On the effect of alkaline pH and cofactor availability in the conformational and oligomeric state of Escherichia coli glutamate decarboxylase

Escherichia coli glutamate decarboxylase (EcGad) is a homohexameric pyridoxal 5'-phosphate (PLP)-dependent enzyme. It is the structural component of the major acid resistance system that protects E. coli from strong acid stress (pH < 3), typically encountered in the mammalian gastrointestinal tract. In fact EcGad consumes one proton/catalytic cycle while yielding γ-aminobutyrate and carbon dioxide from the decarboxylation of l-glutamate. Two isoforms of Gad occur in E. coli (GadA and GadB) that are 99% identical in sequence. GadB is the most intensively investigated. Prompted by the observation that some transcriptomic and proteomic studies show EcGad to be expressed in conditions far from acidic, we investigated the structural organization of EcGadB in solution in the pH range 7.5-8.6. Small angle X-ray scattering, combined with size exclusion chromatography, and analytical ultracentrifugation analysis show that the compact and entangled EcGadB hexameric structure undergoes dissociation into dimers as pH alkalinizes. When PLP is not present, the dimeric species is the most abundant in solution, though evidence for the occurrence of a likely tetrameric species was also obtained. Trp fluorescence emission spectra as well as limited proteolysis studies suggest that PLP plays a key role in the acquisition of a folding necessary for the canonical catalytic activity.

Higher blood pressure load (baric impact) in normotensives with endothelial dysfunction: a paraphysiological status of "pre-hypertension"

PURPOSE

The present study investigated the blood pressure (BP) load (L), namely Baric Impact (BI), in normotensives with and without endothelial dysfunction (ED). The aim was to detect baric differences supporting the thesis that the ED is associated with vasopressant effects that are responsible for a paraphysiological condition of higher BP (pre-hypertension) even in normotensives.

MATERIALS AND METHODS

Thirty-eight normotensives were investigated in their endothelium-dependent vasomotricity by mean of the non-invasive post-ischemic brachial artery vasodilation test. Additionally, their underwent a non-invasive ambulatory (A) BP monitoring (M) over the 24-h span in order to confirm that they were not hypertensive. The ABPM served also to compute the systolic (S) and diastolic (D) BI.

RESULTS

The ED was detected in eight normotensives of the investigated group. These cases with ED were found to show a significantly higher SBI and DBI as compared to the normotensives without ED.

CONCLUSIONS

The significant elevation of the SBI and DBI in normotensives with ED is an evidence convincing that a dysfunctional endothelium is responsible for vasopressant effects that cause a paraphysiological status of "pre-hypertension".

The ambulatory monitoring documents a more elevated blood pressure regimen (pre-hypertension) in normotensives with endothelial dysfunction

PURPOSE

The present study investigates the blood pressure (BP) 24-h values in normotensives with and without endothelial dysfunction (ED). The scope is to detect differences in BP regimen supporting the hypothesis that the ED is associated with vasopressant effects that can cause a condition of "pre-hypertension".

MATERIALS AND METHODS

Thirty-eight normotensives were investigated in their endothelial function by mean of the non-invasive post-ischemic brachial artery vasodilation test (endothelium-dependent vasomotricity). Their were also automatically and non-invasively monitored in their systolic (S) and diastolic (D) BP over the 24-h period in order to confirm that they were not hypertensive.

RESULTS

Eight of the investigated normotensives were found to show an ED. A significantly higher daily mean level as well as a more prominent nychtohemeral variability in SBP and DBP 24-h values were observed in the normotensives with ED as compared to the normotensives without ED. The higher BP regimen in the normotensives with ED was found to maintain a circadian rhythm. However, a significant amplification the second harmonic component, with a 12-h period, was observed. The different structure of the BP 24-h pattern in the normotensives with ED was confirmed by the detection of additional ultradian components at the linear-in-period spectral analysis.

CONCLUSIONS

The present study documented a significant elevation of BP 24-h values in normotensives with ED that is the reflex of consistent changes in the frequency organization of the BP circadian pattern. The elevation of BP regimen suggests that the ED is associated with vasopressant effects even in normotensives. Such a condition of higher BP in normotensives with ED can be regarded as a status of "pre-hypertension".

