Posttranslational phosphorylation and ubiquitination of the Saccharomyces cerevisiae Poly(A) polymerase at the S/G(2) stage of the cell cycle.

The poly(A) polymerase of the budding yeast Saccharomyces cerevisiae (Pap1) is a 64-kDa protein essential for the maturation of mRNA. We have found that a modified Pap1 of 90 kDa transiently appears in cells after release from alpha-factor-induced G(1) arrest or from a hydroxyurea-induced S-phase arrest. While a small amount of modification occurs in hydroxyurea-arrested cells, fluorescence-activated cell sorting analysis and microscopic examination of bud formation indicate that the majority of modified enzyme is found at late S/G(2) and disappears by the time cells have reached M phase. The reduction of the 90-kDa product upon phosphatase treatment indicates that the altered mobility is due to phosphorylation. A preparation containing primarily the phosphorylated Pap1 has no poly(A) addition activity, but this activity is restored by phosphatase treatment. A portion of Pap1 is also polyubiquitinated concurrent with phosphorylation. However, the bulk of the 64-kDa Pap1 is a stable protein with a half-life of 14 h. The timing, nature, and extent of Pap1 modification in comparison to the mitotic phosphorylation of mammalian poly(A) polymerase suggest an intriguing difference in the cell cycle regulation of this enzyme in yeast and mammalian systems.

The Uba2 and Ufd1 proteins of Saccharomyces cerevisiae interact with poly(A) polymerase and affect the polyadenylation activity of cell extracts.

Poly(A) polymerase is responsible for the addition of the adenylate tail to the 3' ends of mRNA. Using the two-hybrid system we have identified two proteins which interact specifically with the Saccharomyces cerevisiae poly(A) polymerase, Pap1. Uba2 is a homolog of ubiquitin-activating (E1) enzymes and Ufd1 is a protein whose function is probably also linked to the ubiquitin-mediated protein degradation pathway. These two proteins interact with Pap1 and with each other, but not with eight other target proteins which were tested in the two-hybrid system. The last 115 amino acids of Uba2, which contains an 82-amino acid region not present in previously characterized E1 enzymes, is sufficient for the interaction with Pap1. Both Uba2 and Ufd1 can be co-immunoprecipitated from extracts with Pap1, confirming in vitro the interaction identified by two-hybrid analysis. Depletion of Uba2 from cells produces extracts which polyadenylate precursor RNA with increased efficiency compared to extracts from nondepleted cells, while depletion of Ufd1 yields extracts which are defective in processing. These two proteins are not components of polyadenylation factors, and instead may have a role in regulating poly(A) polymerase activity.

Peripheral metabolism of [35S]parathyroid hormone in vivo: influence of alterations in calcium availability and parathyroid status.

Parathyroid hormone (PTH) is rapidly metabolized, mainly by liver and kidney, to smaller fragments that are believed to be biologically inactive. The significance of this peripheral metabolism in the overall actions of PTH is unclear. Generation of circulating biologically active amino-terminal PTH fragments during metabolism in vivo has been suggested by certain observations in vitro, and what are believed to be amino-terminal fragments may be detectable in blood under pathological circumstances in vivo (such as renal failure and coexistent hyperparathyroidism) when highly sensitive assays are employed. We recently reported, however, that administration to normal rats of [35S]bovine PTH ([35S]bPTH) directly labelled at amino-terminal methionines, followed by high-resolution chromatographic analysis of extracted [35S]peptides, does not result in appearance of radioactive amino-terminal fragments in blood, even when the tracer is continuously infused to near-physiological plasma concentrations. We have now employed these techniques to address a second question regarding hormonal metabolism: is hormonal metabolism modified during metabolic perturbations such as changing calcium availability or altered levels of calciotrophic hormones? Metabolism of [35S-Met]bPTH (900 Ci/mmol), either alone or together with [3H-Pro]bPTH, however, did not lead to alterations in the rate of hormonal clearance nor to detectable circulating amino-terminal fragments, either in calcium-deprived or thyroparathyroidectomized rats or when animals were first rendered intoxicated with vitamin D or maintained on a high calcium intake. Likewise, tissue localization and specific cleavage patterns of intact hormone in liver or kidney were all unaltered by these various manoeuvres.(ABSTRACT TRUNCATED AT 250 WORDS)

Peripheral metabolism of PTH: fate of biologically active amino terminus in vivo.

Clearance of intact parathyroid hormone (PTH) from blood is associated with rapid uptake by liver and kidney, limited proteolysis by tissue endopeptidases and, within minutes, appearance of circulating carboxyl-(COOH)-terminal PTH fragments. The fate of the corresponding amino(NH2)-terminal portion of the hormone during this peripheral metabolism is still unknown, however. To determine this, we have employed [35S]bovine PTH (bPTH) labeled to high specific activity at NH2-terminal methionines, which permits direct monitoring of the fate of the PTH NH2-terminus during metabolism in vivo. The [35S]PTH was administered by bolus or continuous intravenous infusion to anesthetized normal rats, to rats subjected to acute ablation of the liver, the kidneys, or both, and to rats receiving co-infusions of excess synthetic bPTH(1-34) NH2-terminal fragments. Analysis by high-resolution chromatographic techniques sensitive to 10(-13) M [35S]PTH peptides in plasma yields no evidence that peripheral metabolism of PTH generates circulating NH2-terminal fragments, even when special measures are taken to block clearance of such putative fragments from blood. We find that the NH2-terminus of PTH is rapidly degraded in situ by the liver but that both liver and especially kidney nevertheless contain low levels of NH2-terminal PTH fragments that, although not released into the blood, are large enough to be potentially active. Thus, the peripheral metabolism of PTH in normal animals does not normally lead to the formation of circulating amino terminal fragments of the hormone that might act independently of intact PTH on peripheral target tissues.

Rapid growth of human cancer cells in a mouse model with fibrin clot subrenal capsule assay.

Rapid in vivo growth of cultured human cancer or leukemia cells was achieved by implantation into the subrenal capsule of mice. A solid structure, necessary for accurate implantation and measurement of tumor growth in this model, was provided by stepwise addition of fibrinogen and thrombin to the tumor cells, leading to rapid enzymatic formation of a solid tumor-fibrin matrix. Human leukemia and epithelial cancers increased in volume between 6- and 40-fold when measured 6-10 days after implantation into normal or immunosuppressed mice. Immunosuppression of host CD-1 mice was achieved by cyclosporine given daily after tumor implantation, cyclophosphamide given preimplantation combined with cyclosporine, or whole-body irradiation given preimplantation. Confirming the validity of tumor measurements, tumor histology in the immunosuppressed mice revealed cell proliferation, invasion, and neovascularization. Similarly, no artifactual measurement of tumor growth was observed by nonviable cancer cells, implanted after in vitro exposure to a known cytotoxic concentration of thiotepa. This model provides an economical, short-term technique for the in vivo study of human tumor growth, for the evaluation of new cancer therapies, and for in vitro - in vivo drug activity correlations in specific types of human cancer or leukemia cell lines.