Chemical modification of the Ca(2+)-ATPase of rabbit skeletal muscle sarcoplasmic reticulum: identification of sites labeled with aryl isothiocyanates and thiol-directed conformational probes

Nucleotide activation of the Ca-ATPase

We have used fluorescence spectroscopy, molecular modeling, and limited proteolysis to examine structural dynamics of the sarcoplasmic reticulum Ca-ATPase (SERCA). The Ca-ATPase in sarcoplasmic reticulum vesicles from fast twitch muscle (SERCA1a isoform) was selectively labeled with fluorescein isothiocyanate (FITC), a probe that specifically reacts with Lys-515 in the nucleotide-binding site. Conformation-specific proteolysis demonstrated that FITC labeling does not induce closure of the cytoplasmic headpiece, thereby assigning FITC-SERCA as a nucleotide-free enzyme. We used enzyme reverse mode to synthesize FITC monophosphate (FMP) on SERCA, producing a phosphorylated pseudosubstrate tethered to the nucleotide-binding site of a Ca(2+)-free enzyme (E2 state to prevent FMP hydrolysis). Conformation-specific proteolysis demonstrated that FMP formation induces SERCA headpiece closure similar to ATP binding, presumably due to the high energy phosphoryl group on the fluorescent probe (ATP·E2 analog). Subnanosecond-resolved detection of fluorescence lifetime, anisotropy, and quenching was used to characterize FMP-SERCA (ATP·E2 state) versus FITC-SERCA in Ca(2+)-free, Ca(2+)-bound, and actively cycling phosphoenzyme states (E2, E1, and EP). Time-resolved spectroscopy revealed that FMP-SERCA exhibits increased probe dynamics but decreased probe accessibility compared with FITC-SERCA, indicating that ATP exhibits enhanced dynamics within a closed cytoplasmic headpiece. Molecular modeling was used to calculate the solvent-accessible surface area of FITC and FMP bound to SERCA crystal structures, revealing a positive correlation of solvent-accessible surface area with quenching but not anisotropy. Thus, headpiece closure is coupled to substrate binding but not active site dynamics. We propose that dynamics in the nucleotide-binding site of SERCA is important for Ca(2+) binding (distal allostery) and phosphoenzyme formation (direct activation).

Regulation of cardiovascular cellular processes by S-nitrosylation

BACKGROUND

Nitric oxide (NO), a highly versatile signaling molecule, exerts a broad range of regulatory influences in the cardiovascular system that extends from vasodilation to myocardial contractility, angiogenesis, inflammation, and energy metabolism. Considerable attention has been paid to deciphering the mechanisms for such diversity in signaling. S-nitrosylation of cysteine thiols is a major signaling pathway through which NO exerts its actions. An emerging concept of NO pathophysiology is that the interplay between NO and reactive oxygen species (ROS), the nitroso/redox balance, is an important regulator of cardiovascular homeostasis.

SCOPE OF REVIEW

ROS react with NO, limit its bioavailability, and compete with NO for binding to the same thiol in effector molecules. The interplay between NO and ROS appears to be tightly regulated and spatially confined based on the co-localization of specific NO synthase (NOS) isoforms and oxidative enzymes in unique subcellular compartments. NOS isoforms are also in close contact with denitrosylases, leading to crucial regulation of S-nitrosylation.

MAJOR CONCLUSIONS

Nitroso/redox balance is an emerging regulatory pathway for multiple cells and tissues, including the cardiovascular system. Studies using relevant knockout models, isoform specific NOS inhibitors, and both in vitro and in vivo methods have provided novel insights into NO- and ROS-based signaling interactions responsible for numerous cardiovascular disorders.

GENERAL SIGNIFICANCE

An integrated view of the role of nitroso/redox balance in cardiovascular pathophysiology has significant therapeutic implications. This is highlighted by human studies where pharmacologic manipulation of oxidative and nitrosative pathways exerted salutary effects in patients with advanced heart failure. This article is part of a Special Issue entitled Regulation of Cellular Processes by S-nitrosylation.

