Structural Characterization of Maize SIRK1 Kinase Domain Reveals an Unusual Architecture of the Activation Segment.

Kinases are primary regulators of plant metabolism and excellent targets for plant breeding. However, most kinases, including the abundant receptor-like kinases (RLK), have no assigned role. SIRK1 is a leucine-rich repeat receptor-like kinase (LRR-RLK), the largest family of RLK. In , SIRK1 (SIRK1) is phosphorylated after sucrose is resupplied to sucrose-starved seedlings and it modulates the sugar response by phosphorylating several substrates. In maize, the SIRK1 expression is altered in response to drought stress. In neither Arabidopsis nor in maize has the function of SIRK1 been completely elucidated. As a first step toward the biochemical characterization of SIRK1, we obtained its recombinant kinase domain, demonstrated that it binds AMP-PNP, a non-hydrolysable ATP-analog, and solved the structure of SIRK1- AMP-PNP co-crystal. The SIRK1 crystal structure revealed a unique conformation for the activation segment. In an attempt to find inhibitors for SIRK1, we screened a focused small molecule library and identified six compounds that stabilized SIRK1 against thermal melt. ITC analysis confirmed that three of these compounds bound to SIRK1 with low micromolar affinity. Solving the 3D structure of SIRK1-AMP-PNP co-crystal provided information on the molecular mechanism of SIRK1 activity. Furthermore, the identification of small molecules that bind this kinase can serve as initial backbone for development of new potent and selective SIRK1 antagonists.