The Lys 280 → Gln mutation mimicking disease‐linked acetylation of Lys 280 in tau extends the structural core of fibrils and modulates their catalytic properties

Amyloid fibrillar aggregates isolated from the brains of patients with neurodegenerative diseases invariably have post‐translational modifications (PTMs). The roles that PTMs play in modulating the structures and polymorphism of amyloid aggregates, and hence their ability to catalyze the conversion of monomeric protein to their fibrillar structure is, however, poorly understood. This is particularly true in the case of tau aggregates, where specific folds of fibrillar tau have been implicated in specific tauopathies. Several PTMs, including acetylation at Lys 280, increase aggregation of tau in the brain, and increase neurodegeneration. In this study, tau‐K18 K280Q, in which the Lys 280 → Gln mutation is used to mimic acetylation at Lys 280, is shown, using HX‐MS measurements, to form fibrils with a structural core that is longer than that of tau‐K18 fibrils. Measurements of critical concentrations show that the binding affinity of monomeric tau‐K18 for its fibrillar counterpart is only marginally more than that of monomeric tau‐K18 K280Q for its fibrillar counterpart. Quantitative analysis of the kinetics of seeded aggregation, using a simple Michaelis–Menten‐like model, in which the monomer first binds and then undergoes conformational conversion to β‐strand, shows that the fibrils of tau‐K18 K280Q convert monomeric protein more slowly than do fibrils of tau‐K18. In contrast, monomeric tau‐K18 K280Q is converted faster to fibrils than is monomeric tau‐K18. Thus, the effect of Lys 280 acetylation on tau aggregate propagation in brain cells is expected to depend on the amount of acetylated tau present, and on whether the propagating seed is acetylated at Lys 280 or not.     

Some differences in the properties of carnitine palmitoyltransferase activities of the mitochondrial outer and inner membranes.

Recent evidence has shown that the outer, overt, malonyl-CoA-inhibitable carnitine palmitoyltransferase (CPTo) activity resides in the mitochondrial outer membrane [Murthy & Pande (1987) Proc. Natl. Acad. Sci. U.S.A. 84, 378-382]. A comparison of CPTo activity of rat liver mitochondria with the inner, initially latent, carnitine palmitoyltransferase (CPTi) of the mitochondrial inner membrane has revealed that the presence of digitonin and several other detergents inactivates CPTo activity. The CPTi activity, in contrast, was markedly stimulated by various detergents and phospholipid liposomes. These findings explain why in previous studies, which used digitonin or other detergents to expose, separate and purify the CPT activities, the inferences were drawn that (a) the ratio of latent to overt CPT was quite high, (b) both the CPT activities could be ascribed to one active protein recovered, and (c) the observed lack of malonyl-CoA inhibition indicated possible loss/separation of a putative malonyl-CoA-inhibition-conferring protein. Although both CPTo and CPTi were found to catalyse the forward and the backward reactions, CPTo showed greater capacity for the forward reaction and CPTi for the backward reaction. The easily solubilizable CPT, released on sonication of mitoplasts or of intact mitochondria under hypo-osmotic conditions, resembled CPTi in its properties. When octyl glucoside was used under appropriate conditions, 40-50% of the CPTo of outer membranes became solubilized, but it showed limited stability and decreased malonyl-CoA sensitivity. Malonyl-CoA-inhibitability of CPTo was decreased also on exposure of outer membranes to phospholipase C. When outer membranes that had been exposed to octyl glucoside or to phospholipase C were subjected to a reconstitution procedure using asolectin liposomes, the malonyl-CoA-inhibitability of CPTo was restored. A role of phospholipids in the malonyl-CoA sensitivity of CPTo is thus indicated.     

