Modification of histidine 5 in sarcoplasmic reticulum Ca2+-ATPase by diethyl pyrocarbonate causes strong inhibition of formation of the phosphoenzyme intermediate from inorganic phosphate

Substrate-induced conformational changes in sarcoplasmic reticulum Ca2+-ATPase probed by surface modification using diethylpyrocarbonate with mass spectrometry

We have identified 15 residues from the surface of sarcoplasmic reticulum Ca(2+)-pump ATPase, by mass spectrometry using diethylpyrocarbonate modification. The reactivity of 9 residues remained high under all the conditions. The reactivity of Lys-515 at the nucleotide site was severely inhibited by ATP, whereas that of Lys-158 in the A-domain decreased by one-half and increased by five-fold in the presence of Ca(2+) and MgF(4), respectively. These are well explained by solvent accessibility, pK(a) and nearby hydrophobicity of the reactive atom on the basis of the atomic structure. However, the reactivity of 4 residues near the interface among A-, N- and P-domain suggested larger conformational changes of these domains in membrane upon binding of Ca(2+) (Lys-436), ATP (Lys-158) and MgF(4) (His-5, -190, Lys-436).

Diethyl pyrocarbonate, a histidine-modifying agent, directly stimulates activity of ATP-sensitive potassium channels in pituitary GH(3) cells

The ATP-sensitive K(+) (K(ATP)) channels are composed of sulfonylurea receptor and inwardly rectifying K(+) channel (Kir6.2) subunit. These channels are regulated by intracellular ADP/ATP ratio and play a role in cellular metabolism. Diethyl pyrocarbonate (DEPC), a histidine-specific alkylating reagent, is known to modify the histidine residues of the structure of proteins. The objective of this study was to determine whether DEPC modifies K(ATP)-channel activity in pituitary GH(3) cells. Steady-state fluctuation analyses of macroscopic K(+) current at -120 mV produced power spectra that could be fitted with a single Lorentzian curve in these cells. The time constants in the presence of DEPC were increased. Consistent with fluctuation analyses, the mean open time of K(ATP)-channels was significantly increased during exposure to DEPC. However, DEPC produced no change in single-channel conductance, despite the ability of this compound to enhance K(ATP)-channel activity in a concentration-dependent manner with an EC(50) value of 16 microM. DEPC-stimulated K(ATP)-channel activity was attenuated by pretreatment with glibenclamide. In current-clamp configuration, DEPC decreased the firing of action potentials in GH(3) cells. A further application of glibenclamide reversed DEPC-induced inhibition of spontaneous action potentials. Intracellullar Ca(2+) measurements revealed the ability of DEPC to decrease Ca(2+) oscillations in GH(3) cells. Simulation studies also demonstrated that the increased conductance of K(ATP)-channels used to mimic DEPC actions reduced the frequency of spontaneous action potentials and fluctuation of intracellular Ca(2+). The results indicate that chemical modification with DEPC enhances K(ATP)-channel activity and influences functional activities of pituitary GH(3) cells.

Mutations of either or both Cys876 and Cys888 residues of sarcoplasmic reticulum Ca2+-ATPase result in a complete loss of Ca2+ transport activity without a loss of Ca2+-dependent ATPase activity. Role of the CYS876-CYS888 disulfide bond

Disulfide-containing peptides in pepsin digest of sarcoplasmic reticulum vesicles were identified by using a fluorogenic thiol-specific reagent 4-fluoro-7-sulfamoylbenzofurazan and a reductant tributylphosphine. Sequencing of the purified peptides revealed the presence of a Cys(876)-Cys(888) disulfide bond on the luminal loop connecting the 7th and 8th transmembrane helices (loop 7-8) of the Ca(2+)-ATPase (SERCA1a). We substituted either or both of these cysteine residues with alanine and made three mutants (C876A, C888A, C876A/C888A), in which the disulfide bond is disrupted. The mutants and the wild type were expressed in COS-1 cells, and functional analysis was performed with the microsomes isolated from the cells. Electrophoresis performed under reducing and non-reducing conditions confirmed the presence of Cys(876)-Cys(888) disulfide bond in the expressed wild type. All the three mutants possessed high Ca(2+)-ATPase activity. In contrast, no Ca(2+) transport activity was detected with these mutants. These mutants formed almost the same amount of phosphoenzyme intermediate as the wild type from ATP and from P(i). Detailed kinetic analysis showed that the three mutants hydrolyze ATP in the mechanism well accepted for the Ca(2+)-ATPase; activation of the catalytic site upon high affinity Ca(2+) binding, formation of ADP-sensitive phosphoenzyme, subsequent rate-limiting transition to ADP-insensitive phosphoenzyme, and hydrolysis of the latter phosphoenzyme. It is likely that the pathway for delivery of Ca(2+) from the binding sites into the lumen of vesicles is disrupted by disruption of the Cys(876)-Cys(888) disulfide bond, and therefore that the loop 7-8 having the disulfide bond is important for formation of the proper structure of the Ca(2+) pathway.

