Changes in the Steady-state Fluorescence Anisotropy of N-Iodoacetyl-N′-(5-sulfo-1-naphthyl)ethylenediamine Attached to the Specific Thiol of Sarcoplasmic Reticulum Ca2+-ATPase throughout the Catalytic Cycle

High Affinity Heme Binding to a Heme Regulatory Motif on the Nuclear Receptor Rev-erbβ Leads to Its Degradation and Indirectly Regulates Its Interaction with Nuclear Receptor Corepressor

Rev-erbα and Rev-erbβ are heme-binding nuclear receptors (NR) that repress the transcription of genes involved in regulating metabolism, inflammation, and the circadian clock. Previous gene expression and co-immunoprecipitation studies led to a model in which heme binding to Rev-erbα recruits nuclear receptor corepressor 1 (NCoR1) into an active repressor complex. However, in contradiction, biochemical and crystallographic studies have shown that heme decreases the affinity of the ligand-binding domain of Rev-erb NRs for NCoR1 peptides. One explanation for this discrepancy is that the ligand-binding domain and NCoR1 peptides used for in vitro studies cannot replicate the key features of the full-length proteins used in cellular studies. However, the combined in vitro and cellular results described here demonstrate that heme does not directly promote interactions between full-length Rev-erbβ (FLRev-erbβ) and an NCoR1 construct encompassing all three NR interaction domains. NCoR1 tightly binds both apo- and heme-replete FLRev-erbβ·DNA complexes; furthermore, heme, at high concentrations, destabilizes the FLRev-erbβ·NCoR1 complex. The interaction between FLRev-erbβ and NCoR1 as well as Rev-erbβ repression at the Bmal1 promoter appear to be modulated by another cellular factor(s), at least one of which is related to the ubiquitin-proteasome pathway. Our studies suggest that heme is involved in regulating the degradation of Rev-erbβ in a manner consistent with its role in circadian rhythm maintenance. Finally, the very slow rate constant (10(-6) s(-1)) of heme dissociation from Rev-erbβ rules out a prior proposal that Rev-erbβ acts as an intracellular heme sensor.

Distinct types of abnormality in kinetic properties of three Darier disease-causing sarco(endo)plasmic reticulum Ca2+-ATPase mutants that exhibit normal expression and high Ca2+ transport activity

The possible functional abnormalities in three different Darier disease-causing Ca(2+)-ATPase (SERCA2b) mutants, Ile(274) --> Val at the lumenal end of M3, Leu(321) --> Phe on the cytoplasmic part of M4, and Met(719) --> Ile in P domain, were explored, because they exhibited nearly normal expression and localization in COS-1 cells and the high ATPase and coupled Ca(2+) transport activities that were essentially identical (L321F) or slightly lower (I274V by approximately 35% and M719I by approximately 30%) as compared with those of the wild type. These mutations happened to be in Japanese patients found previously by us. Kinetic analyses revealed that each of the mutants possesses distinct types of abnormalities; M719I and L321F possess the 2-3-fold reduced affinity for cytoplasmic Ca(2+), whereas I274V possesses the normal high affinity. L321F exhibited also the remarkably reduced sensitivity to the feedback inhibition of the transport cycle by accumulated lumenal Ca(2+), as demonstrated with the effect of Ca(2+) ionophore on ATPase activity and more specifically with the effects of Ca(2+) (up to 50 mm) on the decay of phosphoenzyme intermediates. The results on I274V and M719I suggest that the physiological requirement for Ca(2+) homeostasis in keratinocytes to avoid haploinsufficiency is very strict, probably much more than considered previously. The insensitivity to lumenal Ca(2+) in L321F likely brings the lumenal Ca(2+) to an abnormally elevated level. The three mutants with their distinctively altered kinetic properties will thus likely cause different types of perturbation of intracellular Ca(2+) homeostasis, but nevertheless all types of perturbation result in Darier disease. It might be possible that the observed unique feature of L321F could possibly be associated with the specific symptoms in the pedigree with this mutation, neuropsychiatric disorder, and behavior problems. The results also provided further insight into the global nature of conformational changes of SERCAs for ATP-driven Ca(2+) transport.

