Ion pathways in proteins of the sarcoplasmic reticulum

Sarcolipin alters SERCA1a interdomain communication by impairing binding of both calcium and ATP

Sarcolipin (SLN), a single-spanning membrane protein, is a regulator of the sarco-endoplasmic reticulum Ca2+-ATPase (SERCA1a). Chemically synthesized SLN, palmitoylated or not (pSLN or SLN), and recombinant wild-type rabbit SERCA1a expressed in S. cerevisiae design experimental conditions that provide a deeper understanding of the functional role of SLN on the regulation of SERCA1a. Our data show that chemically synthesized SLN interacts with recombinant SERCA1a, with calcium-deprived E2 state as well as with calcium-bound E1 state. This interaction hampers the binding of calcium in agreement with published data. Unexpectedly, SLN has also an allosteric effect on SERCA1a transport activity by impairing the binding of ATP. Our results reveal that SLN significantly slows down the E2 to Ca2.E1 transition of SERCA1a while it affects neither phosphorylation nor dephosphorylation. Comparison with chemically synthesized SLN deprived of acylation demonstrates that palmitoylation is not necessary for either inhibition or association with SERCA1a. However, it has a small but statistically significant effect on SERCA1a phosphorylation when various ratios of SLN-SERCA1a or pSLN-SERCA1a are tested.

Calcium Uptake in Crude Tissue Preparation

The various isoforms of the sarco/endoplasmic reticulum Ca(2+) ATPase (SERCA) are responsible for the Ca(2+) uptake from the cytosol into the endoplasmic or sarcoplasmic reticulum (ER/SR). In some tissues, the activity of SERCA can be modulated by binding partners, such as phospholamban and sarcolipin. The activity of SERCA can be characterized by its apparent affinity for Ca(2+) as well as maximal enzymatic velocity. Both parameters can be effectively determined by the protocol described here. Specifically, we describe the measurement of the rate of oxalate-facilitated (45)Ca uptake into the SR of crude mouse ventricular homogenates. This protocol can easily be adapted for different tissues and animal models as well as cultured cells.

Phospholamban and sarcolipin: Are they functionally redundant or distinct regulators of the Sarco(Endo)Plasmic Reticulum Calcium ATPase?

In muscle, the Sarco(Endo)plasmic Reticulum Calcium ATPase (SERCA) activity is regulated by two distinct proteins, PLB and SLN, which are highly conserved throughout vertebrate evolution. PLB is predominantly expressed in the cardiac muscle, while SLN is abundant in skeletal muscle. SLN is also found in the cardiac atria and to a lesser extent in the ventricle. PLB regulation of SERCA is central to cardiac function, both at rest and during extreme physiological demand. Compared to PLB, the physiological relevance of SLN remained a mystery until recently and some even thought it was redundant in function. Studies on SLN suggest that it is an uncoupler of the SERCA pump activity and can increase ATP hydrolysis resulting in heat production. Using genetically engineered mouse models for SLN and PLB, we showed that SLN, not PLB, is required for muscle-based thermogenesis. However, the mechanism of how SLN binding to SERCA results in uncoupling SERCA Ca(2+) transport from its ATPase activity remains unclear. In this review, we discuss recent advances in understanding how PLB and SLN differ in their interaction with SERCA. We will also explore whether structural differences in the cytosolic domain of PLB and SLN are the basis for their unique function and physiological roles in cardiac and skeletal muscle.

Sarcolipin uncouples hydrolysis of ATP from accumulation of Ca2+ by the Ca2+-ATPase of skeletal-muscle sarcoplasmic reticulum

