are found in virtually all eukaryotic cells and also in bacteria, and they are transporters of a broad variety of ions. So far, a crystal structure with atomic resolution is available only for one species, the SR Ca-ATPase. However, biochemical and biophysical studies provide an abundance of details on the function of this class of ion pumps. The aim of this review is to summarize the results of preferentially biophysical investigations of the three best-studied ion pumps, the Na,K-ATPase, the gastric H,K-ATPase, and the SR Ca-ATPase, and to compare functional properties to recent structural insights with the aim of contributing to the understanding of their structure-function relationship.

Different functional properties of troponin T mutants that cause dilated cardiomyopathy

The effects of Troponin T (TnT) mutants R141W and DeltaK210, the only two currently known mutations in TnT that cause dilated cardiomyopathy(DCM) independent of familial hypertrophic cardiomyopathy (FHC), and TnT-K273E, a mutation that leads to a progression from FHC to DCM, were investigated. Studies on the Ca2+ sensitivity of force development in porcine cardiac fibers demonstrated that TnT-DeltaK210 caused a significant decrease in Ca2+ sensitivity, whereas the TnT-R141W did not result in any change in Ca2+ sensitivity when compared with human cardiac wild-type TnT (HCWTnT). TnT-DeltaK210 also caused a decrease in maximal force when compared with HCWTnT and TnT-R141W. In addition, the TnT-DeltaK210 mutant decreased maximal ATPase activity in the presence of Ca2+. However, the TnT-K273E mutation caused a significant increase in Ca2+ sensitivity but behaved similarly to HCWTnT in actomyosin activation assays. Inhibition of ATPase activity in reconstituted actin-activated myosin ATPase assays was similar for all three TnT mutants and HCWTnT. Additionally, circular dichroism studies suggest that the secondary structure of all three TnT mutants was similar to that of the HCWTnT. These results suggest that a rightward shift in Ca2+ sensitivity is not the only determinant for the phenotype of DCM.

Identification of a functionally critical protein kinase C phosphorylation residue of cardiac troponin T

Cardiac Troponin T (cTnT) is one prominent substrate through which protein kinase C (PKC) exerts its effect on cardiomyocyte function. To determine the specific functional effects of the cTnT PKC-dependent phosphorylation sites (Thr197, Ser201, Thr206, and Thr287) we first mutated these residues to glutamate (E) or alanine (A). cTnT was selectively mutated to generate single, double, triple, and quadruple mutants. Bacterially expressed mutants were evaluated in detergent-treated mouse left ventricular papillary muscle fiber bundles where the endogenous troponin was replaced with a recombinant troponin complex containing either cTnT phosphorylated by PKC-alpha or a mutant cTnT. We simultaneously determined isometric tension development and actomyosin Mg-ATPase activity of the exchanged fiber bundles as a function of Ca2+ concentration. Our systematic analysis of the functional role of the multiple PKC phosphorylation sites on cTnT identified a localized region that controls maximum tension, ATPase activity, and Ca2+ sensitivity of the myofilaments. An important and novel finding of our study was that Thr206 is a functionally critical cTnT PKC phosphorylation residue. Its exclusive phosphorylation by PKC-alpha or replacement by Glu (mimicking phosphorylation) significantly decreased maximum tension, actomyosin Mg-ATPase activity, myofilament Ca2+ sensitivity, and cooperativity. On the other hand the charge modification of the other three residues together (T197/S201/T287-E) had no functional effect. Fibers bundles containing phosphorylated cTnT-wt (but not the T197/S201/T206/T287-E) exhibited a significant decrease of tension cost as compared with cTnT-wt.

Structure and dynamics of the C-domain of human cardiac troponin C in complex with the inhibitory region of human cardiac troponin I

Cardiac troponin C is the Ca2+-dependent switch for heart muscle contraction. Troponin C is associated with various other proteins including troponin I and troponin T. The interaction between the subunits within the troponin complex is of critical importance in understanding contractility. Following a Ca2+ signal to begin contraction, the inhibitory region of troponin I comprising residues Thr128-Arg147 relocates from its binding surface on actin to troponin C, triggering movement of troponin-tropomyosin within the thin filament and thereby freeing actin-binding site(s) for interactions with the myosin ATPase of the thick filament to generate the power stroke. The structure of calcium-saturated cardiac troponin C (C-domain) in complex with the inhibitory region of troponin I was determined using multinuclear and multidimensional nuclear magnetic resonance spectroscopy. The structure of this complex reveals that the inhibitory region adopts a helical conformation spanning residues Leu134-Lys139, with a novel orientation between the E- and H-helices of troponin C, which is largely stabilized by electrostatic interactions. By using isotope labeling, we have studied the dynamics of the protein and peptide in the binary complex. The structure of this inhibited complex provides a framework for understanding into interactions within the troponin complex upon heart contraction.