Optimization and applications of CDAP labeling for the assignment of cysteines

PURPOSE

The aim of the study is to provide a methodology for assigning unpaired cysteine residues in proteins formulated in a variety of different conditions to identify structural heterogeneity as a potential cause for protein degradation.

METHODS

1-Cyano-4-dimethylaminopyridinium tetrafluoroborate (CDAP) was employed for cyanylating free cysteines in proteins and peptides. Subsequent basic cleavage of the peptide bond at the N-terminal side of the cyanylated cysteines provided direct information about their location.

RESULTS

CDAP was successfully employed to a wide variety of labeling conditions. CDAP was reactive between pH 2.0 and 8.0 with a maximum labeling efficiency at pH 5.0. Its reactivity was not affected by excipients, salt or denaturant. Storing CDAP in an organic solvent increased its intrinsic stability. It was demonstrated that CDAP can be employed as a thiol-directed probe to investigate structural heterogeneity of proteins by examining the accessibility of unpaired cysteine residues.

CONCLUSION

CDAP is a unique cysteine-labeling reagent because it is reactive under acidic conditions. This provides an advantage over other sulfhydryl labeling reagents as it avoids potential thiol-disulfide exchange. Optimization of the cyanylation reaction allowed the utilization of CDAP as a thiol-directed probe to investigate accessibility of sulfhydryl groups in proteins under various formulation conditions to monitor structural heterogeneity.

Synthesis and biological activity of new lodoacetamide derivatives on mutants of squalene-hopene cyclase

New iodoacetamide derivatives, containing a dodecyl or a squalenyl moiety, were synthesized. The effect of these new thiol-reacting molecules was studied on two mutants of Alicyclobacillus acidocaldarius squalene-hopene cyclase constructed especially for this purpose. In the quintuple mutant, all five cysteine residues of the enzyme are substituted with serine; in the sextuple mutant, this quintuple substitution is accompanied by the substitution of aspartate D376, located at the enzyme's active site, with a cysteine. N-Dodecyliodoacetamide had little activity toward either mutant, whereas N-squalenyliodoacetamide showed a stronger effect on the sextuple than on the quintuple mutant, as expected.

Peptide mapping and disulfide bond analysis of the cytoplasmic region of an intrinsic membrane protein by mass spectrometry

Intrinsic membrane proteins pose substantial obstacles to analysis by common analytical techniques due to their hydrophobic nature and solubilization requirements. This is the case for studies involving HPLC coupled to mass spectrometry. We have developed an HPLC/mass spectrometry approach to explore and map the peptide sequence of the SERCA1a Ca(2+)-ATPase from the sarcoplasmic reticulum an integral membrane protein of 110 kDa. After extensive proteolysis of the protein, the mass of the proteolytic fragments was analyzed by HPLC/mass spectrometry. Only part of the cytoplasmic fragments was recovered under nondenaturing conditions. On the other hand, peptide fragments obtained under denaturing conditions were found to cover nearly all the cytoplasmic region. Sarcoplasmic reticulum (SR) Ca(2+)-ATPase contains 24 cysteine residues, 18 of which are in the cytosolic or lumenal region of the protein. Peptides containing free cysteines were identified by a mass increase resulting from carboxyamidomethylation of the cysteines with iodoacetamide. Alkylation reactions were executed either before or after reduction of the peptide fragments by dithiothreitol. Analysis of the mass of the fragments indicates that no disulfide bonds exist in the cytoplasmic portion of SR Ca(2+)-ATPase.

Labeling the Ca2+-ATPase of skeletal muscle sarcoplasmic reticulum with maleimidylsalicylic acid

