Inactivation of calcium uptake by EGTA is due to an irreversible thermotropic conformational change in the calcium binding domain of the Ca(2+)-ATPase

Cellular mechanisms underlying temperature-induced bleaching in the tropical sea anemone Aiptasia pulchella

Temperature-induced bleaching in symbiotic cnidarians is a result of the detachment and loss of host cells containing symbiotic algae. We tested the hypothesis that host cell detachment is evoked through a membrane thermotropic event causing an increase in intracellular calcium concentration, [Ca2+]i, which could then cause collapse of the cytoskeleton and perturb cell adhesion. Electron paramagnetic resonance measurements of plasma membranes from the tropical sea anemone Aiptasia pulchella and the Hawaiian coral Pocillopora damicornis labeled with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) revealed no membrane thermotropic event. In addition, intracellular imaging using Fura-2AM as well as labeling anemones with 45Ca revealed no significant change in [Ca2+]i. However, bleaching could be evoked at ambient temperature with 25 mmol l–1 caffeine without affecting [Ca2+]i. [Ca2+]i could be altered with ionomycin in isolated host cells, but ionomycin could not induce bleaching in A. pulchella. As caffeine can affect levels of intracellular protein phosphorylation, the ability of other agents that alter intracellular levels of protein phosphorylation to evoke bleaching was investigated. The protein phosphatase inhibitor vanadate could induce bleaching in A. pulchella. Two-dimensional gels of 32P-labeled proteins from cold-shocked, caffeine-treated and control anemones show that both temperature shock and caffeine alter the array of phosphorylated host soluble proteins. We conclude that cnidarian bleaching is linked to a temperature-induced alteration in protein phosphorylation.

Slippage and uncoupling in P-type cation pumps; implications for energy transduction mechanisms and regulation of metabolism

P-type ATPases couple scalar and vectorial events under optimized states. A number of procedures and conditions lead to uncoupling or slippage. A key branching point in the catalytic cycle is at the cation-bound form of E(1)-P, where isomerization to E(2)-P leads to coupled transport, and hydrolysis leads to uncoupled release of cations to the cis membrane surface. The phenomenon of slippage supports a channel model for active transport. Ability to occlude cations within the channel is essential for coupling. Uncoupling and slippage appear to be inherent properties of P-type cation pumps, and are significant contributors to standard metabolic rate. Heat production is favored in the uncoupled state. A number of disease conditions, include ageing, ischemia and cardiac failure, result in uncoupling of either the Ca(2+)-ATPase or Na(+)/K(+)-ATPase.

Interaction of D-600 with the transmembrane domain of the sarcoplasmic reticulum Ca(2+)-ATPase

Experiments were performed to determine whether the organic Ca(2+) channel blocker D-600 (gallopamil), which penetrates into muscle cells, affects sarcoplasmic reticulum (SR) Ca(2+) uptake by directly inhibiting the light SR Ca(2+)-ATPase. We have previously shown that at 10 microM, D-600 inhibits LSR ATP-dependent Ca(2+) uptake by 50% but has no effect on ATPase activity (21). These data suggest that the SR Ca(2+)-ATPase might be a potential target for D-600. The ATPase activity of the enzyme is associated with its hydrophilic cytoplasmic domain, whereas Ca(2+) binding and translocation are associated with the transmembrane domain (18). In the present experiments, we determined which of the two domains of the ATPase is affected by D-600. Thermal inactivation experiments using the SR Ca(2+)-ATPase demonstrated that D-600 decreased the thermal stability of Ca(2+) transport but had no effect on the stability of ATPase activity. In addition, D-600 at a concentration of 160 microM did not have any leaking effect of Ca(2+) on the Ca(2+)-loaded SR. Thermal denaturation profiles of SR membranes revealed that D-600 interacts directly with the transmembrane domain of the Ca(2+)-ATPase. No evidence for interaction with the nucleotide domain was obtained. We conclude that the Ca(2+) blocker D-600 inhibits the SR Ca(2+) pump specifically by interacting with the transmembrane Ca(2+)-binding domain of the Ca(2+)-ATPase.