Small-angle neutron scattering with contrast variation reveals spatial relationships between the three subunits in the ternary cardiac troponin complex and the effects of troponin I phosphorylation

Small-angle neutron scattering with contrast variation has been used to determine the shapes and dispositions of the three subunits of cardiac troponin and to study the influence of phosphorylation on the structure. Three contrast variation series were collected on three different isotopically labeled variants of the cTnC/cTnI/cTnT(198-298) complex, one of which contained deuterated and bisphosphorylated cTnI. Analysis of the scattering data shows cTnT(198-298) interacting with a single lobe of a somewhat compacted cTnC that sits at one end of an elongated rodlike cTnI, covering about one-third of its length. The cTnT(198-298) sits near the center of the long cTnI axis. The components undergo significant conformational changes and reorientations in response to protein kinase A phosphorylation of cTnI. The rodlike cTnI bends sharply at the end interacting with the cTnC/cTnT(198-298) component, which reorients so as to maintain its contacts with cTnI while undergoing only a relatively small change in shape.

NMR structure of a bifunctional rhodamine labeled N-domain of troponin C complexed with the regulatory "switch" peptide from troponin I: implications for in situ fluorescence studies in muscle fibers

The structure of the calcium-saturated regulatory domain of skeletal troponin C (sNTnC) complexed with the switch peptide comprising residues 115-131 of troponin I (TnI), and with a bifunctional rhodamine fluorescent label attached to residues 56 (E56C) and 63 (E63C) on the C helix of sNTnC, has been determined using nuclear magnetic resonance (NMR) spectroscopy. The structure shows that the integrity of the C helix is not altered by the E(56,63)C mutations or by the presence of the bifunctional rhodamine and that the label does not interact with the hydrophobic cleft of sNTnC. Moreover, the overall fold of the protein and the position of the TnI peptide are similar to those observed previously with related cardiac NTnC complexes with residues 147-163 of cardiac TnI [Li et al. (1999) Biochemistry 38, 8289-8298] and including the drug bepridil [Wang et al. (2002) J. Biol. Chem. 277, 31124-31133]. The degree of opening of the structure is reduced as compared to that of calcium-saturated sNTnC in the absence of the switch peptide [Gagné et al. (1995) Nat. Struct. Biol. 2, 784-789]. The switch peptide is bound in a shallow and complementary hydrophobic surface cleft largely defined by helices A and B and also has key ionic interactions with sNTnC. These results show that bifunctional rhodamine probes can be attached to surface helices via suitable pairs of solvent-accessible residues that have been mutated to cysteines, without altering the conformation of the labeled domain. A set of such probes can be used to determine the orientation and motion of the target domain in the cellular environment [Corrie et al. (1999) Nature 400, 425-430; Ferguson et al. (2003) Mol. Cell 11(4), in press].

Toward an understanding of ion transport through the Na,K-ATPase

In the Na,K-ATPase the charge-translocating reaction steps were found to be binding of the third Na(+) ion to the cytoplasmic side and the release of all three Na(+) ions to the extracellular side as well as binding of the two K(+) ions on the extracellular side. The conformation transition E(1) --> E(2) was only of minor electrogenicity; all other reaction steps produced no significant charge movements. In the SR Ca-ATPase and the gastric H,K-ATPase, all ion-binding and -release steps were identified to move charge through the membrane. The high-resolution structure of the SR Ca-ATPase in state E(1) revealed the position of the ion-binding sites in the transmembrane part of the protein. If the same arrangement is assumed for the Na pump, the missing expected charge movements in state E(1) may to be assumed to be apparent effects. With the proposal that binding of 2 Na(+) or 2 K(+) is compensated correspondingly by H(+) ions, agreement between structural and functional aspects is obtained. Investigations of the pH-dependence of ion-binding steps indicate competition between the ions and electrogenic H(+) binding in support of this concept.

Ion occlusion/deocclusion partial reactions in individual palytoxin-modified Na/K pumps

In P-type ion-motive ATPases, transported ions approach their binding sites from one membrane surface, become buried deep within "occluded" conformations in which the sites are inaccessible from either membrane side, and are then deoccluded and released to the opposite membrane surface. This describes an alternating-gate transport mechanism, in which the pump acts like an ion channel with two gates that open and close alternately. The occluded states ensure that one gate closes before the other can open, thus preventing the large electrodiffusive ion fluxes that would otherwise quickly undo the pump's electrochemical work. High-resolution crystal structures of two conformations of the SERCA (sarcoplasmic and endoplasmic reticulum Ca(2+)) P-type ATPase, together with mutagenesis results and analyses of structural models based on homology, have begun to provide a picture of the ion coordination sites in related P-type ATPases, including the Na/K pump. However, in no P-type ATPase are the structures and mechanisms of the gates known. The marine toxin, palytoxin (PTX), is known to bind to the Na/K pump and elicit a nonselective cation leak pathway, possibly by disrupting the strict coupling between the pump's inner and outer gates, allowing them to both be open. We recently found that ion flow through PTX-modified Na/K pump-channels appears to be modulated by two gates that can be regulated by the pump's physiological ligands in a manner suggesting that gating reflects underlying ion occlusion/deocclusion partial reactions. We review that work here and provide evidence that the pore of the PTX-induced pump-channel has a diameter > 6 A.

