MCB Accepts, published online ahead of print on 5 October 2009

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Mol. Cell. Biol. doi:10.1128/MCB.00794-09
Copyright (c) 2009, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Mechanism of Mpk1 MAPK binding to the Swi4 transcription factor and its regulation by a novel caffeine-induced phosphorylation

Andrew W. Truman, Ki-Young Kim, and David E. Levin^*

Department of Biochemistry and Molecular Biology, Johns Hopkins Bloomberg School of Public Health, Baltimore, Maryland 21205; Department of Molecular and Cell Biology, Goldman School of Dental Medicine, Boston University, Boston, Massachusetts 02118

^* To whom correspondence should be addressed. Email: delevin{at}bu.edu.

The Mpk1 MAPK of the cell wall integrity signaling pathway uses a non-catalytic mechanism to activate the SBF (Swi4/Swi6) transcription factor. Active Mpk1 forms a complex with Swi4, the DNA-binding subunit of SBF, conferring the ability to bind DNA. Because SBF activation is independent of Mpk1 catalytic activity, but requires Mpk1 to be in an active conformation, we sought to understand how Mpk1 interacts with Swi4. Mutational analysis revealed that binding and activation of Swi4 by Mpk1 requires an intact D motif-binding site, a docking surface common to MAPKs that resides distal to the phosphorylation loop, but does not require the substrate-binding site, revealing a novel mechanism for MAPK target regulation. Additionally, we found that Mpk1 binds near the autoinhibitory C-terminus of Swi4, suggesting an activation mechanism in which Mpk1 substitutes for Swi6 in promoting Swi4 DNA binding. Finally, we show that caffeine is an atypical activator of CWI signaling, because it induces phosphorylation of the Mpk1 C-terminal extension at Ser423 and Ser428. These phosphorylations were dependent on the DNA damage checkpoint kinases, Mec1/Tel1 and Rad53. Phosphorylation of Ser423 specifically blocked SBF activation by preventing Mpk1 association with Swi4, revealing a novel mechanism for regulating MAPK target specificity.