Localization of Carbamyl Phosphate-metabolizing Enzymes in Different Organelles within Tomato (Lycopersicon esculentum) Cells

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.

Cerium-based ultracytochemical localization of aspartate transcarbamylase activity in the cell membrane complex of Saccharomyces cerevisiae

Aspartate transcarbamylase (ATCase) activity was localized ultracytochemically in the yeast Saccharomyces cerevisiae by precipitation of its reaction product orthophosphate as cerium phosphate. We prefixed yeast cells with ice-cold 1% glutaraldehyde for 30 min which preserved 80% of ATCase activity. Cells were washed and incubated with ATCase substrates (aspartate, carbamyl phosphate) plus cerium chloride, and postfixed by osmium tetroxide. In cells from exponential batch cultures, deposits of cerium phosphate delineated simultaneously or alternatively membranes of the secretory pathway: nuclear envelope, endoplasmic reticulum, Golgi complex and the plasmalemma; mitochondrial membranes and intramitochondrial fibrous component were labelled as well. Deposits of cerium phosphate were never observed in the nucleoplasm. Cells incubated in the absence of cerium or ATCase substrates and mutant S. cerevisiae cells lacking ATCase activity served as controls. Small round electron-dense condensates were found to be randomly distributed within some cells, both in control and experimental runs, in the nucleoplasm, cytoplasm and mitochondrial matrix and represented undefined osmicated endogenous compounds. Our results suggest that the synthesis of pyrimidine precursors occurs in membranes, where compounds such as UDP-glucose and CDP-diglycerides are needed for membrane and/or yeast cell wall synthesis. The possible contribution of ATCase activity found in the nuclear envelope to nucleic acid synthesis remains to be clarified.

Endogenous polypeptide-chain length and partial sequence of aspartate transcarbamoylase from wheat, characterised by immunochemical and cDNA methods

Aspartate transcarbamoylase (ATCase) is purified from wheat germ as a monofunctional trimer of 36 kDa chains. The possibility that this may be a proteolytic fragment of a large endogenous complex in which ATCase is covalently fused to other pyrimidine-pathway enzymes (such as exists in animals or fungi) was tested. Examination of a rabbit antiserum raised against the purified enzyme confirmed the presence of anti-(wheat ATCase) antibodies by several independent methods. Extracts of wheat seedlings prepared under non-proteolysing conditions were challenged by the antiserum, and in some cases by purified anti-(36 kDa ATCase) antibodies, using immunoblotting techniques. The 36 kDa species was the dominant immunopositive polypeptide. However, the extract also contained small amounts of two larger (45 and 55 kDa) immunopositive polypeptides, as well as traces of polypeptides smaller than 36 kDa, which were assumed to be minor proteolytic products. Neither of the 45 or 55 kDa polypeptides is large enough to also incorporate a carbamoyl phosphate synthetase or dihydroorotase of the sizes found in other organisms. They may be targeted precursors of ATCase with intact leader sequences. A screen of a wheat cDNA expression library by the anti-(ATCase) serum yielded a single positive clone which was shown, by DNA sequencing, to be a concatemer of two cDNAs, one of which encoded a partial ATCase. Northern analysis using this clone as probe identified two transcripts of about 1.3 and 1.0 kbp, but showed no evidence of a transcript of 2 kbp or greater which would be required to encode a bifunctional polypeptide. These results confirm that wheat ATCase is not translationally fused to dihydroorotase or carbamoylphosphate synthetase, as it is in animals and fungi. The deduced amino-acid sequence of the partial wheat ATCase is compared with the catalytic chain of the ATCase from Escherichia coli and with other ATCases.