An analysis of alterations in ribosomal conformation using reductive methylation

The path of murine serum amyloid A through peritoneal macrophages

Serum amyloid A (SAA) is a family of proteins encoded by four related genes. Of the four, isoforms 1.1 and 2.1 are acute phase proteins synthesized by the liver. They become major components of the HDL plasma fraction during acute tissue injury and the HDL/SAA complex is readily taken up by macrophages. Herein we investigated the path SAA follows when presented to macrophages as HDL/SAA or in liposomes. Using antibodies specific to SAA and confocal microscopy, or EM autoradiography where only SAA is radio-labeled, we show that HDL/SAA is taken up rapidly by macrophages and within 30 min SAA, or fragments thereof, proceeds through the cytoplasm to the peri-nuclear region and then the nucleus. Within 45-60 min SAA, or fragments thereof, is found back in the cytoplasm and at the plasma membrane where it is subsequently extruded. The observation that SAA, or fragments thereof, traverse the nucleus is a novel finding and may implicate SAA in macrophage gene regulation. It also raises questions by what mechanism SAA enters and leaves the nucleus. We further investigated if both SAA isoforms traffic through the macrophage in a similar manner. Isoform differences were observed. Both isoforms bind well to the plasma membrane of macrophages at 4 degrees C, but at 37 degrees C only SAA2.1 is taken up by the cell in significant quantity, and is observed in the nucleus, suggesting that the two isoforms are handled differently and that they may have discrete physiological roles.

Influence of sulphate ions on the structure of AA amyloid fibrils

To explore the possible interaction of sulphated GAG with AA amyloid peptides, human AA amyloid fibrils were exposed to buffers containing various salts, and the accessibility of free amino groups on the peptides to reductive methylation was examined. Sodium chloride had little effect except at concentrations of 1 M, where it reduced the accessibility of AA peptides to labelling. In contrast 70 mM Na2SO4 led to a significant increase in peptide accessibility to labelling. The results suggest that, at least in part, GAG interact with AA peptides through their sulphate moieties.

Inhibition of the initiation of hepatic protein synthesis during ethionine mediated ATP depletion in vivo: modification to ribosomal subunits, evidence of impaired ternary complex formation and a subcellular redistribution of eIF-2 alpha

Acute ethionine intoxication is known to induce a reversible hepatic injury in female rats by reducing the level of hepatic ATP. The injury indirectly impairs the initiation of hepatic protein synthesis, with resultant polysome disaggregation. Administration of adenine rapidly restores the ATP levels and protein synthesis. Analysis of liver polysome and ribosomal subunits reveals that polysome disaggregation occurs following 3 h of the intoxication, and reaggregation occurs following the administration of adenine. Inactive hepatic ribosomes accumulate as monomers and disomes when analysed by sucrose gradient sedimentation in low-salt buffers. High-salt buffers dissociate the inactive ribosomes into the component 40 S and 60 S subunits. The level of higher density, 1.48 g/cc, 40 S subunit increases during the inhibition of protein synthesis, while the lower density, 1.41 g/cc, 40 S subunit species does not change significantly. Hepatic microsomal and cytosolic extracts examined for their ability to support the formation of the ternary complex of eIF-2-GTP and [35S]Met-tRNAi demonstrate that during acute ethionine intoxication, ternary complex formation in the two extracts decrease 65% and 85%, respectively. These changes are coincident with polysome disaggregation. Administration of adenine to reverse the intoxication restores the ternary complex forming ability of the cytosolic extract, but does not affect the activity of the microsomal salt wash extracts. Mixing experiments indicate the accumulation of an inhibitor of ternary complex formation in the microsomal salt wash fraction. The application of quantitative western blotting demonstrates that the level of antigenic eIF-2 alpha in the microsomal salt wash extract increases 31% during the inhibition. These observations are consistent with the idea that the inhibition of the initiation of hepatic protein synthesis induced by ethionine is mediated by eIF-2 alpha phosphorylation. The latter results in an inhibition of ternary complex formation, redistribution of eIF-2 to the microsome fraction, polysomal disaggregation, and accumulation of inactive ribosomal subunits.

