Résumé
Pseudokinases lack essential residues for kinase activity, yet are emerging as important regulators of signal transduction networks. The pseudokinase STRAD activates the LKB1 tumour suppressor by forming a heterotrimeric complex with LKB1 and the scaffolding protein MO25. Here, we describe the structure of STRAD alpha in complex with MO25 alpha. The structure reveals an intricate web of interactions between STRAD alpha and involving the alpha C-helix of STRAD alpha, reminiscent of the mechanism by which CDK2 interacts with cyclin A. Surprisingly, STRAD alpha binds ATP and displays a closed conformation and an ordered activation loop, typical of active protein kinases. Inactivity is accounted for by nonconservative substitution of almost all essential catalytic residues. We demonstrate that binding of ATP enhances the affinity of STRAD alpha for MO25 alpha, and conversely, binding of MO25 alpha promotes interaction of STRAD alpha with ATP. Mutagenesis studies reveal that association of STRAD alpha with either ATP or MO25 alpha is essential for LKB1 activation. We conclude that ATP and MO25 alpha cooperate to maintain STRAD alpha in an ''active'' closed conformation required for LKB1 activation. It has recently been demonstrated that a mutation in human STRAD alpha that truncates a C-terminal region of the pseudokinase domain leads to the polyhydramnios, megalencephaly, symptomatic epilepsy (PMSE) syndrome. We demonstrate this mutation destabilizes STRAD alpha and prevents association with LKB1. In summary, our findings describe one of the first structures of a genuinely inactive pseudokinase. The ability of STRAD alpha to activate LKB1 is dependent on a closed "active" conformation, aided by ATP and MO25 alpha binding. Thus, the function of STRAD alpha is mediated through an active kinase conformation rather than kinase activity. It is possible that other pseudokinases exert their function through nucleotide binding and active conformations.