Two different crystal forms of sorcin, a penta-EF-hand Ca2+-binding protein

Sorcin is a 198 amino-acid Ca(2+)-binding protein that belongs to the penta-EF-hand family. Its Ca(2+)-binding domain (residues 33-198) has been crystallized in the absence of Ca(2+) in two different crystal forms. Two complete data sets have been collected on a synchrotron source under cryocooling conditions from crystals grown using ammonium sulfate as precipitant: monoclinic crystals in space group C2, with unit-cell parameters a = 130.93, b = 103.85, c = 78.55 A, beta = 118.0 degrees, diffracting to 2.1 A, and tetragonal crystals in space group P42(1)2, with unit-cell parameters a = b = 103.33, c = 79.15, diffracting to 2.7 A. Crystals were also grown using PEG 6000 as precipitating agent. They also belong to space group C2, diffract to 2.8 A and their unit-cell parameters are very similar to the first form. Structure determination by molecular replacement has been initiated. Structural information should be useful for elucidating the interaction of sorcin with membrane targets.

Breakdown of cytoskeletal proteins during meiosis of starfish oocytes and proteolysis induced by calpain

Meiosis reinitiation in starfish oocytes is characterized by Ca(2+) transients in the cytosol and in the nucleus and is accompanied by the disassembly of the nuclear envelope, a process which is likely to be mediated by the cleavage of selected proteins. We have used mass spectrometry analysis (mass profile fingerprinting) on 2D polyacrylamide gels of extracts of oocytes in which meiosis resumption was induced by 1-methyladenine and have identified five proteins that were specifically degraded: alpha-tubulin, lamin B, dynamin, and two kinds of actin. They are all components of the cytoskeleton or associated with it. We then investigated whether calpain, which is activated by the increase in cell Ca(2+), could cleave the same proteins that became degraded under the influence of 1-methyladenine and thus be involved in nuclear membrane breakdown. The investigation was prompted by the finding that microinjection of calpain into the nuclei of prophase arrested oocytes induced meiosis in the absence of 1-methyladenine. Incubation of prophase arrested (disrupted) oocytes with calpain produced a 2D gel protein pattern in which some of the degradation products coincided with those seen in oocytes challenged with 1-methyladenine.

The sorcin-annexin VII calcium-dependent interaction requires the sorcin N-terminal domain

Surface plasmon resonance experiments show that at neutral pH the stability of the complex between sorcin and annexin VII (synexin) increases dramatically between 3 and 6 microM calcium; at the latter cation concentration the K(D) value is 0.63 microM. In turn, the lack of complex formation between the sorcin Ca(2+) binding domain (33-198) and synexin maps the annexin binding site to the N-terminal region of the sorcin polypeptide chain. Annexin VII likewise employs the N-terminal domain, more specifically the first 31 amino acids, to interact with sorcin [Brownawell, A.M. and Creutz, C.E. (1997) J. Biol. Chem. 272, 22182-22190]. The interaction may involve similar structural motifs in the two proteins, namely GGYY and GYGG in sorcin and GYPP in synexin.

Structure-function relationships in sorcin, a member of the penta EF-hand family. Interaction of sorcin fragments with the ryanodine receptor and an Escherichia coli model system

Sorcin, a 21.6 kDa cytosolic EF-hand protein which undergoes a Ca(2+)-induced translocation from cytoplasm to membranes, has been assigned to the newly defined penta EF-hand family. A molecular model of the C-terminal Ca(2+)-binding domain has been generated using as a template the X-ray coordinates of the corresponding domain in the calpain light subunit, the family prototype [Lin, G., et al. (1997) Nat. Struct. Biol. 4, 539-546]. The model indicates that in sorcin the three-dimensional structure is conserved and in particular that of EF1, the novel EF-hand motif characteristic of the family. On this basis, two stable fragments have been obtained and characterized. Just like the native protein, the sorcin Ca(2+)-binding domain (residues 33-198) is largely dimeric, interacts with the ryanodine receptor at physiological calcium concentrations, and undergoes a reversible, Ca(2+)-dependent translocation from cytosol to target proteins on Escherichia coli membranes. In contrast, the 90-198 fragment (residues 90-198), which lacks EF1 and EF2, does not bind Ca(2+) with high affinity and is unable to translocate. Binding of calcium to the EF1-EF2 pair is therefore required for the activation of sorcin which uses the C-terminal calcium-binding domain for interaction with the ryanodine receptor, a physiological target in muscle cells.

Expression and localisation of annexin VII (synexin) isoforms in differentiating myoblasts

Annexin VII exists in a 47 kDa and a 51 kDa isoform with the 51 kDa protein being the only isoform present in skeletal muscle. Expression of the 51 kDa isoform during myogenesis and localization was studied in cells after conversion into myogenic cells by transduction with MyoD and in mouse and human myogenic cell lines. MyoD expression in NIH3T3 and C3H10T1/2 fibroblasts led to disappearance of the mRNA specific for the 47 kDa isoform and appearance of the 51 kDa isoform-specific mRNA. The overall amount of annexin VII protein was reduced in myogenic converted cells. Both in undifferentiated and differentiated cells annexin VII was localized by immunofluorescence microscopy to punctate structures which were distributed all over the cell. A GFP annexin VII fusion protein showed a similar distribution. Cell fractionation studies indicated that annexin VII is equally distributed between cytosol and membrane fractions in undifferentiated cells, while in differentiated cells it is exclusively present in the membrane fraction. By sucrose gradient centrifugation of postnuclear supernatants we identified two distinct annexin VII-containing membrane populations that cofractionated with caveolin 3- and sorcin-containing membranes.

