Calcium-dependent non-equivalent characteristics of calcium binding sites of the sarcoplasmic reticulum Ca2+-ATPase

Structural basis for calcium and phosphatidylserine regulation of phospholipase C δ1

Many membrane-associated enzymes, including those of the phospholipase C (PLC) superfamily, are regulated by specific interactions with lipids. Previously, we have shown that the C2 domain of PLC δ1 is required for phosphatidylserine (PS)-dependent enzyme activation and that activation requires the presence of Ca(2+). To identify the site of interaction and the role of Ca(2+) in the activation mechanism, we mutagenized three highly conserved Ca(2+) binding residues (Asp-653, Asp-706, and Asp-708) to Gly in the C2 domain of PLC δ1. The PS-dependent Ca(2+) binding affinities of the mutant enzymes D653G, D706G, and D708G were reduced by 1 order of magnitude, and the maximal level of Ca(2+) binding was reduced to half of that of the native enzyme. The level of Ca(2+)-dependent PS binding was also reduced in the mutant enzymes. Under basal conditions, the Ca(2+) dependence and the maximal level of hydrolysis of phosphatidylinositol 4,5-bisphosphate were not altered in the mutants. However, the Ca(2+)-dependent PS stimulation was severely defective. PS reduces the K(m) of the native enzyme almost 20-fold, but far less for the mutants. Replacing Asp-653, Asp-706, and Asp-708 simultaneously with glycine in the C2 domain of PLC δ1 leads to a complete and selective loss of the stimulation and binding by PS. These results show that D653, D706, and D708 are required for Ca(2+) binding in the C2 domain and demonstrate a mechanism by which C2 domains can mediate regulation of enzyme activity by specific lipid ligands.

Spectroscopic characterization of the EF-hand domain of phospholipase C delta1: identification of a lipid interacting domain

The interaction of the isolated EF-hand domain of phospholipase C delta1 with arachidonic acid (AA) was characterized using circular dichroism (CD) and fluorescence spectroscopy. The far-UV CD spectral changes indicate that AA binds to the EF domain. The near-UV CD spectra suggest that the orientations of aromatic residues in the peptide are affected when AA binds to the protein. The fluorescence of the single intrinsic tryptophan located in EF1 was enhanced by the addition of dodecylmaltoside (DDM) and AA suggesting that this region of the protein is involved in hydrophobic interactions. In the presence of a low concentration of DDM it was found that AA induced a change in fluorescence resonance energy transfer, which is indicative of a conformational change. The lipid induced conformational change may play a role in calcium binding because the isolated EF-hand domain did not bind Ca2+ in the absence of lipids, but Ca2+-dependent changes in the intrinsic tryptophan emission were observed when free fatty acids were present. These studies identify specific EF-hand domains as allosteric regulatory domains that require hydrophobic ligands such as lipids.

Activation of phospholipase C delta1 through C2 domain by a Ca(2+)-enzyme-phosphatidylserine ternary complex

The concentration of free Ca(2+) and the composition of nonsubstrate phospholipids profoundly affect the activity of phospholipase C delta1 (PLCdelta1). The rate of PLCdelta1 hydrolysis of phosphatidylinositol 4,5-bisphosphate was stimulated 20-fold by phosphatidylserine (PS), 4-fold by phosphatidic acid (PA), and not at all by phosphatidylethanolamine or phosphatidylcholine (PC). PS reduced the Ca(2+) concentration required for half-maximal activation of PLCdelta1 from 5.4 to 0.5 microM. In the presence of Ca(2+), PLCdelta1 specifically bound to PS/PC but not to PA/PC vesicles in a dose-dependent and saturable manner. Ca(2+) also bound to PLCdelta1 and required the presence of PS/PC vesicles but not PA/PC vesicles. The free Ca(2+) concentration required for half-maximal Ca(2+) binding was estimated to be 8 microM. Surface dilution kinetic analysis revealed that the K(m) was reduced 20-fold by the presence of 25 mol % PS, whereas V(max) and K(d) were unaffected. Deletion of amino acid residues 646-654 from the C2 domain of PLCdelta1 impaired Ca(2+) binding and reduced its stimulation and binding by PS. Taken together, the results suggest that the formation of an enzyme-Ca(2+)-PS ternary complex through the C2 domain increases the affinity for substrate and consequently leads to enzyme activation.

