Yeast Mpk1 Mitogen-Activated Protein Kinase Activates Transcription through Swi4/Swi6 by a Noncatalytic Mechanism That Requires Upstream Signal

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Abstract
The cell wall integrity mitogen-activated protein kinase (MAPK) cascade of Saccharomyces cerevisiae drives changes in gene expression in response to cell wall stress. We show that the MAPK of this pathway (Mpk1) and its pseudokinase paralog (Mlp1) use a noncatalytic mechanism to activate transcription of the FKS2 gene. Transcriptional activation of FKS2 was dependent on the Swi4/Swi6 (SBF) transcription factor and on an activating signal to Mpk1 but not on protein kinase activity. Activated (phosphorylated) Mpk1 and Mlp1 were detected in a complex with Swi4 and Swi6 at the FKS2 promoter. Mpk1 association with Swi4 in vivo required phosphorylation of Mpk1. Promoter association of Mpk1 and the Swi4 DNA-binding subunit of SBF were codependent but did not require Swi6, indicating that the MAPK confers DNA-binding ability to Swi4. Based on these data, we propose a model in which phosphorylated Mpk1 or Mlp1 forms a dimeric complex with Swi4 that is competent to associate with the FKS2 promoter. This complex then recruits Swi6 to activate transcription. Finally, we show that human ERK5, a functional ortholog of Mpk1, is similarly capable of driving FKS2 expression in the absence of protein kinase activity, suggesting that this mammalian MAPK may also have a noncatalytic function in vivo.