Post mortem changes in Ca2+ transporting proteins of sarcoplasmic reticulum in dependence on malignant hyperthermia status in pigs

Meat quality of pigs is dependent on biochemical and biophysical processes in the time course post mortem (p.m.) and is associated with the intracellular Ca2+ homeostasis. However, there is little known about changes in the Ca2+ transporting proteins controlling the Ca2+ uptake of sarcoplasmic reticulum (SR) in the time course p.m. In this study changes in the Ca2+ transporting proteins were investigated in homogenates of longissimus muscles of 4 malignant hyperthermia susceptible (MHS) and 6 malignant hyperthermia resistant (MHR) Pietrain pigs. Muscle samples were obtained at different time intervals: biopsy 2 h prior slaughtering and from the carcass immediately after exsanguination (0 h), 45 min, 4 h, and 22 h p.m. The SR Ca2+ uptake rate was measured immediately after homogenization with closed calcium release channel (CRC), with opened CRC and without manipulation of CRC. Additionally the SR Ca2+ ATPase activity was determined. The results show: (i) The ability of SR to sequester Ca2+ declined to about 60% in the first 45 min p.m. in MHS samples irrespective of CRC state, whereas in MHR samples this decline was about 5%; (ii) Ca2+ uptake and Ca2+ ATPase activity were not different between the biopsy and 0 h samples, i.e. the stress of slaughter was of no immediate influence; (iii) The Ca2+ ATPase activity of the SR declined at about the same rate as the Ca2+ uptake in both MHS and MHR pig samples in the course of time p.m.; (iv) In samples, taken immediately after exsanguination, the Ca2+ ATPase activity of MHS pigs was higher than that of MHR pigs. However, in samples taken 4 h p.m. Ca2+ ATPase activity of MHS pigs has declined to about 30% of the value at 0 h; (v) The CRC can be closed and opened in all samples up to 22 h p.m. and seems to be fully functional at all sampling times; (vi) The CRC of MHS pigs is almost fully open, whereas the CRC of MHR pigs is only partially open at all sampling times; (vii) The permeability of the SR membrane to Ca2+ (determined as the ratio of SR Ca2+ ATPase with and without ionophore A23187) is the same in both MHS and MHR and did not change with ongoing time; (viii) No uncoupling of uptake from ATP hydrolysis occurred up to 4 h p.m., but the coupling differed between MHS and MHR for all time intervals with lower values for MHS pigs. The results suggest that the decreasing Ca2+ uptake rate of homogenates, sampled at different times p.m., is essentially caused by changes in the Ca2+ pump and not by changes in the CRC or an increased phospholipid membrane permeability to Ca2+.

Detection of membrane packing defects by time-resolved fluorescence depolarization

Packing defects in lipid bilayer play a significant role in the biological activities of cell membranes. Time-resolved fluorescence depolarization has been used to detect and characterize the onset of packing defects in binary mixtures of dilinoleoylphosphatidylethanolamine/1-palmitoyl-2- oleoylphosphatidylcholine (PE/PC). These PE/PC mixtures exhibit mesoscopic packing defect state (D), as well as one-dimensional lambellar liquid crystalline (L alpha) and two-dimensional inverted hexagonal (HII) ordered phases. Based on previous electron microscopic investigations, this D state is characterized by the presence of interlamellar attachments and precursors of HII phase between the lipid layers. Using a rotational diffusion model for rod-shaped fluorophore in a curved matrix, rotational dynamics parameters, second rank order parameter, localized wobbling diffusion, and curvature-dependent rotational diffusion constants of dipyenylhexatriene (DPH)-labeled PC (DPH-PC) in the host PE/PC matrix were recovered from the measured fluorescence depolarization decays of DPH fluorescence. At approximately 60% PE, abrupt increases in these rotational dynamics parameters were observed, reflecting the onset of packing defects in the host PE/PC matrix. We have demonstrated that rotational dynamics parameters are very sensitive in detecting the onset of curvature-associating packing defects in lipid membranes. In addition, the presence of the D state can be characterized by the enhanced wobbling diffusional motion and order packing of lipid molecules, and by the presence of localized curvatures in the lipid layers.