Maleimidylsalicylic acid reacts with the Ca(2+)-ATPase of skeletal muscle sarcoplasmic reticulum with high affinity and inhibits the ATPase activity following a pseudo-first-order kinetic with a rate constant of 8.3 m(-1) s(-1). Calcium binding remains unaffected in the maleimide-inhibited ATPase. However, the presence of ATP, ADP, and, to a lesser extent, AMP protects the enzyme against inhibition. Furthermore, ATPase inhibition is accompanied by a concomitant decrease in ATP binding. The stoichiometry of the nucleotide-dependent maleimidylsalicylic acid binding is 6-10 nmol/mg ATPase, which corresponds to the binding of up to one molecule of maleimide/molecule of ATPase. The stoichiometry of maleimide binding is decreased in the presence of nucleotides and in the ATPase previously labeled with fluorescein-5'-isothiocyanate or N-ethylmaleimide A fluorescent peptide was isolated by high performance liquid chromatography after trypsin digestion of the maleimide-labeled ATPase. Analysis of the sequence and mass spectrometry of the peptide leads us to propose Cys(344) as the target for maleimidylsalicylic acid in the inhibition reaction. The effect of Cys(344) modification on the nucleotide site is discussed.

Nitric oxide-dependent modification of the sarcoplasmic reticulum Ca-ATPase: localization of cysteine target sites

Skeletal muscle contraction and relaxation is modulated through the reaction of sarcoplasmic reticulum (SR) protein thiols with reactive oxygen and nitrogen species. Here, we have utilized high-performance liquid chromatography-electrospray mass spectrometry and a specific thiol-labeling procedure to identify and quantify cysteine residues of the SR Ca-ATPase that are modified by exposure to nitric oxide (NO). NO and/or NO-derived species inactivate the SR Ca-ATPase and modify a broad spectrum of cysteine residues with highest reactivities towards Cys364, Cys670, and Cys471. The selectivity of NO and NO-derived species towards the SR Ca-ATPase thiols is different from that of peroxynitrite. The efficiency of NO at thiol modification is significantly higher compared with that of peroxynitrite. Hence, NO has the potential to modulate muscle contraction through chemical reaction with the SR Ca-ATPase in vivo.

Characterization of phenylmaleimide inhibition of the Ca(2+)-ATPase from skeletal-muscle sarcoplasmic reticulum

The Ca(2+)-ATPase from sarcoplasmic reticulum reacts with phenylmaleimide, producing the inhibition of the ATPase activity following a pseudo-first-order kinetic with a rate constant of 19 M(-1) s(-1). Calcium and ATP binding are not altered upon phenylmaleimide inhibition. However, the presence of millimolar calcium, and to a lesser extent magnesium, in the inhibition medium enhances the effect of phenylmaleimide, causing a higher degree of inhibition. Solubilization with C(12)E(8) does not affect the ATPase inhibition, excluding any kind of participation of the lipid bilayer. Phosphorylation with ATP in steady-state conditions as well as phosphorylation with inorganic phosphate in equilibrium conditions were strongly inhibited. Conversely, we have found that the occupancy of the phosphorylation site by ortovanadate fully protects against the inhibitory effect of phenylmaleimide, indicating a conformational transition associated with the phosphorylation reaction.

Peroxynitrite modification of protein thiols: oxidation, nitrosylation, and S-glutathiolation of functionally important cysteine residue(s) in the sarcoplasmic reticulum Ca-ATPase

Skeletal muscle contraction and relaxation is efficiently modulated through the reaction of reactive oxygen-nitrogen species with sarcoplasmic reticulum protein thiols in vivo. However, the exact locations of functionally important modifications are at present unknown. Here, we determine by HPLC-MS that the modification of one (out of 24) Cys residue of the sarcoplasmic reticulum (SR) Ca-ATPase isoform SERCA1, Cys(349), by peroxynitrite is sufficient for the modulation of enzyme activity. Despite the size and nature of the SR Ca-ATPase, a 110 kDa membrane protein, identification and quantitation of Cys modification was achieved through labeling with 4-(dimethylamino)phenylazophenyl-4'-maleimide (DABMI) and/or N-(2-iodoethyl)trifluoroacetamide (IE-TFA) followed by an exhaustive tryptic digestion and on-line HPLC-UV-electrospray MS analysis. The reaction with IE-TFA generates aminoethylcysteine, a new trypsin cleavage site, which allows the production of specific peptide fragments that are diagnostic for IE-TFA labeling, conveniently identified by mass spectrometry. Exposure of the SR Ca-ATPase to low concentrations (0.1 mM) of peroxynitrite resulted in the fully reversible chemical modification of Cys at positions 344, 349, 471, 498, 525, and 614 (nitrosylation of Cys(344) and Cys(349) was seen), whereas higher concentrations of peroxynitrite (0.45 mM) additionally affected Cys residues at positions 636, 670, and 674. When the SR Ca-ATPase was exposed to 0.45 mM peroxynitrite in the presence of 5.0 mM glutathione (GSH), thiol modification became partially reversible and S-glutathiolation was detected for Cys residues at positions 344, 349, 364, 498, 525, and 614. The extent of enzyme inactivation (determined previously) quantitatively correlated with the loss of labeling efficiency (i) of a single Cys residue and (ii) of the tryptic fragment containing both Cys(344) and Cys(349). Earlier results had shown that the independent selective modification of Cys(344) is functionally insignificant [Kawakita, M., and Yamashita, T. (1987) J. Biochem. (Tokyo) 102, 103-109]. Thus, we conclude that modification of only Cys(349) is responsible for the modulation of the SR Ca-ATPase activity by peroxynitrite.

