Expression data from wt, pop2, pbp1, and mpt5-deletion yeast cells.

The S. cerevisiae Pop2 protein is an exonuclease in the Ccr4-Not complex that is a conserved regulator of gene expression. Pop2 regulates gene expression post-transcriptionally by shortening the poly(A) tail of mRNA. A previous study has shown that Pop2 is phosphorylated at threonine 97 (T97) by Yak1 protein kinase in response to glucose limitation. However, the physiological importance of Pop2 phosphorylation remains unknown. In this study, we found that Pop2 is phosphorylated at serine 39 (S39) under unstressed conditions. The dephosphorylation of S39 was occurred within 1 min after glucose depletion, and the addition of glucose to the glucose-deprived culture recovered this phosphorylation, suggesting that Pop2 phosphorylation at S39 is regulated by glucose. We previously reported that Pop2 takes a part in the cell wall integrity pathway by regulation of LRG1 mRNA; however, S39 phosphorylation of Pop2 is not involved in LRG1 expression. On the other hand, Pop2 phosphorylation at S39 is involved in the expression of HSP12 and HSP26, encoding small heat shock proteins. In medium supplemented with glucose, Pop2 might be phosphorylated at S39 by Pho85 kinase to repress the expression of HSP12 and HSP26. Glucose starvation inactivated Pho85, which resulted in the derepression of HSP12 and HSP26. Thus, Pop2 phosphorylation at S39 is important for Pop2 to repress the expression of stress response genes, HSP12 and HSP26, in the presence of glucose. Our results suggest that Pop2 phosphorylation by Pho85 kinase is a part of the glucose sensing system in yeast. Wild type, pop2, pbp1, mpt5 cells were grown in YPD medium to mid-log phase, then shifted to YPGL medium. Total RNAs ere extracted and subjected to microarray analysis.