Sarcolipin (SLN) is a small peptide found in the sarcoplasmic reticulum of skeletal muscle. It is predicted to contain a single hydrophobic transmembrane alpha-helix. Fluorescence emission spectra for the single Trp residue of SLN suggest that SLN incorporates fully into bilayers of dioleoylphosphatidylcholine, but only partially into bilayers of phosphatidylcholines with long (C(22) or C(24)) fatty acyl chains. The fluorescence of SLN is quenched in bilayers of dibromostearoylphosphatidylcholine, also consistent with incorporation into the lipid bilayer. SLN was reconstituted with the Ca(2+)-ATPase of skeletal-muscle sarcoplasmic reticulum. Even at a 50:1 molar ratio of SLN/ATPase, SLN had no significant effect on the rate of ATP hydrolysis by the ATPase or on the Ca(2+)-dependence of ATP hydrolysis. However, at a molar ratio of SLN/ATPase of 2:1 or higher the presence of SLN resulted in a marked decrease in the level of accumulation of Ca(2+) by reconstituted vesicles. The effect of SLN was structurally specific and did not result from a breakdown in the vesicular structure or from the formation of non-specific ion channels. Vesicles were impermeable to Ca(2+) in the absence of ATP in the external medium. The effects of SLN on accumulation of Ca(2+) can be simulated assuming that SLN increases the rate of slippage on the ATPase and the rate of passive leak of Ca(2+) mediated by the ATPase. It is suggested that the presence of SLN could be important in non-shivering thermogenesis, a process in which heat is generated by hydrolysis of ATP by skeletal-muscle sarcoplasmic reticulum.

Cells from chronic myelogenous leukaemia patients at presentation exhibit multidrug resistance not mediated by either MDR1 or MRP1

Tetramethylrosamine (TMR) is excluded from P-glycoprotein (MDR1)-enriched cell lines, but it stains efficiently MDR1-poor parent lines. Application of the TMR resistance assay to cells obtained from chronic myelogenous leukaemia (CML) patients revealed, in all individuals, a significant resistance compared with healthy donors (P < 0.001). Cells from the same patients at later phases exhibited a further increase in TMR resistance. Doxorubicin was excluded from all cell samples obtained from CML patients at presentation. The resistance to TMR and doxorubicin was energy-dependent, and was not modulated by inhibitors of MDR1 and multidrug-resistance protein-1 (MRP1). Transcription of mRNAs suspected as relevant to multidrug resistance was assessed using comparative reverse transcription polymerase chain reaction. All cells from the CML patients transcribed high levels of MRP3, MRP4 and MRP5 compared with healthy donors. Low levels of MDR1, MRP1, MRP2, MRP6, lung resistance-related protein and anthracycline resistance-associated protein were equally transcribed in cells from healthy donors and CML patients. These results indicated that neither MDR1 nor MRP1 mediate the resistance in these cells. Our results shed light on a resistance mechanism operative in CML patients, which, together with the resistance to apoptosis, is responsible for the lack of response of CML patients to induction-type protocols used to treat acute myeloid leukaemia patients.

Sarcolipin regulates the activity of SERCA1, the fast-twitch skeletal muscle sarcoplasmic reticulum Ca2+-ATPase

The 31-amino acid proteolipid, sarcolipin (SLN), is associated with the fast-twitch skeletal muscle sarcoplasmic reticulum Ca2+-ATPase (SERCA1). Constructs of human and rabbit SLN and of rabbit SLN with the FLAG epitope at its N terminus (NF-SLN) or its C terminus (SLN-FC) were coexpressed with SERCA1 in HEK-293 T-cells. Immunohistochemistry was used to demonstrate colocalization of NF-SLN and SERCA1 in the endoplasmic reticulum membrane and to demonstrate the cytosolic orientation of the N terminus of SLN. Coexpression of native rabbit SLN or NF-SLN with SERCA1 decreased the apparent affinity of SERCA1 for Ca2+ but stimulated maximal Ca2+ uptake rates (Vmax). The N terminus of SLN is not well conserved among species, and the addition of an N-terminal FLAG epitope did not alter SLN function. Anti-FLAG antibody reversed both the inhibition of Ca2+ uptake by NF-SLN at low Ca2+ concentrations and the stimulatory effect of NF-SLN on Vmax. Addition of the FLAG epitope to the highly conserved C terminus decreased the apparent affinity of SERCA1 for Ca2+ relative to native SLN and decreased Vmax significantly. Mutations in the C-terminal domain showed that this sequence is critical for SLN function. Mutational analysis of the transmembrane helix, together with the additive regulatory effects of coexpression of both SLN and phospholamban (PLN) with SERCA1, provided evidence for different mechanisms of interaction of SLN and PLN with SERCA molecules. Ca2+ uptake rates in sarcoplasmic reticulum vesicles, isolated from rabbit fast-twitch muscle (tibialis anterior) subjected to chronic low frequency stimulation, were reduced by approximately 40% in 3- and 4-day stimulated muscle, with a marginal increase in apparent affinity of SERCA1 for Ca2+. SERCA1 mRNA and protein levels were unaltered after stimulation. In contrast, SLN mRNA was decreased by 15%, and SLN protein was reduced by 40%. Reduced SLN expression could explain the decrease in SERCA1 activity observed in these muscles and might represent an early functional adaptation to chronic low frequency stimulation.