Deletions of any single residues in Glu40-Ser48 loop connecting a domain and the first transmembrane helix of sarcoplasmic reticulum Ca(2+)-ATPase result in almost complete inhibition of conformational transition and hydrolysis of phosphoenzyme intermediate

Possible roles of the Glu40-Ser48 loop connecting A domain and the first transmembrane helix (M1) in sarcoplasmic reticulum Ca(2+)-ATPase (SERCA1a) were explored by mutagenesis. Deletions of any single residues in this loop caused almost complete loss of Ca(2+)-ATPase activity, while their substitutions had no or only slight effects. Single deletions or substitutions in the adjacent N- and C-terminal regions of the loop (His32-Asn39 and Leu49-Ile54) had no or only slight effects except two specific substitutions of Asn39 found in SERCA2b in Darier's disease pedigrees. All the single deletion mutants for the Glu40-Ser48 loop and the specific Asn39 mutants formed phosphoenzyme intermediate (EP) from ATP, but their isomeric transition from ADP-sensitive EP (E1P) to ADP-insensitive EP (E2P) was almost completely or strongly inhibited. Hydrolysis of E2P formed from Pi was also dramatically slowed in these deletion mutants. On the other hand, the rates of the Ca(2+)-induced enzyme activation and subsequent E1P formation from ATP were not altered by the deletions and substitutions. The results indicate that the Glu40-Ser48 loop, with its appropriate length (but not with specific residues) and with its appropriate junction to A domain, is a critical element for the E1P to E2P transition and formation of the proper structure of E2P, therefore, most likely for the large rotational movement of A domain and resulting in its association with P and N domains. Results further suggest that the loop functions to coordinate this movement of A domain and the unique motion of M1 during the E1P to E2P transition.

Organization of cytoplasmic domains of sarcoplasmic reticulum Ca(2+)-ATPase in E(1)P and E(1)ATP states: a limited proteolysis study

In order to characterize the domain organization of sarcoplasmic reticulum Ca(2+)-ATPase in different physiological states, limited proteolysis using three proteases (proteinase K (prtK), V8 and trypsin) was conducted systematically and quantitatively. The differences between E(2) and E(2)P were examined in our previous study and E(2)P was characterized by the complete resistance to all three proteases (except for trypsin attack at the very top of the molecule (T1 site)). The same strategies were employed in this study for E(1)ATP, E(1)PADP and E(1)P states. Because of the transient nature of these states, they were either stabilized by non-hydrolyzable analogues or made predominant by adjusting buffer conditions. Aluminum fluoride (without ADP) was found to stabilize E(1)P. All these states were characterized by strong (E(1)ATP) to complete (E(1)PADP and E(1)P) resistance to prtK and to V8 but only weak resistance to trypsin at the T2 site. Because prtK and V8 primarily attack the loops connecting the A domain to the transmembrane helices whereas the trypsin T2 site (Arg(198)) is located on the outermost loop in the A domain, these results lead us to propose that the A domain undergoes a large amount of rotation between E(1)P and E(2)P. Combined with previous results, we demonstrated that four states can be clearly distinguished by the susceptibility to three proteases, which will be very useful for establishing the conditions for structural studies.

Fluorometric study on conformational changes in the catalytic cycle of sarcoplasmic reticulum Ca(2+)-ATPase

Changes in the fluorescence of N-acetyl-N'-(5-sulfo-1-naphthyl)ethylenediamine (EDANS), being attached to Cys-674 of sarcoplasmic reticulum Ca(2+)-ATPase without affecting the catalytic activity, as well as changes in the intrinsic tryptophan fluorescence were followed throughout the catalytic cycle by the steady-state measurements and the stopped-flow spectrofluorometry. EDANS-fluorescence changes reflect conformational changes near the ATP binding site in the cytoplasmic domain, while tryptophan-fluorescence changes most probably reflect conformational changes in or near the transmembrane domain in which the Ca2+ binding sites are located. Formation of the phosphoenzyme intermediates (EP) was also followed by the continuous flow-rapid quenching method. The kinetic analysis of EDANS-fluorescence changes and EP formation revealed that, when ATP is added to the calcium-activated enzyme, conformational changes in the ATP binding site occur in three successive reaction steps; conformational change in the calcium.enzyme.substrate complex, formation of ADP-sensitive EP, and transition of ADP-sensitive EP to ADP-insensitive EP. In contrast, the ATP-induced tryptophan-fluorescence changes occur only in the latter two steps. Thus, we conclude that conformational changes in the ATP binding site in the cytoplasmic domain are transmitted to the Ca(2+)-binding sites in the transmembrane domain in these latter two steps.