Apolipoprotein A-I and apolipoprotein SAA half-lives during acute inflammation and amyloidogenesis

The half-lives of two apolipoproteins of mouse high-density lipoproteins, apo A-I and apo SAA, were determined in normal animals and compared with those having an inflammatory condition, or inflammation leading to AA amyloid deposition. The apo A-I half-life was considerably shorter in animals with inflammation and in those that were preamyloidotic, than in controls (t1/2 of 3-3.5 h vs. 10-12 h). The average loss of apo A-I in controls over the first 10 h was 31.1 micrograms/ml per h, while that in inflamed animals was 58.7 micrograms/ml per h a 2-fold increase in apo A-I clearance. The apo SAA half-life was similar in all groups of animals and was of the order of 1.5 h. The concentration of apo SAA during inflammation is however considerably higher (500-1000-fold) than in controls, which implies a much greater clearance rate during inflammation and involving a process which is apparently not saturable. In addition to hypertriglyceridemia and Tangier's disease, ordinary acute inflammation can now be added to those pathological conditions which lead to a significant decrease in apo A-I half-life.

A reproducible model for the study of factor X kinetics in AA amyloidosis

Factor X clearance was examined in a model of rapid AA amyloid deposition. Accelerated equilibration with extravascular compartments and accelerated removal postequilibration mimic features seen in patients with AL amyloidosis. The handling of Factor X was different from that of two other proteins, mouse albumin and IgG. Each protein had its own specific characteristic clearance properties, although in amyloidotic animals all proteins were cleared more rapidly in the postequilibration phase. The liver was by far the major site of Factor X clearance but this was true in all control groups as well. No significant difference was seen in tissue clearance site in any of the treatment groups, perhaps because the amount of AA amyloid in each tissue 3 days into the protocol was not yet large. Nevertheless, a reproducible model that possesses accelerated Factor X clearance is now available to study the mechanism of coagulation factor abnormalities in amyloidosis.

Effects of culture substrates and normal hepatic sinusoidal cells on in vitro hepatocyte synthesis of Apo-SAA

Primary hepatocyte cultures synthesize apo-SAA upon stimulation with supernatant from lipopolysaccharide (LPS)-treated macrophages. The matrices on which the hepatocytes were grown influence their basal apo-SAA synthetic capability. Fibronectin was superior. Coculturing hepatocytes with hepatic sinusoidal cells did not adversely affect the ability of hepatocytes to synthesize and secrete apo-SAA into the culture medium. In 72 h, clear islands of endothelial cells nestled in layers of hepatocytes. Both apo-SAA and apo-SAA were made in considerable quantities but no evidence could be obtained that the apo-SAA were free of apo-A-1. The coculturing of hepatocytes with liver sinusoidal cells, the site of ultimate AA deposition, is a first step in establishing an in vitro system for AA amyloidogenesis.

A comparison of accessibility of ribosomal proteins on free and membrane-bound ribosomes: the ribosomal proteins potentially involved in ribosome-membrane binding

The relative accessibility of rat liver ribosomal proteins to reductive methylation was examined using membrane-bound and free ribosomes. Comparisons indicated that 12-13 large ribosomal proteins are masked by ribosomal association with membranes. These consisted of L8, L10, L17, L26-28, L31 and L36, and probably also include L4, L5, L7 and L29. These proteins seem to surround a region centered about L3 and may partly define a ribosomal channel through which the nascent peptide emerges. Approx. 10-20% of the large ribosomal subunit surface area is shielded by the membrane.

Cyclic AMP binding activity in liver supernatants during acute ethionine intoxication

There is a significant increase in hepatic cAMP binding activity 4 hr following ethionine intoxication. This activity is localized to the 100, 000 g supernatant. The binding of cAMP by the 100, 000 g supernatant of controls can be increased to experimental levels simply by dialysis. The increased binding activity seen during ethionine intoxication can be reversed by the administration of adenine. Neither S-adenosyl ethionine, S-adenosyl methionine, S-adenosyl homocysteine, nor AMP compete effectively with cAMP for the binding protein. Increasing ADP concentrations stimulates cAMP binding whereas increasing ATP concentration inhibits cAMP binding. At concentrations seen during ethionine intoxication the effects of ADP and ATP were equal but opposite in direction.

Hepatic adenylate cyclase and phosphodiesterase activity during acute ethionine intoxication

Four hours after the administration of ethionine and in the face of an 80-90% depletion of hepatic ATP there is a twofold increase in cAMP. This increase in cAMP is shown not to be effected by a reduction in phosphodiesterase activity. Both the low Km and high Km forms of the enzyme retain their activity. The purification of liver plasma membranes is not affected by ethionine. There is a significant 20% increase in plasma membrane adenylate cyclase activity following acute ethionine intoxication. This was demonstrable using either ATP or 5'-adenylyl-imidodiphosphate as substrates. The plasma membranes, isolated in the presence of Ca2+, have a 20-fold higher basal adenylate cyclase activity then previously reported and are not further stimulated by GTP or NaF. A modified protocol for isolating cAMP from the adenylate cyclase reaction is described.