Characterization of the gene encoding human sarcolipin (SLN), a proteolipid associated with SERCA1: absence of structural mutations in five patients with Brody disease

Sarcolipin (SLN) is a low-molecular-weight protein that copurifies with the fast-twitch skeletal muscle sarcoplasmic reticulum Ca2+ ATPase (SERCA1). Genomic DNA and cDNA encoding human sarcolipin (SLN) were isolated and characterized and the SLN gene was mapped to chromosome 11q22-q23. Human, rabbit, and mouse cDNAs encode a protein of 31 amino acids. Homology of SLN with phospholamban (PLN) suggests that the first 7 hydrophilic amino acids are cytoplasmic, the next 19 hydrophobic amino acids form a single transmembrane helix, and the last 5 hydrophilic amino acids are lumenal. The cytoplasmic and transmembrane sequences are not well conserved among the three species, but the lumenal sequence is highly conserved. Like SERCA1, SLN is highly expressed in rabbit fast-twitch skeletal muscle, but it is expressed to a lower extent in slow-twitch muscle and to an even lower extent in cardiac muscle, where SERCA2a and PLN are highly expressed. It is expressed in only trace amounts in pancreas and prostate. SLN and PLN genes resemble each other in having two small exons, with their entire coding sequences lying in exon 2 and a large intron separating the two segments. Brody disease is an inherited disorder of skeletal muscle function, characterized by exercise-induced impairment of muscle relaxation. Mutations in the ATP2A1 gene encoding SERCA1 have been associated with the autosomal recessive inheritance of Brody disease in three families, but not with autosomal dominant inheritance of the disease. A search for mutations in the SLN gene in five Brody families, four of which were not linked to ATP2A1, did not reveal any alterations in coding, splice junction or promoter sequences. The homozygous deletion of C438 in the coding sequence of ATP2A1 in Brody disease family 3, leading to a frameshift and truncation following Pro147 in SERCA1, is the fourth ATP2A1 mutation to be associated with autosomal recessive Brody disease.

Uptake of Ca2+ mediated by the (Ca2+ + Mg2+)-ATPase in reconstituted vesicles

The (Ca2+ + Mg2+)-ATPase was purified from skeletal muscle sarcoplasmic reticulum and reconstituted into sealed phospholipid vesicles by solution in cholate and deoxycholate followed by detergent removal on a column of Sephadex G-50. The level of Ca2+ accumulated by these vesicles, either in the presence or absence of phosphate within the vesicles, increased with increasing content of phosphatidylethanolamine in the phospholipid mixture used for the reconstitution. The levels of Ca2+ accumulated in the absence of phosphate were very low for vesicles reconstituted with egg yolk phosphatidylcholine alone at pH 7.4, but increased markedly with decreasing pH to 6.0. Uptake was also relatively low for vesicles reconstituted with dimyristoleoyl- or dinervonylphosphatidylcholine, and addition of cholesterol had little effect. The level of Ca2+ accumulated increased with increasing external K+ concentration, and was also increased by the ionophores FCCP and valinomycin. Vesicle sizes changed little with changing phosphatidylethanolamine content, and the sidedness of insertion of the ATPase was close to random at all phosphatidylethanolamine contents. It is suggested that the effect of phosphatidylethanolamine on the level of Ca2+ accumulation follows from an effect on the rate of Ca2+ efflux mediated by the ATPase.

Distances between the functional sites of sarcoplasmic reticulum (Ca2+ + Mg2+)-ATPase and the lipid/water interface

Measurements of fluorescence energy transfer have been performed to determine the distance between the lipid-water interface and the ATP-binding site in the (Ca2+ + Mg2+)-ATPase from sarcoplasmic reticulum. The calculated distance between the donor, FITC bound to the protein (nucleotide binding-site marker), and the acceptor, rhodamine-5'-isothiocyanyldipalmitoylphosphatidylethanolamine (RITC-DPPE) incorporated in the membrane, was in the range of 34-42 A. In addition the distance between the high affinity Ca2+-binding sites and the lipid/water interface has been calculated by luminescence energy transfer from Tb3+ bound to the Ca2+ sites to RITC-DPPE included in the membrane, and it was approx. 10 A.

