The N-terminal half of Cdc25 is essential for processing glucose signalingin Saccharomyces cerevisiae

Saccharomyces cerevisiae Cdc25 is the prototype Ras GDP/GTP exchange protei n. Its C-terminal catalytic domain was found to be highly conserved in the homologues p140(ras-GRF) and Sos. The regulatory domains in each Ras exchan ger mediate the signals arriving from upstream elements such as tyrosine ki nases for Sos, or Ca2+ and G proteins for p140.(Ras-GRF). In this study, we show that the N-terminal half (NTH) of S. cerevisiae Cdc25, as well as the C-terminal 37 amino acids, is essential for processing the elevation of cA MP in response to glucose. The mammalian p140(ras-GRF) catalytic domain (CG RF)restores glucose signaling in S. cerevisiae only if tethered between the N-terminal half (NTI-I) of S. cerevisiae Cdc25 and the C-terminal 37 amino acids. The glucose-induced transient elevation in cAMP is nullified or sev erely hampered by the deletion of domains within the NTH of Cdc25. These de letions, however, do not modify the intrinsic GDP/GTP exchange activity of mutant proteins as compared to native Cdc25. We also show that 7 Ser to Ala mutations at the cAMP-dependent protein kinase putative phosphorylation si tes within the NTH of Cdc25 eliminate the descending portion of the glucose response curve, responsible for signal termination. These findings support a dual role of the NTH of Cdc25 in both enabling the glucose signal and be ing responsible for its attenuation.