Amino acid sequence of a Ca(2+)-transporting ATPase from the sarcoplasmic reticulum of the cross-striated part of the adductor muscle of the deep sea scallop: comparison to serca enzymes of other animals

The RT PCR approach was used to obtain the nucleotide sequence of the mRNA of a sarco/endoplasmic reticulum calcium transporting ATPase (SERCA) from the cross-striated (phasic) part of the adductor muscle of the deep sea scallop. Initially, degenerate primers based on consensus sequences among SERCAs and tryptic fragments of the scallop Ca-ATPase were used. The sequence was then extended using homologous primers and the 5' and 3' ends of the transcript determined by 5' and 3' RACE. The mRNA codes for a polypeptide chain 994 amino acid residues long (coded for by 2982 nucleotides) and has a 195 bp 5' untranslated region, with a 697 bp 3' untranslated region. The scallop enzyme shows strongest amino acid similarity to the SERCA enzyme of Loligo, followed by those of Drosophila and Artemia. It resembles the vertebrate SERCA3 in that it does not possess the phospholamban binding motif and so is unlikely to be regulated by protein kinase A mediated signals.

Erythrosin isothiocyanate selectively labels lysine464 within an ATP-protectable binding site on the Ca-ATPase in skeletal sarcoplasmic reticulum membranes

Conditions that permit the selective modification of an ATP-protectable site on the Ca-ATPase in skeletal sarcoplasmic reticulum (SR) membranes using erythrosin isothiocyanate (Er-ITC) have been identified. The major labeling site for Er-ITC has been identified using reversed-phase HPLC and positive FAB mass spectrometry after exhaustive tryptic digestion of the Er-ITC-modified Ca-ATPase. An ATP-protectable peptide corresponding to M452NVFNTEVRNLSK464VER467 is modified by Er-ITC, the average mass of which is 2830.1 +/- 0.3 Da. The exclusive modification of lysine residues indicates Lys464 as the site of Er-ITC modification. Derivatization with Er-ITC diminishes the secondary activation of steady-state ATPase activity and the rate of dephosphorylation by millimolar concentrations of ATP. In contrast, in the presence of micromolar ATP concentrations Er-ITC modification of the Ca-ATPase does not affect (i) the apparent affinity of ATP, (ii) the maximal extent of phosphoenzyme formation by ATP, (iii) the rate of steady-state ATP hydrolysis, or (iv) the rate of dephosphorylation of the Ca-ATPase. Furthermore, ATP utilization by the Ca-ATPase is unaffected by detergent solubilization, irrespective of Er-ITC modification, indicating that the secondary activation of ATP hydrolysis involves a single Ca-ATPase polypeptide chain. Therefore, Er-ITC does not interfere with the normal structural transitions associated with phosphoenzyme decay. Rather, these results indicate that Er-ITC bound to Lys464 interferes with either ATP binding to a low-affinity site or the associated structural transitions that modulate the rate of enzyme dephosphorylation.

