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Human p53 inhibits growth in Schizosaccharomyces pombe
headlines
Human p53 and CDC2Hs genes combine to inhibit the proliferation of
Saccharomycescerevisiae J. M. Nigro, R. Sikorski, S. I. Reed and B. Vogelstein Mol. Cell. Biol. 12, 1 357-1 363
Human p53 inhibits growth in Schizosaccharomycespombe J. R. Bischoff, D. Casso and D. Beach Mol. Cell. Biol. 12, 1405-1411 The tumour suppressor p53, a nuclear phosphoprotein, is involved in negative regulation of cell proliferation in several types of mammalian cells. Indeed, mutation of the gene encoding p53 is often a key step in the development of malignant tumours. Attempts to identify a p53 homoIogue in yeast have been unsuccessful so far. However, these papers now report that expression of the wildtype human p53 gene in both budding and fission yeast cells can strongly inhibit their growth rate. Mutant p53 genes from human tumours had a far weaker effect. Expression of the wild-type p53 gene
in Saccharomyces cerevisiae caused a partial growth arrest in G1, a result analogous to the effect of overexpressing p53 in mammalian cells; the phase of the cell cycle in which the Schizosaccharomyces pombe cells were arrested, however, could not be determined. In human cells, p53 is a substrate for the protein kinase p34 cDc2m, which is required for passage through the G1-S and G2-M transitions. Nigro et al. found that cotransformation of S. cerevisiae with genes encoding p53 and p34CDC2Hs enhanced the inhibitory effect of p53 on growth, but this apparently was not due to
Mutation of protein kinase A causes heterochronic development in
A conditional yeast m u t a n t deficient in mRNA transport from nucleus to cytoplasm T. Kadowaki, Y. Zhao and A. M. Tartakoff Proc. Natl Acad. Sci. USA 89, 2312-2316
Dictyostelium M-N. Simon, O. Pelegrini, M. Veron and R. R. Kay Nature 356, 1 71-1 72
The rde class of Dictyostelium mutants undergo rapid but perturbed development: spore and stalk differentiation occur before morphogenesis is complete. Furthermore, mutant cells undergo spore formation as individual ceils in the absence of other developmental processes. These authors have established that the rdeC mutant phenotype results from inactivation of the regulatory (R) subunit of cAMP-dependent protein kinase (PKA). Normally, the catalytic subunit of PKA is inactive when associated with the R subunit, and cAMP binding by the R subunit causes dissociation of the complex. in two rdeC strains the phenotype results from complete loss of R subunit expression; in a third, the R subunit is still present but has undergone an amino acid substitution. Interestingly, the substitution (Ala to Thr) lies within the R subunit
differential phosphorylation of p53. Bischoff et aL showed that in S. pombe p53 was located in the nucleus and phosphorylated on sites identified as phosphorylation targets in mammalian cells. While the action of p53 may differ in yeast and mammalian cells, it is at least clear that the human protein can be recognized as a substrate for phosphorylation by endogenous yeast enzymes. This system thus has considerable potential for elucidating the mechanism of action of p53, for example by identifying mutations in other genes that can reverse the growth inhibition by p53.
Transport of mRNA from the nucleus to the cytoplasm occurs through large protein complexes in the nuclear envelope called nuclear pore complexes, which are thought to mediate all molecular traffic between these compartments. Specific proteins associated with the mRNA are likely to play major roles in the transport process. However, due to intractable technical difficulties, few proteins involved in mRNA export have been identified. Kadowaki et al. isolated temperature-sensitive mutants of Saccharomyces cerevisiae in a two-step screen designed to identify genes encoding these elusive and long-sought proteins. They reasoned that cells unable to export mRNA at the restrictive temperature would deplete cytoplasmic mRNA stocks and eventually cease protein synthesis. These cells would thus tolerate exposure to tritiated amino acids much better than their wild-type counterparts, and could be enriched by such 'suicide selection'. From the many mutants that showed temperature-sensitive defects in protein synthesis, a second round of screening identified a few that accumulated poly(A) ÷ RNA in the nucleus, as visualized by in situ hybridization to oligo(dT) probes. The first of these, mtrl-1, displays several features expected of a genuine mRNA export mutant.