Phosphorylation or glutamic acid substitution at protein kinase C sites on cardiac troponin I differentially depress myofilament tension and shortening velocity

There is evidence that multi-site phosphorylation of cardiac troponin I (cTnI) by protein kinase C is important in both long- and short-term regulation of cardiac function. To determine the specific functional effects of these phosphorylation sites (Ser-43, Ser-45, and Thr-144), we measured tension and sliding speed of thin filaments in reconstituted preparations in which endogenous cTnI was replaced with cTnI phosphorylated by protein kinase C-epsilon or mutated to cTnI-S43E/S45E/T144E, cTnI-S43E/S45E, or cTnI-T144E. We used detergent-skinned mouse cardiac fiber bundles to measure changes in Ca(2+)-dependence of force. Compared with controls, fibers reconstituted with phosphorylated cTnI, cTnI-S43E/S45E/T144E, or cTnI-S43E/S45E were desensitized to Ca(2+), and maximum tension was as much as 27% lower, whereas fibers reconstituted with cTnI-T144E showed no change. In the in vitro motility assay actin filaments regulated by troponin complexes containing phosphorylated cTnI or cTnI-S43E/S45E/T144E showed both a decrease in Ca(2+) sensitivity and maximum sliding speed compared with controls, whereas filaments regulated by cTnI-S43E/S45E showed only decreased maximum sliding speed and filaments regulated by cTnI-T144E demonstrated only desensitization to Ca(2+). Our results demonstrate novel site specificity of effects of PKC phosphorylation on cTnI function and emphasize the complexity of modulation of the actin-myosin interaction by specific changes in the thin filament.

Ca2+-induced conformational transition in the inhibitory and regulatory regions of cardiac troponin I

Cardiac muscle activation is initiated by the binding of Ca(2+) to the single N-domain regulatory site of cardiac muscle troponin C (cTnC). Ca(2+) binding causes structural changes between cTnC and two critical regions of cardiac muscle troponin I (cTnI): the regulatory region (cTnI-R, residues 150-165) and the inhibitory region (cTnI-I, residues130-149). These changes are associated with a decreased cTnI affinity for actin and a heightened affinity for cTnC. Using Förster resonance energy transfer, we have measured three intra-cTnI distances in the deactivated (Mg(2+)-saturated) and Ca(2+)-activated (Ca(2+)-saturated) states in reconstituted binary (cTnC-cTnI) and ternary (cTnC-cTnI-cTnT) troponin complexes. Distance A (spanning cTnI-R) was unaltered by Ca(2+). Distances B (spanning both cTnI-R and cTnI-I) and C (from a residue flanking cTnI-I to a residue in the center of cTnI-R) exhibited Ca(2+)-induced increases of >8 A. These results compliment our previous determination of the distance between residues flanking cTnI-I alone. Together, the data suggest that Ca(2+) activation causes residues within cTnI-I to switch from a beta-turn/coil to an extended quasi-alpha-helical conformation as the actin-contacts are broken, whereas cTnI-R remains alpha-helical in both Mg(2+)- and Ca(2+)-saturated states. We have used the data to construct a structural model of the cTnI inhibitory and regulatory regions in the Mg(2+)- and Ca(2+)-saturated states.

Folding and function of the troponin tail domain. Effects of cardiomyopathic troponin T mutations

Troponin contains a globular Ca(2+)-binding domain and an elongated tail domain composed of the N terminus of subunit troponin T (TnT). The tail domain anchors troponin to tropomyosin and actin, modulates myosin function, and is a site of cardiomyopathy-inducing mutations. Critical interactions between tropomyosin and troponin are proposed to depend on tail domain residues 112-136, which are highly conserved across phyla. Most cardiomyopathy mutations in TnT flank this region. Three such mutations were examined and had contrasting effects on peptide TnT-(1-156), promoting folding and thermal stability assessed by circular dichroism (F110I) or weakening folding and stability (T104V and to a small extent R92Q). Folding of both TnT-(1-156) and whole troponin was promoted by replacing bovine TnT Thr-104 with human TnT Ala-104, further indicating the importance of this cardiomyopathy site residue for protein folding. Mutation F110I markedly stabilized the troponin tail but weakened binding of holo-troponin to actin-tropomyosin 8-fold, suggesting that loss of flexibility impairs troponin tail function. The effect of the F110I mutation on troponin-tropomyosin binding to actin was much less, indicating this flexibility is particularly important for the interactions of troponin with tropomyosin. We suggest that most cardiomyopathic mutations in the troponin tail alter muscle function indirectly, by perturbing interactions between troponin and tropomyosin requisite for the complex effects of these proteins on myosin.