Independence of two conformations of sarcoplasmic reticulum Ca2+-ATPase molecules in hydrolyzing acetyl phosphate. A two-pair model of the ATPase structural unit

The sarcoplasmic reticulum Ca2+-ATPase molecules have been shown to exist in two conformations (A and B) that result from intermolecular interaction of ATPase molecules (Nakamura, J., and Tajima, G. (1995) J. Biol. Chem. 270, 17350-17354). The A form binds two calcium ions noncooperatively, whereas the B form binds the calcium ions cooperatively. Here, we examined the independence of these two forms in the calcium-activated hydrolysis of acetyl phosphate (AcP) under asynchronous and synchronous conditions of their E1-E2 transitions at 0-5 and 25 degrees C. Irrespective of their synchronism and temperature, the two forms hydrolyzed AcP due to calcium that was bound to each of the forms, indicating the independence of the two forms in hydrolyzing AcP. Taking into account the monomer-dimer transition of the ATPase molecules on the sarcoplasmic reticulum membrane accompanying E1-E2 transition of the molecules (Dux, L., Taylor, K. A., Ting-Beall, H. P., and Martonosi, A. (1985) J. Biol. Chem. 260, 11730-11743), the two types of molecules seem to independently carry out such monomer-dimer transition of the same type of molecules. Two pairs, each consisting of the same type of molecules, are suggested to be the structural unit of the ATPase molecules.

How do Ca2+ ions pass through the sarcoplasmic reticulum membrane

We propose an overview of the mechanism of Ca2+ transport through the sarcoplasmic reticulum membrane via the Ca(2+)-ATPase. We describe cytoplasmic calcium binding, calcium occlusion in the membrane and lumenal calcium dissociation. A channel-like structure is discussed and related to structural data on the membranous domain of the Ca(2+)-ATPase.

Lumenal Ca2+ dissociation from the phosphorylated Ca(2+)-ATPase of the sarcoplasmic reticulum is sequential

Once two radioactive Ca2+ coming from the cytoplasm are bound to the transport sites of the nonphosphorylated ATPase, excess EGTA induces rapid dissociation of both ions, whereas excess nonradioactive Ca2+ only reaches one of the two bound Ca2+. This difference has been explained assuming that the two Ca2+ sites are in a single file channel in which the superficial Ca2+ is freely exchangeable from the cytoplasm, whereas the deeper Ca2+ is exchangeable only when the superficial site is vacant. The same experiment was done using phosphorylated ATPase to determine whether Ca2+ dissociation toward the lumen is sequential as well. Under conditions that allow ADP-sensitive phosphoenzyme to accumulate (leaky vesicles, 5 degrees C, pH 8, 300 mM KC1), we found the same two pools of Ca2+. Excess EGTA induced dissociation of both ions together with dephosphorylation. Excess nonradioactive Ca2+ induced the exchange of half the radioactive Ca2+ without any effect on the phosphoenzyme level. Our results show a close similarity between the transport sites of the nonphosphorylated and the phosphorylated enzymes, although the orientation, affinities, and dissociation rate constants are different.

Characterization of ruthenium red-binding sites of the Ca(2+)-ATPase from sarcoplasmic reticulum and their interaction with Ca(2+)-binding sites

Sarcoplasmic reticulum Ca(2+)-ATPase has previously been shown to bind and dissociate two Ca2+ ions in a sequential mode. This behaviour is confirmed here by inducing sequential Ca2+ dissociation with Ruthenium Red. Ruthenium Red binds to sarcoplasmic reticulum vesicles (6 nmol/mg) with a Kd = 2 microM, producing biphasic kinetics of Ca2+ dissociation from the Ca(2+)-ATPase, decreasing the affinity for Ca2+ binding. Studies on the effect of Ca2+ on Ruthenium Red binding indicate that Ruthenium Red does not bind to the high-affinity Ca(2+)-binding sites, as suggested by the following observations: (i) micromolar concentrations of Ca2+ do not significantly alter Ruthenium Red binding to the sarcoplasmic reticulum; (ii) quenching of the fluorescence of fluorescein 5'-isothiocyanate (FITC) bound to Ca(2+)-ATPase by Ruthenium Red (resembling Ruthenium Red binding) is not prevented by micromolar concentrations of Ca2+; (iii) quenching of FITC fluorescence by Ca2+ binding to the high-affinity sites is achieved even though Ruthenium Red is bound to the Ca(2+)-ATPase; and (iv) micromolar Ca2+ concentrations prevent inhibition of the ATP-hydrolytic capability by dicyclohexylcarbodi-imide modification, but Ruthenium Red does not. However, micromolar concentrations of lanthanides (La3+ and Tb3+) and millimolar concentrations of bivalent cations (Ca2+ and Mg2+) inhibit Ruthenium Red binding as well as quenching of FITC-labelled Ca(2+)-ATPase fluorescence by Ruthenium Red. Studies of Ruthenium Red binding to tryptic fragments of Ca(2+)-ATPase, as demonstrated by ligand blotting, indicate that Ruthenium Red does not bind to the A1 subfragment. Our observations suggest that Ruthenium Red might bind to a cation-binding site in Ca(2+)-ATPase inducing fast release of the last bound Ca2+ by interactions between the sites.