Amino-acid sequence of the catalytic subunit of the (Na+ + K+)ATPase deduced from a complementary DNA

We have isolated and characterized a complementary DNA for the catalytic subunit of the sheep kidney sodium/potassium-dependent ATPase. The 1,016-amino-acid protein seems to have eight transmembrane domains. The apparent ouabain binding site is located at the extracellular junction of two transmembrane domains and is linked to the phosphorylation site by a 60-amino-acid conserved sequence that may be a major channel for energy transduction.

Amino-acid sequence of a Ca2+ + Mg2+-dependent ATPase from rabbit muscle sarcoplasmic reticulum, deduced from its complementary DNA sequence

We have cloned and sequenced complementary DNA encoding a Ca2+-ATPase of rabbit muscle sarcoplasmic reticulum. We propose a model of the protein which has 3 cytoplasmic domains joined to a set of 10 transmembrane helices by a narrow, penta-helical stalk. In this model, ATP bound to one cytoplasmic domain would phosphorylate an aspartate in an adjoining cytoplasmic domain, inducing translocation of Ca2+ from binding sites on the stalk.

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.

Phospholipid asymmetry in the isolated sarcoplasmic reticulum membrane

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

Ionophorous properties of the 13 000-Da fragment from sarcoplasmic reticulum (Ca2+ + Mg2+)-ATPase

The 25 000-Da tryptic fragment from rabbit muscle sarcoplasmic reticulum (Ca2+ + Mg2+)-ATPase was subjected to cyanogen bromide digestion, and the four fragments isolated. Only the 13 000-Da fragment induced ionophorous activity in planar thin lipid membranes made with 5:1 (w/w) phosphatidylcholine/cholesterol in decane. The membranes became cation selective, with a selectivity sequence among divalent of Mn2+ greater than Ca2+ greater than Ba2+ greater than Sr2+ greater than Mg2+. This is different from that of the 25 000-Da fragment (A.E. Shamoo, 1978, J. Memb. Biol. 43, 227-242), it's 'parent' 55 000-Da fragment, and the intact enzyme, all of which have the same selectivity sequence. The inhibitory effects of Hg2+, Cd2+ and Zn2+ were also examined. All were inhibitory, with Zn2+ being the most effective of these. The heavy-metal-induced inhibition of Ca2+ conductance could be reversed by selective chelation of the heavy metals by EDTA. From changes in the selectivity as well as changes in heavy-metal-induced inhibition behavior, we conclude that the ion transport site of the 13 000-Da fragment may not be the same site as that of the parent fragment. It is either a different site altogether or has been physically modified by peptide cleavage.

Annular and non-annular binding sites on the (Ca2+ + Mg2+)-ATPase

Quenching of the fluorescence of the (Ca2+ + Mg2+)-ATPase purified from muscle sarcoplasmic reticulum can be used to measure relative binding constants of hydrophobic compounds to the phospholipid-protein interface. We show that the binding constant for cholesterol is considerably less than that for phosphatidylcholine, so that cholesterol is effectively excluded from the phospholipid annulus around the ATPase. However, dibromocholestan-3 beta-ol causes quenching of the fluorescence of the ATPase, and so has access to other, non-annular sites. We suggest that these non-annular sites could be at protein/protein interfaces in ATPase oligomers. Oleic acid can bind at the phospholipid/protein interface, although its binding constant is less than that for a phosphatidylcholine, and it can also bind at the postulated non-annular sites. The effects of these compounds on the activity of the ATPase depend on the structure of the phospholipid present in the systems.