Calcium- and pH-linked oligomerization of sorcin causing translocation from cytosol to membranes

Sorcin, a cytosolic calcium-binding protein containing a pair of EF-hand motifs, undergoes a Ca2(+)-dependent translocation to the cell membrane. The underlying conformational change is similar at pH 6.0 and 7.5 and consists in an increase in overall hydrophobicity that involves the aromatic residues and in particular the two tryptophan residues which become less exposed to solvent. The concomitant association from dimers to tetramers indicates that the tryptophan residues, which are located between the EF-hand sites, become buried at the dimer-dimer interface. Ca2(+)-bound sorcin displays a striking difference in solubility as a function of pH that has been ascribed to the formation of calcium-stabilized aggregates.

Effect of the vinyl-globin interactions on the temperature-dependent broadening of the Soret spectra: a study with horse myoglobin and Scapharca dimeric hemoglobin reconstituted with unnatural 2,4-heme derivatives

The temperature dependence of the Soret absorption spectra has been measured over the range 80 to 300 K on deoxygenated and carbonmonoxy horse heart myoglobin and Scapharca inaequivalvis dimeric hemoglobin reconstituted with proto- or with meso- and deutero-heme, in which the vinyl groups have been replaced with ethyl groups or hydrogen atoms, respectively. In the meso- and deutero-derivatives of both proteins the linewidth of the absorption spectra is narrower and less sensitive to thermal broadening effects than in the proto-derivatives. Moreover, the broadening effects are larger in the deoxygenated proteins with respect to the liganded adducts. The quantitative analysis of these effects shows that the change in linewidth is due to a marked decrease in the extent of coupling between the heme vibronic transitions and the protein low-frequency motions. The relevance of the vinyl groups in the dynamics of the heme-globin interaction is highlighted by this experimental approach which shows that the protein is capable of transmitting structural information to the heme by coupling the ensemble of the low-frequency modes to the stereochemistry of the vinyl itself. This mechanism, which entails adjustment of the equilibrium between vinyl torsional conformers, represents an additional pathway for the control of the heme reactivity in addition to the iron-histidine link.

The unique heme-heme interactions of the homodimeric Scapharca inaequivalvis hemoglobin as probed in the protein reconstituted with unnatural 2,4 heme derivatives

In the homodimeric hemoglobin from Scapharca, HbI, functional communication between the two heme groups is based on their direct structural linkage across the subunit interface through the heme propionates. The heme-protein interactions have been altered in deutero- and meso-HbI by substituting the vinyl groups at positions 2 and 4 of protoheme with hydrogen and ethyl groups, respectively. In meso-HbI the introduction of the ethyl groups in the heme pocket induces significant alterations in the conformation of the heme peripheral substituents, including the propionates, and in the structure of bound CO, as revealed by the resonance Raman spectra. The functional counterpart of these structural changes is the loss of cooperativity in carbon monoxide binding and in the rate of oxygen dissociation. Oxygen pulse and flash photolysis experiments indicate that meso-HbI is locked in the liganded conformation. It is postulated that the ethyl groups, which occupy a larger volume than vinyl ones, impair the ligand-linked movement of the heme relative to its pocket and in turn the expression of cooperativity. In deutero-HbI structural alterations have not been monitored. Functionally, cooperativity in the CO binding kinetics is increased as if hydrogen atoms at positions 2 and 4 permitted more marked movements of the heme than in the native protein.

Calcium-dependent translocation of sorcin to membranes: functional relevance in contractile tissue

Sorcin, a 22 kDa calcium binding protein present in abundance in cardiac tissue and in multi-drug resistant cells and previously described as a soluble protein, is now shown to undergo a calcium-dependent translocation process from the cytosol to cellular membranes in both systems. The translocation process takes place also in E. coli BL21 cells that express recombinant sorcin, r-sorcin, and can be exploited in the purification of the protein. Calcium binding to purified r-sorcin occurs at micromolar concentrations of the metal and is accompanied by a conformational change that renders the protein soluble in the non-ionic detergent Triton X-114. This finding suggests that lipids are the target of sorcin on cellular membranes. The possible significance of the calcium-dependent translocation of sorcin in the specialized functions of sorcin-expressing cells is discussed.