Life-cycle-dependent changes of aspartate carbamoyltransferase localization in membranes of Saccharomyces cerevisiae--centrifugal elutriation and ultracytochemical study

Enzymatic activities of Ura2 and Ura1 proteins (aspartate carbamoyltransferase and dihydro-orotate dehydrogenase) are present in both isolated membranes and cytoplasm of Saccharomyces cerevisiae

Computational analysis predicted three potential hydrophobic transmembrane alpha-helices within the Ura2 multidomain protein of Saccharomyces cerevisiae, the C-terminal subdomain of which catalyses the second step of uridine-monophosphate biosynthesis by its L-aspartate carbamoyltransferase activity (EC 2.1.3.2). The fourth step of pyrimidine biosynthesis is catalysed by dihydro-orotate dehydrogenase (Ura1 protein; EC 1.3.99.11), which was similarly characterized as a peripheral membrane protein. Ex situ, the activities of the investigated enzymes were associated both with isolated yeast membranes, fractionated by differential centrifugation to remove intact nuclei, and with soluble cytoplasmic proteins.

Immuno-electron localization of DNA in chondriolites of Saccharomyces cerevisiae mitochondria

Under electron microscope, the matrix of sectioned mitochondria exhibits ribosomes and an oval, electron-transparent zone which is devoid of ribosomes and is named chondriolite. Fine fibers or clumps of an electron-dense material appeared in this zone after several fixation and contrasting steps and were identified with mitochondrial DNA by cytologists. To verify this assumption, we labeled DNA by a monoclonal antibody and a secondary antibody coupled to immunogold. The label was observed in the nucleus and in the chondriolite zone of sectioned mitochondria. Because the ultrastructure of chondriolites resembles that of nucleoids of prokaryotes, we suggest the term mitochondrial nucleoid for the zone of mitochondrial matrix devoid of ribosomes and containing DNA.

The yeast Ura2 protein that catalyses the first two steps of pyrimidines biosynthesis accumulates not in the nucleus but in the cytoplasm, as shown by immunocytochemistry and Ura2-green fluorescent protein mapping

The Ura2 multidomain protein catalyses the first two steps of pyrimidines biosynthesis in Saccharomyces cerevisiae. It consists of a 240 kDa polypeptide which contains carbamyl phosphate synthetase and aspartate transcarbamylase domains. The Ura2 protein was believed to be nucleoplasmic, since one of the aspartate transcarbamylase reaction products, monophosphate, was reported to be precipitated by lead ions inside nuclei. However, this ultracytochemical approach was recently shown to give artifactual lead polyphosphate precipitates, and the use of cerium instead of lead failed to reveal this nucleoplasmic localization. Ura2 localization has therefore been undertaken by means of three alternative approaches based on the detection of the protein itself: (a) indirect immunofluorescence of yeast protoplasts; (b) immunogold labelling of ultrathin sections of embedded yeast cells (both approaches using affinity purified primary antibodies directed against the 240 kDa Ura2 polypeptide chain, or against a 22 residue peptide specific of the carbamyl phosphate synthetase domain); and (c) direct fluorescence of cells expressing an Ura2-green fluorescent protein hybrid. All three approaches localize the bulk of Ura2 to the cytoplasm, whereas the signals associated with the nucleus, mitochondria or vacuoles are close to or at the background level.