Effects of ranitidine on gastric vesicles containing H+,K(+)-adenosine triphosphatase in rats

BACKGROUND

To ascertain the mechanism for rebound acid hypersecretion after treatment with an H2-receptor blocker, we investigated the effects of ranitidine on gastric H+,K(+)-adenosine triphosphatase (ATPase) in rats.

METHODS

Male Wistar rats received ranitidine (1-50 mg/kg body weight intraperitoneally twice a day for 5 days). The rats were starved for 15 h after the last treatment and then killed, and gastric vesicles containing H+,K(+)-ATPase were prepared.

RESULTS

Treatment with ranitidine dose-dependently increased protein content in the gastric vesicular fraction purified from the gastric mucosa without changing total protein content. Ranitidine also increased the content of a 94,000-dalton protein, the catalytic subunit of H+,K(+)-ATPase. On the other hand, ranitidine did not affect the specific activity of the enzyme (mumol/min/mg of the gastric vesicular protein). Since gastric vesicles in the fasting state mainly consist of the tubulovesicular membrane, these results suggest that ranidine administration increases total tubulovesicular H+,K(+)-ATPase content (mumol/min/rat) by increasing the number of tubulovesicles per parietal cell. The ranitidine-induced increase in total tubulovesicular H+,K(+)-ATPase activity was still evident 1 week after treatment and returned to control level 1 month later.

CONCLUSIONS

All these findings suggest that the increased content and total activity of tubulovesicular H+,K(+)-ATPase after ranitidine treatment may contribute to the mechanism for acid rebound after H2-blocker therapy.

Two states of the nucleotide-binding site of sarcoplasmic reticulum adenosine triphosphatase detected by the calcium-dependent reaction with adenosine 5'-[gamma-imidazolidate]triphosphate and adenosine 5'-[beta-imidazolidate]diphosphate

The Ca(2+)-ATPase from sarcoplasmic reticulum can be inhibited by adenosine 5'-[gamma-imidazolidate]triphosphate through the formation of an intramolecular cross-link at the active site which is dependent on the presence of Ca2+ [Bill, E., Gutowski, Z. & Bämert, H.G. (1988). Calcium-dependent inactivation of the Ca(2+)-ATPase from sarcoplasmic reticulum by chemically reactive adenosine triphosphate, Eur. J. Biochem. 176, 119-124] In the present study we show that adenosine 5'-[beta-imidazolidate]diphosphate is likewise an inhibitor of the Ca(2+)-ATPase effecting a similar inhibition pattern on phosphate release and Ca2+ transport. The overall reaction is Ca2+ dependent and produces a protein band that in SDS/PAGE is indistinguishable from that seen with ATP[imidazolidate]. This shows that the side chain of Asp351 which is claimed to be involved in the cross-linking reaction must be in reach of both the beta and the gamma phosphate moiety of the respective nucleotides. The cross-linked product is formed by a two-step reaction. The first step is the fast reaction of nucleotide imidazolidate presumably at the phosphorylation site (Asp351) under-formation of a mixed anhydride that covalently links nucleotide and protein. Subsequently, the nucleotide is released by a substitution reaction with a second amino acid side chain. This cross-linking reaction is strictly Ca2+ dependent and, remarkably, requires Ca2+ to be added before addition of the inhibitor. It proceeds at two rates and suggests that there are two states of the nucleotide-bindings site. This is also supported by the fact that in the absence of CA2+, ATP[imidazolidate] reacts only in approximately 50% of the calculated ATP-binding sites (based on 80-90% ATPase of total sarcoplasmic reticulum protein) with no subsequent cross-linking reaction.

