The effects of acetylation on the binding region of cartilage proteoglycans to hyaluronic acid

Summary Proteoglycans in cartilage are found as aggregates and as monomers. Evidence has been obtained indicating that hyaluronic acid, normally present in this tissue, binds together monomers into large molecular weight aggregates. In this investigation, the interacting region of the protein backbone has been studied. The results unequivocally demonstrated that the epsilon amino groups of lysine are involved in hyaluronic acid binding to proteoglycans and that their blocking by acetylation either prevents reaggregation or disaggregates the high mol. w aggregates.Supported by a grant from the Consiglio Nazionale delle Ricerche, Rome, Italy, and U. S. P. H. S., grants DE-02670 and HL 11310.     

Interaction of anaesthetics with electrical synapses

Studies of the interaction of anaesthetics with various preparations, from whole animals to organic solvents, have been continuing since Overton and Meyer found a correlation between anaesthetic potency and solubility in olive oil. Although the physiological basis of anaesthesia is far from clear, one popular hypothesis is that anaesthetics act primarily by interfering with the normal functioning of chemical synapses. This hypothesis is supported by experiments showing that these synapses are more sensitive to both local and general anaesthetics than are axons. The effects of anaesthetics on electrical synapses (gap-junctions or nexus) have not previously been studied. These ubiquitous structures, presumably responsible for cell-to-cell communication, are found in most vertebrate and invertebrate tissues. We report here the effects of several anaesthetics on electronic coupling between nerve cells, and show that electrical synapses are less sensitive to most anaesthetics than are chemical synapses and axonal membranes.     

Effect of glucose administration on bilirubin excretion in the rabbit

Summary The effect of i.v. infusion of glucose on the hepatic handling of bilirubin was examined in rabbits. A significant increase in the excretion of conjugated bilirubin into the bile was observed, accompanied by a decrease in bilirubinemia. Hepatic bilirubin concentrations were lowered and the UDP-glucose concentrations and liver UDP-glucuronosyl and UDP-glucosyl transferase activities increased.     

The RNA required in the first step of chlorophyll biosynthesis is a chloroplast glutamate tRNA

A molecule of chlorophyll is synthesized from eight molecules of delta-aminolevulinate (DALA), the universal precursor of porphyrins. The light-regulated conversion of glutamate to delta-aminolevulinate in the stroma of greening plastids involves the reduction of glutamate to glutamate-1-semialdehyde and its subsequent transamination. The components performing this conversion have been isolated from barley and Chlamydomonas and separated into three fractions by serial affinity chromatography on Blue Sepharose and haem- or chlorophyllin-Sepharose. The complete reaction can be performed in vitro in a reconstituted assay by combining all three fractions. An RNA is the essential component of the chlorophyllin-Sepharose-bound fraction. By nucleotide sequence analysis, we have now identified this RNA as a chloroplast glutamate acceptor RNA. Glutamate attached by an aminoacyl bond to the 3'-terminal adenosine of this RNA is a substrate for the enzyme(s) which perform the subsequent reactions. This reaction represents a novel role for transfer RNA: participation in the metabolic conversion of its cognate amino acid into another metabolite of low relative molecular mass which subsequently is not used in peptide bond synthesis.     

Protein biosynthesis in organelles requires misaminoacylation of tRNA

In the course of our studies on transfer RNA involvement in chlorophyll biosynthesis, we have determined the structure of chloroplast glutamate tRNA species. Barley chloroplasts contain in addition to a tRNA(Glu) species at least two other glutamate-accepting tRNAs. We now show that the sequences of these tRNAs differ significantly: they are differentially modified forms of tRNA(Gln) (as judged by their UUG anticodon). These mischarged Glu-tRNA(Gln) species can be converted in crude chloroplast extracts to Gln-tRNA(Gln). This reaction requires a specific amidotransferase and glutamine or asparagine as amide donors. Aminoacylation studies show that chloroplasts, plant and animal mitochondria, as well as cyanobacteria, lack any detectable glutaminyl-tRNA synthetase activity. Therefore, the requirement for glutamine in protein synthesis in these cells and organelles is provided by the conversion of glutamate attached to an 'incorrectly' charged tRNA. A similar situation has been described for several species of Gram-positive bacteria. Thus, it appears that the occurrence of this pathway of Gln-tRNA(Gln) formation is widespread among organisms and is a function conserved during evolution. These findings raise questions about the origin of organelles and about the evolution of the mechanisms maintaining accuracy in protein biosynthesis.     

An endoplasmic reticulum retention signal in the CD3 epsilon chain of the T-cell receptor.

Isolated polypeptide chains of the T-cell antigen receptor complex are degraded or retained in the endoplasmic reticulum (ER). Assembly of the multisubunit complex allows the individual chains to escape retention in the ER and to be expressed on the cell surface. We engineered a series of deletions in the CD3 epsilon subunit of the human T-cell receptor in order to find the sequences responsible for its retention in the ER. Deletion of amino acids 171 to 180 in the cytosolic tail resulted in the cell-surface expression of the isolated chain. This sequence also promotes retention when it is appended to CD4, a plasma membrane protein. Mutagenesis of the 10-amino-acid CD3 epsilon sequence established that the tyrosine and serine residues are important for ER retention. This and other ER retention signals must be hidden when a complete T-cell receptor complex is assembled in order to allow its expression on the cell surface.