We have constructed the yeast strain TS1, with the RAS2 gene replaced by mutant allele encoding a partially defective gene product, and with an inactive RAS1 gene. TS1 cells accumulate as unbudded cells upon temperature shift from 30 to 37 degrees C, thus showing that the RAS1 and RAS2 gene functions are important for progression through the G1 phase of the cell cycle. After the isolation of revertants able to grow at the nonpermissive temperature, we have found that a chromosomal point mutation can bypass the G1 arrest of TS1 and cdc25 cells, and the lethality of ras1 ras2 mutants. The mutation predicts the replacement of threonine by isoleucine at position 1651 of yeast adenylate cyclase. The RAS‐independent, as well as the RAS‐dependent adenylate cyclase activity, is increased by the mutation. Like the wild‐type enzyme, the RAS‐dependent activity of the mutant adenylate cyclase is turned on by the GTP‐bound form of the RAS2 protein. The amino acid sequence surrounding the threonine 1651 shows similarity with protein kinase substrates. Possible implications for the function of adenylate cyclase are discussed.
We have previously shown that a conserved glycine at position 82 of the yeast RAS2 protein is involved in the conversion of RAS proteins from the GDP‐ to the GTP‐bound form. We have now investigated the role of glycine 82 and neighbouring amino acids of the distal switch II region in the physiological mechanism of activation of RAS. We have introduced single and double amino acid substitutions at positions 80–83 of the RAS2 gene, and we have investigated the interaction of the corresponding proteins with a yeast GDP dissociation stimulator (SDC25 C‐domain). Using purified RAS proteins, we have found that the SDC25‐stimulated conversion of RAS from the GDP‐bound inactive state to the GTP‐bound active state was severely impaired by amino acid substitutions at positions 80–81. However, the rate and the extent of conversion from the GDP‐ to the GTP‐bound form in the absence of dissociation factor was unaffected. The insensitivity of the mutated proteins to the dissociation factor in vitro was paralleled by an inhibitory effect on growth in vivo. The mutations did not significantly affect the interaction of RAS with adenylyl cyclase. These findings point to residues 80–82 as important determinants of the response of RAS to GDP dissociation factors. This suggests a molecular model for the enhancement of nucleotide release from RAS by such factors.
Cell division cycle 25 (Cdc25) proteins are highly conserved dual specificity phosphatases that regulate cyclin-dependent kinases and represent attractive drug targets for anticancer therapies. To discover more potent and diverse inhibitors of Cdc25 biological activity, virtual screening was performed by docking 2.1 million compounds into the Cdc25B active site. An initial subset of top-ranked compounds was selected and assayed, and 15 were found to have enzyme inhibition activity at micromolar concentration. Among these, four structurally diverse inhibitors with a different inhibition profile were found to inhibit human MCF-7, PC-3, and K562 cancer cell proliferation and significantly affect the cell cycle progression. A subsequent hierarchical similarity search with the most active reversible Cdc25B inhibitor found led to the identification of an additional set of 19 ligands, three of which were confirmed as Cdc25B inhibitors with IC(50) values of 7.9, 4.2, and 9.9 μM, respectively.
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