Editors' ChoiceStructural Biology

Kinase Conformation, Not Activity, Required

Science Signaling  16 Jun 2009:
Vol. 2, Issue 75, pp. ec199
DOI: 10.1126/scisignal.275ec199

The kinase LKB1, which is a tumor suppressor and activates AMP-activated protein kinase (AMPK), is activated by interacting with two additional proteins, STRAD (Ste-related adaptor) and MO25. STRAD (one of two isoforms) is a pseudokinase, meaning that critical conserved residues required for catalytic activity are missing, and MO25 (one of two isoforms) is a scaffolding protein. Zeqiraj et al. characterized the crystal structure of STRADα (the pseudokinase domain, residues 59 to 431) and full-length MO25α. In the structure, STRADα adopted a conformation similar to that of an active kinase and bound an ATP molecule. In contrast to catalytically active kinases, ATP binding did not require coordination with Mg2+; instead, STRADα appeared to position the ATP through interactions with specific amino acids (Arg215 and His200 for the β-phosphate and Lys197 for the γ-phosphate). In addition to the known interaction between the STRADα WEF motif and MO25α, the structure revealed extensive interactions between MO25α on its concave surface and STRADα; the importance of these interactions for formation of a stable complex was confirmed by mutational analysis. Comparison of the structure of STRADα when bound to MO25α resembled that of CDK1 (cyclin-dependent kinase 1) bound to cyclin A and a complex between an active epidermal growth factor receptor (EGFR) and an EGFR in the inactive conformation, with the conformation characterized as “closed and active.” Various biochemical assays demonstrated that MO25α increases the affinity of STRADα for ATP and that ATP increases the affinity of STRADα for MO25α. Interactions with LKB1 permit MO25α and STRADα to form a heterotrimeric complex even when the interaction between MO25α and STRADα is disrupted by mutation. However, STRADα mutants that could not bind either MO25α or ATP failed to activate LKB1 even though they could form the heterotrimeric complex. Thus, ATP and MO25α appear to allosterically modulate STRADα, allowing it to adopt the closed and active conformation that is necessary for LKB1 activation. A mutation that produces a truncated STRADα is associated with polyhydramnios, megalencephaly, symptomatic epilepsy syndrome, and this would be expected to produce a functionally inactive form of STRADα due to loss of the C-terminal domain of the pseudokinase domain. Expression of this truncated form in cultured cells showed that the protein was less stable than full-length STRADα and failed to interact with or activate LKB1.

E. Zeqiraj, B. M. Filippi, S. Goldie, I. Navratilova, J. Boudeau, M. Deak, D. R. Alessi, D. M. F. van Aalten, ATP and MO25α regulate the conformational state of the STRADα pseudokinase and activation of the LKB1 tumour suppressor. PLoS Biol. 7, e1000126 (2009). [PubMed]