Phosphorylation of phospholamban by cAMP-dependent protein kinase enhances interactions between Ca-ATPase polypeptide chains in cardiac sarcoplasmic reticulum membranes

We have used spin-label EPR spectroscopy to examine possible alterations in protein-protein interactions that accompany the activation of the cardiac sarcoplasmic reticulum (SR) Ca-ATPase following the phosphorylation of phospholamban (PLB). Using a radioactive derivative of a maleimide spin label (MSL), we have developed conditions for the selective spin-labeling of the Ca-ATPase in both native cardiac and skeletal sarcoplasmic reticulum membranes. The rotational dynamics of the cardiac and skeletal Ca-ATPase isoforms in native SR membranes were measured using saturation transfer EPR. We report that the phosphorylation of PLB in cardiac SR results in a (1.8 +/- 0.2)-fold reduction in the overall rotational mobility of the Ca-ATPase. The alteration in the rotational dynamics of the Ca-ATPase is the direct result of the phosphorylation of PLB, and is not related to the phosphorylation of the Ca-ATPase or any other SR proteins since no alteration in the ST-EPR spectrum is observed as a result of conditions that phosphorylate the cardiac Ca-ATPase with ATP. Neither do the use of conditions that activate the Ca-ATPase in cardiac SR result in the alteration of the rotational dynamics or catalytic properties of the Ca-ATPase in skeletal SR where PLB is not expressed. Measurements of the rotational dynamics of stearic acid spin labels (SASL) incorporated into cardiac SR membranes with a nitroxide at the 5- and 12-positions using conventional EPR indicate that there is virtually no difference in the lipid acyl chain dynamics in cardiac SR membranes upon the phosphorylation of PLB. These results indicate that the decrease in the rotational dynamics of the Ca-ATPase in cardiac SR membranes associated with the phosphorylation of PLB is related to enhanced interactions between individual Ca-ATPase polypeptide chains due to (i) an alteration in the spatial arrangement of cardiac Ca-ATPase polypeptide chains within a defined oligomeric state or (ii) increased protein-protein associations. We suggest that altered interactions between Ca-ATPase polypeptide chains and PLB serves to modulate the activation barrier associated with calcium activation of the Ca-ATPase in cardiac SR membranes.

Reaction of 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole with the (Ca2+ + Mg2+)- ATPase protein of sarcoplasmic reticulum at low temperature

Modification of the (Ca2+ + Mg2+)-ATPase protein of rabbit skeletal sarcoplasmic reticulum (SR) with 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole, NBD-Cl, at 4 degrees C for 5 min caused a 63% loss of the Ca(2+)-dependent ATPase activity when 1 mol of the adenine analog was incorporated per 10(5) g of protein. At 25 degrees C, above the lipid phase transition, the extent of labeling was 3-fold higher although the Ca(2+)-ATPase activity was inhibited to the same extent. MgATP protected the ATPase activity at 4 degrees C and 25 degrees C but there was little change in the extent of labeling at 4 degrees C suggesting that changes in the fluidity of the lipid moiety made different sites on the ATPase protein accessible to the reagent. At 4 degrees C, addition of sodium deoxycholate enhanced the inactivation (6% ATPase activity remained) but the labeling of the SR-ATPase protein did not increase significantly. Incubation with MgATP prior to solubilization with deoxycholate resulted in the protection of the Ca(2+)-ATPase activity and only a small decrease in the labeling occurred. At 25 degrees C, a similar pattern was found with deoxycholate but the loss of ATPase activity was less dramatic and the extent of labeling by NBD-Cl was greater than that at 4 degrees C. MgATP induced changes in the conformation of the ATPase protein protecting essential cysteine residues while shifting the reaction of NBD-Cl with the ATPase protein to non-essential sites in the absence or presence of deoxycholate. An analysis of tryptic digests of the NBD-ATPase protein showed that MgATP shifted the labeling from the A2 subfragment to the A1 subfragment in the absence of deoxycholate and from the A1 subfragment to the A2 subfragment in the presence of deoxycholate. The reagent, NBD-Cl, can distinguish between different temperature dependent conformational states of the ATPase protein.