Regulation of the yeast Rlm1 transcription factor by the Mpk1 cell wall integrity MAP kinase

The Mpk1 MAP kinase of the Saccharomyces cerevisiae cell wall integrity signalling pathway phosphorylates and activates the Rlm1 transcription factor in response to cell wall stress. Rlm1 is related to mammalian MEF2 isoforms, and shares a similar DNA-binding specificity. Signalling through Rlm1 regulates the expression of at least 25 genes, most of which have been implicated in cell wall biogenesis. We report here the transcriptional induction by agents of cell wall stress of a set of lacZ reporter plasmids derived from several Rlm1-responsive genes. Analysis of substitution mutations at putative Mpk1 phosphorylation sites within Rlm1 revealed that Ser427 and Thr439 are important for its stress-induced transcriptional activation of these reporter plasmids. Assessment of Rlm1 activation potency when fused to a heterologous DNA-binding domain showed that the identified seryl and threonyl residues are necessary for the Rlm1 transcriptional activation function independently of its DNA binding. We also demonstrate that a MAP kinase docking site, shown recently to mediate activation of MEF2A and MEF2C, is conserved in Rlm1 and is required for its ability to mediate transcriptional activation in response to agents that induce cell wall stress. Finally, intracellular localization analyses show that Rlm1 resides in the nucleus regardless of its activation and phosphorylation status. Together these observations support the inference that Mpk1 regulates the Rlm1 transcriptional activation function by phosphorylation of Ser427 and Thr439.