The vmax of the Ca2+-ATPase of cardiac sarcoplasmic reticulum (SERCA2a) is not altered by Ca2+/calmodulin-dependent phosphorylation or by interaction with phospholamban

Earlier studies (Hawkins, C., Xu, A., and Narayanan, N. (1994) J. Biol. Chem. 269, 31198-31206) have suggested that the Vmax of Ca2+ uptake is enhanced up to 2-fold through phosphorylation of Ser38 in the cardiac Ca2+-ATPase (SERCA2a) by calmodulin-dependent protein kinase (CaM kinase). It is difficult, however, to determine whether stimulation is caused by phosphorylation of the Ca2+-ATPase or by phosphorylation of phospholamban in cardiac microsomes. We have expressed SERCA2a in HEK-293 cells in the presence or absence of phospholamban and measured the effects on Ca2+ uptake activity of phosphorylation of microsomal proteins by CaM kinase or protein kinase A (PKA). We found no effect on the Vmax of Ca2+ uptake following phosphorylation by CaM kinase or PKA in either the presence or absence of phospholamban. The K0.5 for Ca2+ dependence of Ca2+ transport, however, was shifted following phosphorylation by either CaM kinase or PKA in those microsomes containing both SERCA2a and phospholamban, but not in those expressing only SERCA2a. Thus, we cannot confirm earlier reports of stimulation of SERCA2a activity by CaM kinase II phosphorylation of Ser38. Our studies, however, emphasize the need for adequate controls for measurement of Vmax.

Interaction of dibucaine with the transmembrane domain of the Ca(2+)-ATPase of sarcoplasmic reticulum

The site of interaction of dibucaine with the Ca(2+)-ATPase of rabbit sarcoplasmic reticulum, an ion-transporting membrane protein, was investigated by determining the effect of dibucaine on the denaturation of the transmembrane domain and the aqueous domain containing, respectively, the high-affinity Ca2+ binding sites and the site of ATP hydrolysis. In the absence of Ca2+, a single irreversible denaturation transition with Tm approximately equal to 49 degrees C is observed for the Ca(2+)-ATPase by differential scanning calorimetry (DSC). In the presence of Ca2+, but not Mg2+, Sr2+, or Ba2+, a new high-temperature transition is observed that has been shown to be due to stabilization of the transmembrane region [Lepock, J. R., Rodahl, A. M., Zhang, C., Heynen, M. L., Waters, B., & Cheng, K. H. (1990) Biochemistry 29, 681-689]. The maximum stabilization corresponds to a shift in Tm of 13.8 degrees C, and Hill analysis indicates that the Ca2+ binding site yielding stabilization has a Kd = 2.5 x 10(-4) M with a cooperativity (n) of 1. Thus, stabilization is due to Ca2+ binding not to the high-affinity sites but to one of the previously observed sites of low or intermediate affinity, which must be located in the transmembrane or stalk subdomains. Dibucaine has little effect on the Tm of the aqueous domain, but it decreases the Tm of the transmembrane domain with Kd approximately equal to 4.1 x 10(-4) M and a cooperativity of approximately 1.6, implying that destabilization is due to the binding of dibucaine to sites of intermediate or moderately high affinity.(ABSTRACT TRUNCATED AT 250 WORDS)

Thermal uncoupling of the Ca(2+)-transporting ATPase in sarcoplasmic reticulum. Changes in surface properties of light vesicles

It is known that the light fraction of rabbit skeletal muscle sarcoplasmic reticulum vesicles can release Ca2+ from the intravesicular space, although the Ca(2+)-conductive channels are present only in the heavy fraction of sarcoplasmic reticulum vesicles. To study the possible pathways of the Ca2+ leakage from light vesicles we have used a short-term treatment for 4.5 min at 45 degrees C which quickly decreases the efficiency of Ca(2+)-transporting ATPase operation without any visible effects on the hydrolytic activity of the Ca(2+)-ATPase in the membranes. The treatment of the vesicles decreased the negative membrane surface potential created by the Ca(2+)-ATPase. Comparative titration of control and heat-treated preparations of light sarcoplasmic reticulum vesicles by K+, Na+, Mg2+, and Ca2+ revealed clear differences in their surface properties. The short-term heating resulted in release of Ca2+ from the vesicles previously loaded with 45Ca2+, which indicates an increase in passive membrane permeability to Ca2+. Study of Ca(2+)-ATPase protein arrangement in the membrane indicated that the heat treatment induced protein oligomerization and some of the Ca(2+)-ATPase molecules acquired intermolecular and intramolecular covalent bonds. From these data, we have concluded that the changes in the surface and structure properties of the vesicle membranes after the short-term heat treatment were the result of clustering of the Ca(2+)-ATPase molecules. This protein rearrangement may create channels for calcium leakage from light sarcoplasmic reticulum vesicles.