ADP-insensitive phosphoenzyme intermediate of sarcoplasmic reticulum Ca(2+)-ATPase has a compact conformation resistant to proteinase K, V8 protease and trypsin

Sarcoplasmic reticulum Ca(2+)-ATPase was digested with proteinase K, V8 protease and trypsin in the absence of Ca(2+). Unphosphorylated enzyme was rapidly degraded. In contrast, ADP-insensitive phosphoenzyme formed with P(i) and phosphorylated state analogues produced by the binding of F(-) or orthovanadate, were almost completely resistant to the proteolysis except for tryptic cleavage at the T1 site (Arg(505)). The results indicate that the phosphoenzyme and its analogues have a very compact form in the cytoplasmic region, being consistent with large domain motions (gathering of three cytoplasmic domains). Results further show that the structure of the enzyme with bound decavanadate is very similar to ADP-insensitive phosphoenzyme. Thapsigargin did not affect the changes in digestion time course induced by the formation of the phosphorylated state analogues.

Diethyl pyrocarbonate inactivates CD39/ecto-ATPDase by modifying His-59

Diethyl pyrocarbonate (DEPC) in conditions that favour carbethoxylation of histidyl residues strongly inactivated E-type ATPase activity of a rat lung membrane preparation, as well as ecto-ATPase activity of rat vessels and human Epstein-Barr virus-transformed B lymphocytes. Inactivation of the enzyme (up to 70%) achieved at concentrations of DEPC below 0.5 mM could be fully reversed by 200 mM hydroxylamine at pH 7.5, thus confirming histidine-selective modification. UTP effectively protected the enzyme activity from DEPC inactivation. This was taken to indicate that the conformation adopted by the enzyme molecule upon substrate binding was not compatible with DEPC reaching and/or modifying the relevant histidyl residue. Substrate activation curves were interpreted to show the enzyme molecule to be inactive, at all substrate concentrations tested, when the target histidyl residue had been modified by DEPC. Comparison of known sequences of CD39-like ecto-ATP(D)ases with the results on inactivation by DEPC revealed His-59 and His-251 (according to the human CD39 sequence) as equally possible targets of the inactivating DEPC modification. Potato apyrase lacks a homologue for the former residue, while the latter is preserved in the enzyme sequence. Therefore, this enzyme was exposed to DEPC, and since hydrolysis of ATP and ADP by potato apyrase was insensitive to modification with DEPC, it was concluded that His-59 is the essential residue in CD39 that is affected by DEPC modification in a way that causes inactivation of the enzyme.

Deletions or specific substitutions of a few residues in the NH(2)-terminal region (Ala(3) to Thr(9)) of sarcoplasmic reticulum Ca(2+)-ATPase cause inactivation and rapid degradation of the enzyme expressed in COS-1 cells

Amino acid residues in the NH(2)-terminal region (Glu(2) - Ala(14)) of adult fast twitch skeletal muscle sarcoplasmic reticulum Ca(2+)-ATPase (SERCA1a) were deleted or substituted, and the mutants were expressed in COS-1 cells. Deletion of any single residue in the Ala(3)-Ser(6) region or deletion of two or more consecutive residues in the Ala(3)-Thr(9) region caused strongly reduced expression. Substitution mutants A4K, A4D, and H5K also showed very low expression levels. Deletion of any single residue in the Ala(3)-Ser(6) region caused only a small decrease in the specific Ca(2+) transport rate/mg of SERCA1a protein. In contrast, other mutants showing low expression levels had greatly reduced specific Ca(2+) transport rates. In vitro expression experiments indicated that translation, transcription, and integration into the microsomal membranes were not impaired in the mutants that showed very low expression levels in COS-1 cells. Pulse-chase experiments using [(35)S]methionine/cysteine labeling of transfected COS-1 cells demonstrated that degradation of the mutants showing low expression levels was substantially faster than that of the wild type. Lactacystin, a specific inhibitor of proteasome, inhibited the degradation accelerated by single-residue deletion of Ala(3). These results suggest that the NH(2)-terminal region (Ala(3) -Thr(9)) of SERCA1a is sensitive to the endoplasmic reticulum-mediated quality control and is thus critical for either correct folding of the SERCA1a protein or stabilization of the correctly folded SERCA1a protein or both.

Localization of critical histidyl residues required for vinblastine-induced tubulin polymerization and for microtubule assembly

Vinblastine-induced tubulin polymerization is electrostatically regulated and shows pH dependence with a pI approximately 7.0 suggesting the involvement of histidyl residues. Modification of histidyl residues of tubulin with diethylpyrocarbonate (DEPC) at a mole ratio of 0.74 (DEPC/total His residues) for 3 min at 25 degreesC completely inhibited vinblastine-induced polymerization with little effect on microtubule assembly. Under these conditions DEPC reacts only with histidyl residues. For complete inhibition two histidyl residues have to be modified. Demodification of the carboxyethyl histidyl derivatives by hydroxylamine led to nearly complete recovery of polymerization competence. Labeling with [14C]DEPC localized both of these histidyl residues on beta-tubulin at beta227 and beta264. Similarly, tubulin modification with DEPC for longer times (8 min) resulted in complete inhibition of microtubule assembly, at which time approximately 4 histidyl residues had been modified. This inhibition by DEPC was also reversed by hydroxylamine. The third histidyl residue was found on alpha-tubulin at alpha88. Thus, two charged histidyl residues are obligatorily involved in vinblastine-induced polymerization, whereas a different histidyl residue on a different tubulin monomer is involved in microtubule assembly.