Fig. 4. act1-157, a yeast strain whose actin is resistant to latrunculin, shows no inhibition of Gln3 nuclear accumulation when cells are transferred from a good nitrogen source to a poor one after latrunculin treatment.

Yeast strains containing either wild type (WT) ACT1 (DDY 1495) (54) or the act1-157 mutation (DDY 1544) (53) were grown to mid-log in YNB-glutamine and sampled for microscopic examination (0 min, panel A). The cultures were then preincubated with latrunculin B as described in Fig. 2 for 30 min. Aliquots were then removed just before transfer from glutamine to proline medium (0 min, EtOH or Lat in panel A; 0 min, −LAT and +LAT in panel B) and 60 min after this transfer and processed for indirect immunofluorescence. Panel A, the percent of cells in which Gln3 is localized to the nucleus. Panel B, representative wild type and mutant cells treated with either 150 μM latrunculin or the carrier and stained with phalloidin to permit evaluation of latrunculin treatment and the act1-157 phenotype. The genetic background containing the act1-157 mutation is somewhat less latrunculin-sensitive than the one we used for the rest of our experiments. We did not, however, increase the latrunculin concentration in this control experiment because we preferred to maintain a uniform latrunculin concentration throughout all of the experiments reported.