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JCS ePress
online publication date 23 Nov 2004
doi: 10.1242/jcs.01571
Research Article
Analysis of the LKB1-STRAD-MO25 complex
Jérôme Boudeau*,
John W. Scott,
Nicoletta Resta,
Maria Deak,
Agnieszka Kieloch,
David Komander,
D. Grahame Hardie,
Alan R. Prescott,
Daan M.F. van Aalten,
and
Dario R. Alessi
* Author for correspondence (e-mail: d.r.alessi{at}dundee.ac.uk)
Mutations in the LKB1 tumour suppressor threonine kinase cause the inherited Peutz-Jeghers cancer syndrome and are also observed in some sporadic cancers. Recent work indicates that LKB1 exerts effects on metabolism, polarity and proliferation by phosphorylating and activating protein kinases belonging to the AMPK subfamily. In vivo, LKB1 forms a complex with STRAD, an inactive pseudokinase, and MO25, an armadillo repeat scaffolding-like protein. Binding of LKB1 to STRAD-MO25 activates LKB1 and re-localises it from the nucleus to the cytoplasm. To learn more about the inherent properties of the LKB1-STRAD-MO25 complex, we first investigated the activity of 34 point mutants of LKB1 found in human cancers and their ability to interact with STRAD and MO25. Interestingly, 12 of these mutants failed to interact with STRAD-MO25. Performing mutagenesis analysis, we defined two binding sites located on opposite surfaces of MO25
, which are required for the assembly of MO25
into a complex with STRAD
and LKB1. In addition, we demonstrate that LKB1 does not require phosphorylation of its own T-loop to be activated by STRAD
-MO25
, and discuss the possibility that this unusual mechanism of regulation arises from LKB1 functioning as an upstream kinase. Finally, we establish that STRAD
, despite being catalytically inactive, is still capable of binding ATP with high affinity, but that this is not required for activation of LKB1. Taken together, our findings reinforce the functional importance of the binding of LKB1 to STRAD, and provide a greater understanding of the mechanism by which LKB1 is regulated and activated through its interaction with STRAD and MO25.

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© The Company of Biologists Ltd 2004