Ribosome topography in baby hamster kidney cells infected with Sindbis and vesicular stomatitis viruses

The topography of polysomal ribosomes in mock-infected and in Sindbis virus- and vesicular stomatitis virus-infected BHK cells was investigated using a double, radioactive labelling technique. Ribosomal proteins in intact polysomes were surface labelled by reductive methylation using [14C]formaldehyde. Following removal of ribosomal RNA, proteins were denatured in 6 M guanidine and labelled with [3H]borohydride. Labelled ribosomal proteins were separated by electrophoresis in two-dimensional gels and the 3H/14C ratio for each ribosomal protein was taken as an index of its relative surface exposure in intact ribosomes. Comparison of the ratios for individual ribosomal proteins in Sindbis virus-infected vs. control polysomes indicated that proteins L7, L8, L17, L26 and S19 became more 'buried' and others such as L4, L29, L36, S2 and S26 became more 'exposed' in infected cells. Most of the topographical alterations occurred in the large ribosomal subunit. In contrast, infection of BHK cells with vesicular stomatitis virus induced little or no topographical alteration.

Effects of ethionine treatment of protein-synthesizing apparatus of rat liver 80 S ribosomes and 40 S ribosomal subunits

The inhibitory effects of ethionine treatment of female rats for 4 h on the protein-synthesizing machineries of 80 S ribosomes and 40 S ribosomal subunits of the liver were investigated. The following results were obtained. (1) The translation of globin mRNA by 80 S ribosomes or 40 S ribosomal subunits, in combination with mouse 60 S subunits, was markedly inhibited by ethionine treatment in a complete cell-free system containing partially purified initiation factors of rabbit reticulocytes and the rat liver pH 5 fraction. (2) The polysome formation of 80 S ribosomes in the complete system described above was inhibited by ethionine treatment. Similar inhibitions by ethionine treatment were observed in the case of incubation of 40 S subunits with reticulocyte lysate, although the polysome formation was rather low even in the case of control 40 S subunits. (3) The pattern of CsCl isopycnic centrifugation of rat liver native 40 S subunits uniformly labeled with [14C]- or [3H]orotic acid showed that the content of non-ribosomal proteins of native 40 S subunits was decreased by ethionine treatment. The analysis of proteins of native 40 subunits by SDS-polyacrylamide slab gel electrophoresis revealed that eIF-3 subunits and two unidentified protein fractions of molecular weight of 2.3.10(4) and 2.1.10(4) were decreased in ethionine-treated rate liver. (4) 40 S subunits from ethionine-treated or control rat livers were labeled with N-[3H]ethylmaleimide or N-[14C]ethylmaleimide, and the 3H to 14C ratios of individual 40 S proteins on two-dimensional polyacrylamide gel electrophoresis were measured. The results suggested that the conformation of rat liver 40 S subunits was changed by ethionine treatment. (5) These results may indicate that ethionine treatment decreases the activity of rat liver 40 S subunits for the interaction with initiation factors, especially eIF-3, as the results of conformational changes of 40 S subunits.

Photo-induced protein-RNA cross-linking in mammalian 60-S ribosomal subunits

Rat liver 60-S ribosomal subunits were submitted to increasing doses of radiation (253.7 nm), at 4 degrees C and 25 degrees C, as previously reported fro 40-S subunits. The existence of protein-RNA cross-linking was demonstrated by two methods. The first consisted in the separation of protein-RNA complex; the second was indirect, and took into account alteration either in the electrophoretic mobility of cross-linked proteins or the separability of 28-S RNA in a 4 M urea/3 M LiCl buffer. The peptide synthetase activity and the sedimentation characteristics of the particles irradiated at 4 degrees C were well preserved, but at 25 degrees C the large subunits were progressively inactivated and unfolded for doses higher than 2 x 10(18) quanta. The dose-dependent variations of protein cross-linkage determined by two-dimensional gel electrophoresis allowed us to distinguish those proteins which reacted at the lowest doses with a first-order reaction from those which cross-linked to RNA after a subtle modification of the subunit structure. At 25 degrees C, all proteins became low-dose reactive. The curve obtained for 28-S RNA cross-linkage was similar to that of the total protein moiety, while those obtained fro the 5-S and 5.8-S RNA (which were parallel) suggest a lower reactivity of these RNAs. As a general rule, proteins from the large subunits were more reactive to RNA than those from the small subunits. This could indicate differences in the organisation of the two subunits.