Demonstration of structural differences between the two subunits of human-plasma fibronectin in the carboxy-terminal heparin-binding domain

Structural differences between the two subunits of human plasma fibronectin were studied by analyzing the carboxy-terminal heparin-binding domain (Hep-2). Two fragments (29 kDa and 38 kDa) derived from the Hep-2 domain were purified from thermolysin-digested human plasma fibronectin. Identical NH2-terminal sequences were obtained for both fragments through 16 Edman cycles. Neither domain contained the 90-amino-acid extra domain which is predicted by cDNA analysis of the cellular form of fibronectin. We have examined the primary structures of the 29-kDa and 38-kDa Hep-2 domains produced from the two chains of plasma fibronectin by analyzing the tryptic peptides by fast atom bombardment/mass spectrometry and comparison with the predicted fragments deduced from the corresponding cDNA-derived peptide sequences. Peptides that were unique to each domain were further characterized by microsequence analysis. The two domains showed identical amino acid sequences through 274 residues, followed by a region of variability. The 29-kDa domain contains 279 amino acids with an estimated relative molecular mass (Mr) of 30,460. This domain is located in the heavy chain of plasma fibronectin and contains three repeats of type III sequences plus a portion of the connecting segment (IIICS) region. The 38-kDa domain contains 359 amino acids and one O-linked glycosyl unit for an estimated Mr of 39,263. This domain is from the light chain of plasma fibronectin and contains four repeats of type III sequences with the deletion of the entire 120-amino-acid IIICS area. Secondary structure analysis by Chou/Fasman and circular dichroism reveals extensive beta-sheet structure for these domains. Key sulfhydryl and glycosylation sites are located near the mRNA splice junctions for the two chains. It is postulated that the splice junctions are adjacent to a flexible domain joining two regions of extensive beta-sheet structure.     

Monoclonal antibodies in the analysis of fibronectin isoforms generated by alternative splicing of mRNA precursors in normal and transformed human cells

Recent results showing that a single fibronectin gene can give rise to several different mRNAs by alternative splicing have offered an explanation for fibronectin polymorphism. Here we report on monoclonal antibodies that show specificity for a fibronectin segment (ED) that can be included or omitted from the molecule depending on the pattern of splicing of the mRNA precursors. Using these monoclonals, we have quantitatively analyzed the expression of the ED sequence in human fibronectin from different sources. The results demonstrated that, at the protein level, the ED segment is not expressed in plasma fibronectin and that, in fibronectin from the tissue culture medium of tumor-derived or simian virus-40-transformed human cells, the percentage of fibronectin molecules containing the ED segment is about 10 times higher than in fibronectin from normal human fibroblasts. These results suggest that in malignant cells the mechanisms that regulate the splicing of mRNA precursors are altered.     

Interdependence of coenzyme-induced conformational work and binding potential in yeast alcohol and porcine heart lactate dehydrogenases: a hydrogen-deuterium exchange study

Binding of NAD coenzymes to yeast alcohol dehydrogenase (YADH) and porcine heart lactate dehydrogenase (PHLDH) was studied by hydrogen-deuterium exchange with the infrared technique. Conformational changes in the enzymes specific to the coenzymes and their fragments were observed, and the pH dependence of the exchange reaction shows that it conforms to the EX-2 scheme. In both YADH and PHLDH the magnitude of the conformational change of measured by exchange retardation is considerably larger for NAD+ than for NADH. Studies with coenzyme fragments like ADP-ribose, ADP, and AMP also highlight the lack of rigorous correlation between structural features such as charge and size and their influence on exchange behavior. Ternary complexes such as YADH-NAD+-pyrazole, PHLDH-NAD+-oxalate, and PHLDH-NADH-oxamate, which mimic the transition state, have a significantly more pronounced effect on exchange rates than the corresponding binary complexes. The outstanding feature of this study is the demonstration that in the binary enzyme-coenzyme complexes the more loosely bound NAD+ is more effective in retarding exchange than the more firmly bound NADH. These differences are attributed to the unequal structural constraints exerted by the two coenzymes upon the enzymes, which translate to unequal expenditure of transconformational work in the formation of the two complexes. The opposing variation in the free energy of binding and the transconformational work expended can be viewed as an unequal partitioning of the net free energy gain resulting from the protein-ligand interaction into a binding term and that required for conformational change.     