Intramolecular excimer kinetics of fluorescent dipyrenyl lipids: 1. DMPC/cholesterol membranes

The intramolecular dynamics of the excimer forming dipyrenyl lipids (DipynPC) of different chain lengths (n) in ethanol and in dimyristoylphosphatidycholine (DMPC) membranes was investigated by the use of frequency-domain fluorescence intensity decay technique. Based on a 3-state model, the extent of aggregation and rotational rate of the two intralipid pyrene moieties in the dipyrenyl lipids were estimated from the frequency-domain data. In ethanol (20 degrees C), the rotational rate for DipynPC increased progressively as n was varied from 4 to 12. At the gel (L beta)-to-liquid crystalline (L alpha) phase transition of DMPC (approximately 23 degrees C), the rotational rate increased and aggregation decreased significantly for Dipy10PC, whereas only the rotational rate was changed for Dipy4PC. In the presence of 30 mol% cholesterol, significant increases in both the rotational rate and aggregation were observed for Dipy10PC in both L beta and L alpha phases. However, for the case of Dipy4PC, an increase in the rotational rate but a decrease in the aggregation were noticed only in the L beta phase, and no similar changes were detected in the L alpha phase. Our results indicate differential effects of cholesterol on the conformational dynamics of acyl chains at different depths of the membranes.

Interactions of vanadate oligomers with sarcoplasmic reticulum Ca(2+)-ATPase

Upon addition of sarcoplasmic reticulum (SR), the line width of tetrameric vanadate signal of 51V-NMR spectra narrowed in the presence of ATP and Ca2+, whereas monomeric vanadate line widths were broadened. Thus, ATP decreases the affinity of the enzyme for tetravanadate whereas it induces the interaction with monomeric vanadate. In the presence of Ca2+ it was observed that tetrameric and decameric vanadate bind to SR ATPase whereas monomeric vanadate only binds to SR when ATP is present. However, decameric vanadate clearly differs from vanadate oligomers present in monovanadate solutions in preventing the accumulation of Ca2+ by sarcoplasmic reticulum (SR) vesicles coupled to ATP hydrolysis. Mg2+ increased the inhibitory effect promoted by decavanadate whereas a slight enhancement of Ca2+ uptake was observed in the presence of monovanadate. For 5 mM Mg2+, a nominal 2 mM vanadium 'decavanadate' solution containing about 190 to 200 microM decameric and less than 100 microM monomeric species depressed the rate of Ca2+ uptake by 50% whereas a nominal 2 mM monovanadate solution containing about 662 microM monomeric, 143 microM dimeric and 252 microM tetrameric species had no effect on the rate of Ca2+ accumulation. However, 2 mM 'decavanadate' inhibits by 75% the SR Ca(2+)-ATPase activity whereas the presence of 2 mM 'monovanadate' produces an inhibitory effect below 50%. Therefore, the Ca:ATP stoichiometry of Ca2+ transport is enhanced by monovanadate. In the presence of oxalate, inhibition of SR Ca(2+)-ATPase activity by these solutions is enhanced to 97% and 86% whereas in the presence of the ionophore lasalocid, the inhibitory values were 87% and 19% for 2 mM decavanadate and 2 mM monovanadate solutions, respectively. Apparently, the increase of vesicular Ca2+ concentration counteracts monovanadate inhibition of SR Ca(2+)-ATPase activity but it does not significantly affect decavanadate inhibition.