ABSTRACT

In Saccharomyces cerevisiae the cell division cycle gene CDC25 is a fundamental element of the RAS/adenylate cyclase pathway which controls the production of cAMP, an internal signal for the onset of cell division. 1 - 5 Temperature-sensitive mutations of this gene lead to a growth arrest in the Gl phase. Several lines of evidence indicate that the CDC25 gene product, which is probably involved in monitoring environmental signals, acts upstream of the RAS proteins which regulate adenylate cyclase activity. 6 , 7 The two homologous proteins RASI and RAS2, which share considerable similarities with mammalian ras proteins, are GTPases and, as for all members of this class of proteins, their function is controlled by GTP and GDP, which respectively, induce the active and the inactive conformation. 8 - 11 Because of the tight binding of GDP and GTP, the key steps of the RAS-guanine nucleotide cycle are the dissociation of the RAS-GDP complex, which conditions the regeneration of the active complex RAS-GTP, and the hydrolysis of GTP which turns off the active state of RAS. The very low rates of the intrinsic GDP to GTP exchange and GTPase of RAS imply the existence of effectors to assure a rapid transient response to the extracellular stimuli which lead to cAMP production. 12 - 14 In S. cerevisiae the product of IRA2 (and probably also that of IRAI) corresponds functionally to mammalian GTPase-activating protein (GAP). 15 Concerning the GDP to GTP exchange of RAS, there is enough evidence that the CDC25 gene product is a regulator of this reaction. This was first suggested by the observation that dominant mutations of RAS2 increasing the level of the RAS2-GTP complex can bypass the growth arrest caused by thermosensitive cdc25 mutations. 4 , 5 , 16 The use of reconstituted systems for cAMP production in vitro has further supported the involvement of the CDC25 gene product in the nucleotide exchange of RAS proteins. 17 It has been reported that partially purified CDC25-β-galactosidase fusion products or extracts from yeast cells with overexpressed CDC25 gene can accelerate the RAS2-GDP dissociation in vitro. 18