pseudosubstrate site (Arg-Arg-Val-Ala) that inhibits the catalytic activity of the C subunit. However, this change is similar to a difference (Ala to Ser) between the bovine isozymes RI and RII; it is therefore puzzling that in Dictyostefium it results in inactivity of the R subunit. The association of PKA activity
TRENDS IN CELL BIOLOGY VOL 2 JUNE 1992
with the Dictyostelium rdeC phenotype may prove useful for investigations of the control processes associated with this kinase. By isolating new mutants with similar phenotypes to that of rdeC it should be possible to clone novel genes within the cAMP second messenger pathways. 1 57
Human p53 and CDC2Hs genes combine to inhibit the proliferation of
Saccharomycescerevisiae J. M. Nigro, R. Sikorski, S. I. Reed and B. Vogelstein Mol. Cell. Biol. 12, 1 357-1 363
Human p53 inhibits growth in Schizosaccharomycespombe J. R. Bischoff, D. Casso and D. Beach Mol. Cell. Biol. 12, 1405-1411 The tumour suppressor p53, a nuclear phosphoprotein, is involved in negative regulation of cell proliferation in several types of mammalian cells. Indeed, mutation of the gene encoding p53 is often a key step in the development of malignant tumours. Attempts to identify a p53 homoIogue in yeast have been unsuccessful so far. However, these papers now report that expression of the wildtype human p53 gene in both budding and fission yeast cells can strongly inhibit their growth rate. Mutant p53 genes from human tumours had a far weaker effect. Expression of the wild-type p53 gene
in Saccharomyces cerevisiae caused a partial growth arrest in G1, a result analogous to the effect of overexpressing p53 in mammalian cells; the phase of the cell cycle in which the Schizosaccharomyces pombe cells were arrested, however, could not be determined. In human cells, p53 is a substrate for the protein kinase p34 cDc2m, which is required for passage through the G1-S and G2-M transitions. Nigro et al. found that cotransformation of S. cerevisiae with genes encoding p53 and p34CDC2Hs enhanced the inhibitory effect of p53 on growth, but this apparently was not due to
Mutation of protein kinase A causes heterochronic development in
A conditional yeast m u t a n t deficient in mRNA transport from nucleus to cytoplasm T. Kadowaki, Y. Zhao and A. M. Tartakoff Proc. Natl Acad. Sci. USA 89, 2312-2316
Dictyostelium M-N. Simon, O. Pelegrini, M. Veron and R. R. Kay Nature 356, 1 71-1 72
The rde class of Dictyostelium mutants undergo rapid but perturbed development: spore and stalk differentiation occur before morphogenesis is complete. Furthermore, mutant cells undergo spore formation as individual ceils in the absence of other developmental processes. These authors have established that the rdeC mutant phenotype results from inactivation of the regulatory (R) subunit of cAMP-dependent protein kinase (PKA). Normally, the catalytic subunit of PKA is inactive when associated with the R subunit, and cAMP binding by the R subunit causes dissociation of the complex. in two rdeC strains the phenotype results from complete loss of R subunit expression; in a third, the R subunit is still present but has undergone an amino acid substitution. Interestingly, the substitution (Ala to Thr) lies within the R subunit
differential phosphorylation of p53. Bischoff et aL showed that in S. pombe p53 was located in the nucleus and phosphorylated on sites identified as phosphorylation targets in mammalian cells. While the action of p53 may differ in yeast and mammalian cells, it is at least clear that the human protein can be recognized as a substrate for phosphorylation by endogenous yeast enzymes. This system thus has considerable potential for elucidating the mechanism of action of p53, for example by identifying mutations in other genes that can reverse the growth inhibition by p53.
Transport of mRNA from the nucleus to the cytoplasm occurs through large protein complexes in the nuclear envelope called nuclear pore complexes, which are thought to mediate all molecular traffic between these compartments. Specific proteins associated with the mRNA are likely to play major roles in the transport process. However, due to intractable technical difficulties, few proteins involved in mRNA export have been identified. Kadowaki et al. isolated temperature-sensitive mutants of Saccharomyces cerevisiae in a two-step screen designed to identify genes encoding these elusive and long-sought proteins. They reasoned that cells unable to export mRNA at the restrictive temperature would deplete cytoplasmic mRNA stocks and eventually cease protein synthesis. These cells would thus tolerate exposure to tritiated amino acids much better than their wild-type counterparts, and could be enriched by such 'suicide selection'. From the many mutants that showed temperature-sensitive defects in protein synthesis, a second round of screening identified a few that accumulated poly(A) ÷ RNA in the nucleus, as visualized by in situ hybridization to oligo(dT) probes. The first of these, mtrl-1, displays several features expected of a genuine mRNA export mutant.
pseudosubstrate site (Arg-Arg-Val-Ala) that inhibits the catalytic activity of the C subunit. However, this change is similar to a difference (Ala to Ser) between the bovine isozymes RI and RII; it is therefore puzzling that in Dictyostefium it results in inactivity of the R subunit. The association of PKA activity
TRENDS IN CELL BIOLOGY VOL 2 JUNE 1992
with the Dictyostelium rdeC phenotype may prove useful for investigations of the control processes associated with this kinase. By isolating new mutants with similar phenotypes to that of rdeC it should be possible to clone novel genes within the cAMP second messenger pathways. 1 57