Structural dynamics of the Ca2(+)-ATPase of sarcoplasmic reticulum. Temperature profiles of fluorescence polarization and intramolecular energy transfer

The temperature dependence of fluorescence polarization and Förster-type resonance energy transfer (FRET) was analyzed in the Ca2(+)-ATPase of sarcoplasmic reticulum using protein tryptophan and site-specific fluorescence indicators such as 5-[2-[iodoacetyl)amino)ethyl]aminonaphthalene-1-sulfonic acid (IAEDANS), fluorescein 5'-isothiocyanate (FITC), 2',3'-O-(2,4,3-trinitrophenyl)adenosine monophosphate (TNP-AMP) or lanthanides (Pr3+, Nd3+) as probes. The normalized energy transfer efficiency between AEDANS bound at cysteine-670 and -674 and FITC bound at lysine-515 increases with increasing temperature in the range of 10-37 degrees C, indicating the existence of a relatively flexible structure in the region of the ATPase molecule that links the AEDANS to the FITC site. These observations are consistent with the theory of Somogyi, Matko, Papp, Hevessy, Welch and Damjanovich (Biochemistry 23 (1984) 3403-3411) that thermally induced structural fluctuations increase the energy transfer. Structural fluctuations were also evident in the energy transfer between FITC linked to the nucleotide-binding domain and Nd3+ bound at the putative Ca2+ sites. By contrast the normalized energy transfer efficiency between AEDANS and Pr3+ was relatively insensitive to temperature, suggesting that the region between cysteine-670 and the putative Ca2+ site monitored by the AEDANS-Pr3+ pair is relatively rigid. A combination of the energy transfer data with the structural information derived from analysis of Ca2(+)-ATPase crystals yields a structural model, in which the location of the AEDANS-, FITC- and Ca2+ sites are tentatively identified.

Characterization of the substrate-induced conformational change of N-iodoacetyl-N'-(5-sulfo-1-naphthyl)ethylenediamine-labeled sarcoplasmic reticulum Ca2(+)-ATPase by using different kinds of substrate

Cys-674 of the sarcoplasmic reticulum Ca2(+)-ATPase was labeled with N-iodoacetyl-N'-(5-sulfo-1-naphthyl)ethylenediamine without a loss of the catalytic activity, and changes in the fluorescence intensity upon addition of seven kinds of substrate were followed by the stopped-flow method. The steady-state fluorescence intensity and anisotropy were also determined. When Ca2+ was present, the fluorescence intensity and anisotropy decreased greatly upon addition of any substrate used. The observed affinity for each substrate agreed with the previously observed affinity of the catalytic site. The fluorescence drop induced by the adenine nucleotides, ATP and adenosine 5'-(beta, gamma-methylene)triphosphate (a nonhydrolyzable ATP analog), was much faster than that induced by other substrates. The ATP-induced fluorescence drop preceded phosphoenzyme formation when the ATP concentration was high, but the fluorescence drop coincided with phosphoenzyme formation when it was slowed by reducing ATP concentrations. The fluorescence drop induced by ITP or acetyl phosphate was slow even at high concentrations of the substrate, and it coincided with phosphoenzyme formation. When Ca2+ was absent, the fluorescence intensity and anisotropy decreased only slightly upon addition of any substrate other than the adenine nucleotides. They decreased substantially upon addition of the adenine nucleotides, but the kinetics of this fluorescence drop were quite different from that of the fluorescence drop induced by any substrate in the presence of Ca2+. These results show that the conformational change, which makes the bound label less constrained, is induced by substrate binding to the catalytic site of the Ca2(+)-activated enzyme. This change precedes phosphoenzyme formation in the catalytic cycle and is greatly accelerated by the adenine moiety of the substrate.

Fluorescence properties of the Ca2+,Mg2(+)-ATPase protein of sarcoplasmic reticulum labeled with 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole

The fluorescence intensity of the Ca2+,Mg2(+)-ATPase protein of rabbit skeletal sarcoplasmic reticulum that incorporated about 2 mol of 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole (NBD-Cl) was enhanced at high MgATP concentrations with or without 50 microM calcium. The observed enhancement indicates that the fluorophore, NBD-Cl, can detect conformational changes in the ATPase protein.