Operations for portal hypertension due to extrahepatic obstruction: results and 10 year follow up.

Between 1976 and 1984, 136 patients with portal hypertension due to extrahepatic obstruction were operated on. Twenty two patients had emergency and 114 elective operations. The operative mortality was 9% and 1%, respectively. Altogether 117 patients (86%) were followed up for from two to 10 years: 17 rebled, none developed encephalopathy or sepsis after splenectomy, and 90% and 75% were alive at five and 10 years respectively. Unlike endoscopic sclerotherapy and treatment with propranolol, operative treatment of variceal bleeding can usually be completed during one admission and carries a low mortality and a fairly low morbidity. Operation seems to be the best form of treatment for poor patients living far from medical facilities in developing countries and may be the treatment of choice in developed countries as well.     

Effect of lipid-protein interaction on the color of bacteriorhodopsin

Detergent solubilization and subsequent delipidation of bacteriorhodopsin (bR) results in the formation of a new species absorbing maximally at 480 nm (bR480). Upon lowering the pH, its absorption shifts to 540 nm (bR540). The pK of this equilibrium is 2.6, with the higher pH favoring bR480 (Baribeau, J. and Boucher, F. (1987) Biochim. Biophysica Acta, 890, 275-278). Resonance Raman spectroscopy shows that bR480, like the native bR, contains a protonated Schiff base (PSB) linkage between the chromophore and the protein. However, the Schiff base vibrational frequency in bR480, and its shift upon deuteration, are quite different from these in the native bR, suggesting changes in the Schiff base environment upon delipidation. Infrared absorption and circular-dichroism (CD) spectral studies do not show any net change in the protein secondary structure upon formation of bR480. It is shown that deprotonation of the Schiff base is not the only mechanism of producing hypsochromic shift in the absorption maximum of bR-derived pigments, subtle changes in the protein tertiary structure, affecting the Schiff base environment of the chromophore, may play an equally significant role in the color regulation of bR-derived pigments.     

NMR study of the molecular and electronic structure of the heme cavity of Aplysia metmyoglobin. Resonance assignments based on isotope labeling and proton nuclear Overhauser effect measurements

The 1H NMR characteristics of the high-spin metmyoglobin from the mollusc Aplysia limacina have been investigated and compared with those of the myoglobin (Mb) from sperm whale. Aplysia metMb exhibits a normal acid----alkaline transition with pK approximately 7.8. In the acidic form, the heme methyl and meso proton resonances have been assigned by 1H NMR using samples reconstituted with selectively deuterated hemins and in the latter case by 2H NMR as well. On the basis of the methyl peak intensities and shift pattern, heme rotational disorder could be established in Aplysia Mb; approximately 20% of the protein exhibits a reversed heme orientation compared to that found in single crystals. Three meso proton resonances have been detected in the upfield region between -16 and -35 ppm, showing that the chemical shift of such protons can serve as a diagnostic probe for a pentacoordinated active site in hemoproteins, as previously shown to be the case in model compounds. The temperature dependence of the chemical shift of the meso proton signals deviates strongly from the T-1 Curie behavior, reflecting the presence of a thermally accessible Kramers doublet with significant S = 3/2 character. Nuclear Overhauser effect, NOE, measurements on Aplysia metMb have provided the assignment of individual heme alpha-propionate resonances and were used to infer spatial proximity among heme side chains. The hyperfine shift values for assigned resonances, the NOE connectivities, and the NOE magnitudes were combined to reach a qualitative picture of the rotational mobility and the orientation of the vinyl and propionate side chains of Aplysia metMb relative to sperm whale MbH2O.(ABSTRACT TRUNCATED AT 250 WORDS)