Energy coupling to ATP synthesis by the proton-translocating ATPase

This review summarizes recent work on energy coupling to ATP synthesis by the reversible, proton-translocating ATPase to mitochondria, chloroplasts, and bacteria. In the first sections, this enzyme is distinguished from other ATP-linked ion transport systems, and progress in the biochemical analysis is discussed. There is at present a reasonably consistent idea of the overall structure of the enzyme, and one can begin to assign specific functional roles to individual subunits of the complex. The latter half of the review deals with mechanisms of energy coupling, about which there is clear divergence of opinion. An "indirect coupling" model would allow for the possibility that H+ translocation transmits energy for ATP synthesis by driving the enzyme through a sequence of conformational states, so that H+ translocated need not take part in the chemistry of ATP synthesis. By contrast, a "direct coupling" mechanism would specify that H+ translocated must participate in the chemical reaction by combining with oxygen must participate in the chemical reaction by combining with oxygen from phosphate during the synthetic step. Such discussion is preceded by an outlined of the "proton well," since this idea forms the basis of one direct coupling model. In addition, it is suggested that the idea of a proton (ion) well may be of more general significance to the analysis of ion-coupled transport, because it includes the postulate that mechanistically significant ion binding can occur within the profile of the electric field. A proton (ion) well can be derived from both kinetic and equilibrium treatments, and from mechanistic considerations in fields as distinct as biochemistry and neurophysiology. As a result, it illustrates how further advances in formulating mechanisms of energy coupling might profit by a merger of technique and perspective from areas that have as a common goal an understanding of how large proteins catalyze movements of small molecules across a membrane.

Studies of Ca2+ release from sarcoplasmic reticulum

Our studies of Ca2+ release have shown that three classes of compounds--quercetin, which inhibits Ca2+ release by reversal of the Ca2+ pump, H+ ionophores and DCCD, which affect proton gradients, and lipophilic anions, which act to oppose a separation of membrane surface charges--all have potential in probing the mechanism of Ca2+ release from sarcoplasmic reticulum.

Critical effects from lipid-protein interaction in membranes. II. Interpretation of experimental results

The effects arising from lipid-protein and lipid-cholesterol interaction are discussed within the framework of a general theoretical description presented in the preceding paper. Available experimental results are interpreted, and new experiments are proposed. In the fluid lipid phase proteins and cholesterol increase the lipid orientational order in their neighborhood, in the ordered phase they decrease it. This leads to a decrease of the latent heat at the ordered-fluid transition, which vanishes at a critical concentration of protein or cholesterol. Theoretical predictions for the critical concentrations agree with results from calorimetry. The approach to the critical point is accompanied by an increase of thermal fluctuations of the lipid order and an increase of the lipid response on small perturbations. Thus proteins and cholesterol increase the lipid specific heat, lateral compressibility, permeability, and lateral diffusion on both sides of the phase transition. Notions such as decrease of cooperativity or fluidity due to protein or cholesterol are reviewed in this context.

A proton gradient controls a calcium-release channel in sarcoplasmic reticulum

Sarcoplasmic reticulum vesicles from mammalian skeletal muscle have previously been shown to develop a proton gradient (alkaline inside) of 0.15-0.5 pH units during active Ca2+ uptake. We found that dissipation of this gradient by the proton ionophores gramicidin, nigericin, and carbonyl cyanide p-trichloromethoxyphenylhydrazone caused a rapid transient tension in skinned rabbit psoas muscle fibers. Increases, but not decreases, in medium pH of approximately 0.2 units over the range from pH 6.5 to pH 7.5 also elicited transient tensions. In isolated vesicles, physiological levels of Ca2+ (3.3 microM), inhibited pH-induced Ca2+ release. Dicyclohexylcarbodiimide blocked pH- and ionophore-induced Ca2+ release under conditions in which it could bind to sarcoplasmic reticulum proteins but did not inhibit Ca2+ uptake. We propose that a proton gradient generated across sarcoplasmic reticulum membranes during Ca2+ uptake maintains a Ca2+ release channel in a closed conformation and that dissipation of this gradient permits the Ca2+ release channel to open. We further propose that elevated myoplasmic Ca2+ also causes the Ca2+ channel to close, permitting Ca2+ uptake through Ca2+/Mg2+-ATPase to function effectively. As the proteolipids of sarcoplasmic reticulum bind dicyclohexylcarbodiimide under conditions in which Ca2+ release is blocked and as they have previously been shown to have Ca2+ ionophoric activity, we propose that the Ca2+-release channel either resides in the proteolipids or is controlled by H+ fluxes through the proteolipids.