Hsp90 and phosphorylation of the Slt2(Mpk1) MAP kinase activation loop are essential for catalytic, but not non-catalytic, Slt2-mediated transcription in yeast

In yeast, the Slt2 (Mpk1) stress-activated protein kinase directs the activation of two transcription factors, Rlm1 and Swi4/Swi6, in response to cell wall stress. Rlm1 is activated through a phosphorylation by Slt2, whereas the Swi4/Swi6 activation is noncatalytic and triggered by the binding of phosphorylated forms of both Slt2 and a catalytically inactive pseudokinase (Mlp1). Previous studies have delineated a role for the molecular chaperone Hsp90 in the activation of Slt2, but the involvement of Hsp90 in these events of catalytic versus non-catalytic cell integrity signaling has remained elusive. In cells lacking Mlp1, the Hsp90 inhibitor radicicol was found to inhibit the Slt2-mediated catalytic activation of Rlm1, but not the noncatalytic activation of Swi4/Swi6. Mutation of residues in the TEY motif of the Slt2 activation loop strongly impacted both Hsp90 binding and Rlm1-mediated transcription. In contrast, many of these same mutations had only modest effects on Swi4/6 (Slt2-mediated, non-catalytic) transcription, although one that blocked both the Slt2: Hsp90 interaction and Rlm1-mediated transcription (E191G) triggered a hyperactivation of Swi4/6. Taken together, our results cement the importance of the Slt2 activation loop for both the binding of Hsp90 by Slt2 and the catalytic activation of cell integrity signaling.