[Simulation of Mutant P32T Homo- and Heterodimers of Human Inosine Triphosphate Pyrophosphatase hITPA].

The structure of three forms of a dimeric enzyme, human inosine triphosphate pyrophosphatase, is considered to identify the enzyme conformation changes causing the inactivation effect of a P32T mutation. Analysis of a nanosecond molecular dynamics is performed; the mean square deviations of the atoms between the wild-type and mutant homodimers, and also the heterodimer are calculated. A 3 ns modeling shows a greater displacement of atoms in mutant protomers. During molecular dynamics simulation, the strongest changes are observed in the loop between α2 and ß2 (amino acid residues 28-33, an area of the P32T mutation), the loop between ß5 and ß6, and the C-terminal amino acid residues. The loop between (α2 and ß2 has two conformations characterized by different positions of the Phe31 aromatic group. The distance between Cys33 (Cα) and Phe31 (C(z)) for wild-type and mutant protomers was -9 and 5.5 Å, respectively. These conformations were kept constant.

[Involvement of cyclin-dependent kinase CDK1/CDC28 in regulation of cell cycle].

Cyclin-dependent kinases (CDKs) are a family of enzymes essential for the progression of the cells through the cell cycle in eukaryotes. Moreover, genetic stability-maintaining processes, such as checkpoint control and DNA repair, require the phosphorylation of a wide variety of target substrates by CDK. In budding yeast Saccharomyces cerevisiae, the key role in the cell cycle progression is played by CDK1/CDC28 kinase. This enzyme is the most thoroughly investigated. In this review the involvement of CDC28 kinase in regulation of the cell cycle is discussed in the light of newly obtained data.

NET1 and HFI1 genes of yeast mediate both chromosome maintenance and mitochondrial rho(-) mutagenesis.

An increase in the mitochondrial rho(-) mutagenesis is a well-known response of yeast cells to mutations in numerous nuclear genes as well as to various kinds of stress. Despite extensive studies for several decades, the biological significance of this response is still not fully understood. The genetic approach to solving this enigma includes a study of genes that are required for the high incidence of spontaneous rho(-) mutants. We have obtained mutations of a few nuclear genes of that sort and found that mutations in certain genes, including CDC28, the central cell-cycle regulation gene, result in a decrease in spontaneous rho(-) mutability and simultaneously affect the maintenance of the yeast chromosomes and plasmids. Two more genes resembling CDC28 in this respect are identified in the present work as a result of the characterization of four new mutants. These two genes are NET1 and HFI1 which mediate important regulatory protein-protein interactions in the yeast cell. The effects of four mutations, including net1-srm and hfi1-srm, on the maintenance of the yeast mitochondrial genome, chromosomes and plasmids, as well as on the cell's sensitivity to ionizing radiation, are also described. The data presented suggest that the pleiotropic srm mutations determining coordinate changes in the fidelity of mitotic transmission of chromosomes, plasmids and mtDNA molecules identify genes that most probably operate high up in the hierarchy of the general genetic regulation of yeast.

Mutations of the CDC28 gene and the radiation sensitivity of Saccharomyces cerevisiae.

cdc28-srm, a non-temperature-sensitive (ts) mutation in the CDC28 gene of Saccharomyces cerevisiae that affects fidelity of mitotic transmission of both mitochondrial and nuclear genetic structures (Devin et al., 1990), also affected cell growth and sensitivity to lethal effects of ionizing radiation. At 30 degrees C cdc28-13, a ts mutation, was without appreciable effects on spontaneous mitochondrial rho(-)-mutagenesis, cell growth and radiation sensitivity, whereas all three cell characteristics mentioned were affected (although to a lesser degree than by cdc28-srm) by cdc28-1, another ts mutation. cdc28-srm was without any significant effect on the rates of spontaneous nuclear gene mutations and gamma-ray-induced mitotic recombination. An analysis of double mutants as regards their radiation sensitivity has revealed additive or even synergistic interactions between the cdc28-srm mutation and every one of the rad6-1 and rad52-1 mutations. The rad9 delta allele was found to be epistatic to cdc28-srm. These data suggest that the p34CDC28 protein is involved in the RAD9-dependent feedback control of DNA integrity operating at the cell cycle checkpoints.

The start gene CDC28 and the genetic stability of yeast.

The cdc28-srm mutation in Saccharomyces cerevisiae decreases spontaneous and induced mitochondrial rho- mutability and the mitotic stability of native chromosomes and recombinant circular minichromosomes. The effects of cdc28-srm on the genetic stability of cells support the hypothesis that links cell cycle regulation in yeast to changes in chromatin organization dependent on the start gene CDC28 (Hayles and Nurse, 1986).

A nuclear mutation decreasing rho- production modifies the unstable nitrous-acid sensitivity of yeast.

The nuclear mmgl mutation, which reduces rho- mutability in Saccharomyces cerevisiae, renders the rho+ cells less sensitive to inactivation by nitrous acid (NA) but has little or no effect on the NA sensitivity of the rho0 cells devoid of mitochondrial (mt) DNA. Therefore the cells' NA sensitivity seems to be influenced by an interaction of the mmgl mutation and the mt genome rather than the mmgl mutation itself. The clonal variation of NA sensitivity is high in MMG+ yeast and significantly reduced in rho0 mutants and mmgl cells. The results presented suggest that frequent spontaneous heritable changes of the mt genome occur in MMG+ cells, which, (i) unlike rho- mutations, do not damage the respiratory capacity, and (ii) manifest themselves in a high clonal variation of NA sensitivity.

Nuclear mutants of yeast with reduced spontaneous mutability of the mitochondrial genome.

Nuclear mutations in yeast were obtained that reduced rates of spontaneous mutations in the mitochondrial genome. The symbol mmg (mutability of mitochondrial genome) is used to designate these mutations. 2 types of mmg mutation are described: semidominant and recessive ones. Each of the 4 mutations studied is located in a separate mmg locus suggesting that there are probably more than 4 mmg loci in the nuclear genome of a yeast cell.