Expression of p60v-src in Saccharomyces cerevisiae results in elevation of p34CDC28 kinase activity and release of the dependence of DNA replication on mitosis

Single plasmids expressing human steroid hormone receptors and a reporter gene for use in yeast signaling assays

Single plasmids designed to express the six human type I steroid hormone receptors and detect signaling activity are described in this report. These stably replicating plasmids reported ligand-induced transcriptional activation via lacZ assays in Baker's yeast (Saccharomyces cerevisiae). The ligand concentrations needed to activate signaling in yeast expressing these plasmids spanned five orders of magnitude as based on comparisons of EC(50) values. Radicicol, a direct inhibitor of heat shock protein 90 (Hsp90) and an indirect inhibitor of steroid hormone receptor signaling, was used to determine the functional utility of this yeast reporter system. The inhibitory effect of radicicol was similar on the signaling of all six steroid hormone receptors and was distinguishable from cytotoxic effects that occurred with higher concentrations. These yeast plasmids provide a high throughput system for comparative assessment of steroid hormone receptor signaling and may be useful in screening for pharmacological or xenobiotic activities.

Aberrant protein phosphorylation at tyrosine is responsible for the growth-inhibitory action of pp60v-src expressed in the yeast Saccharomyces cerevisiae

Expression of pp60v-src, the transforming protein of Rous sarcoma virus, arrests the growth of the yeast Saccharomyces cerevisiae. To determine the basis of this growth arrest, yeast strains were constructed that expressed either wild-type v-src or various mutant v-src genes under the control of the galactose-inducible, glucose repressible GAL1 promoter. When shifted to galactose medium, cells expressing wild-type v-src ceased growth immediately and lost viability, whereas cells expressing a catalytically inactive mutant (K295M) continued to grow normally, indicating that the kinase activity of pp60v-src is required for its growth inhibitory effect. Mutants of v-src altered in the SH2/SH3 domain (XD4, XD6, SPX1, and SHX13) and a mutant lacking a functional N-terminal myristoylation signal (MM4) caused only a partial inhibition of growth, indicating that complete growth inhibition requires either targeting of the active kinase or binding of the kinase to phosphorylated substrates, or both. Cells arrested by v-src expression displayed aberrant microtubule structures, alterations in DNA content and elevated p34CDC28 kinase activity. Immunoblotting with antiphosphotyrosine antibody showed that many yeast proteins, including the p34CDC28 kinase, became phosphorylated at tyrosine in cells expressing v-src. Both the growth inhibition and the tyrosine-specific protein phosphorylation observed following v-src expression were reversed by co-expression of a mammalian phosphotyrosine-specific phosphoprotein phosphatase (PTP1B). However a v-src mutant with a small insertion in the catalytic domain (SRX5) had the same lethal effect as wild-type v-src, yet induced only very low levels of protein-tyrosine phosphorylation. These results indicate that inappropriate phosphorylation at tyrosine is the primary cause of the lethal effect of pp60v-src expression but suggest that only a limited subset of the phosphorylated proteins are involved in this effect.