Pseudohyphal differentiation defect due to mutations in GPCR and ammonium signaling is suppressed by low glucose concentration: a possible integrated role for carbon and nitrogen limitation

In response to carbon and/or nitrogen limitation, diploid cells of Saccharomyces cerevisiae either sporulate or develop pseudohyphae. Although the signal transduction pathways leading to these developmental changes have been extensively studied, how nutritional signals are integrated is not clearly understood. Results of this study indicate that reducing glucose concentration from 2% (SLAD) to 0.05% (SLALD) causes an increase in the magnitude of filamentation as well as a discernible reduction in the time required for pseudohyphal development. Further, the pseudohyphal defect of gpa2, gpr1and gpa2gpr1 but not the mep2 mutant strain is overcome on SLALD. Low glucose also induced pseudohyphae in mep2gpr1 but not mep2gpa2 strain suggesting that GPR1 inhibits pseudohyphae by inhibiting GPA2 function. Accordingly, deleting GPA2 in mep2gpr1 mutant abrogated pseudohyphae formation in SLALD. Further, replenishment of glucose suppressed pseudohyphal differentiation in wild-type cells grown in SLAD medium. However, in SLALD, glucose replenishment suppressed the filamentation response of gpa2 mutants but not that of strains carrying the wild-type GPA2. Increased trehalose levels correlated with decreased pseudohyphae formation. Results of this study demonstrate that filamentation in response to nitrogen limitation occurs as glucose becomes limiting.