No abstract

In eukaryotes, multienzyme complexes containing five to nine aminoacyl-tRNA synthetase activities have frequently been reported. In this study, we report the existence, in bovine liver cytoplasm, of a multienzyme complex containing at least 16 activities which can be disrupted by homogenization to give rise to smaller complexes and noncomplexed synthetases. Determination of the size and component activity of these complexes and of the molecular weights of all 20 free synthetases suggests that the smaller complexes and free activities normally identified arise from the larger complex by well-defined stages during homogenization. We also show that similar, though not identical, complexes are found in bovine liver mitochondria and give the molecular weights of 16 mitochondrial synthetases.

Recent polyacrylamide gel electrophoresis studies on Cherax destructor hemocyanin have demonstrated the presence of three further constituent fractions in the alkaline dissociation product in addition to the three subunits reported in earlier work. Two of these recently discovered subunits are monomeric with molecular weights around 750000, while the third subunit is of similar size to the previously identified dimeric subunit M3' with a molecular weight near 150 000. The aggregation process is influenced by the presence of calcium ions, particularly in the distribution of hybrid hexameric species. However, the relative proportions, as well as the types of subunits present initially, are of primary importance in determining the oligomer distribution pattern obtained upon reconstitution from alkaline pH to pH 7.8 of selected mixtures of subunits. An additional significant factor in the assembly process has been proposed: the operation of different relative rates of aggregation between different types of subunits. Reconstitution experiments based on these findings substantially explain the complex distribution of oligomeric forms in C. destructor hemolymph.

Insulin from a monotreme, the platypus (Ornithorhynchus anatinus), was isolated and the amino acid sequence determined. It differs from pig insulin at eleven amino acid sites, mainly on the surface of the monomer. Substitutions relative to pig insulin occur in the monomer-monomer interface, the dimer-dimer interface and the receptor binding region. The residues A5 Glu, A8 Lys and A13 Met have not been reported before in any insulin. Multiple sequence comparison studies reveal a relatively close relationship with the nearest group of relatives to the platypus, the mammals. The relationship of the platypus sequence to reptilian insulin sequences (and amphibian and avian insulin sequences in this case) is sufficiently close to support the observation that platypus has retained some ancient reptilian characteristics over the course of evolution. Model building the platypus insulin sequence on the structure of porcine insulin indicates that there may be some interesting differences.