The NAF domain defines a novel protein-protein interaction module conserved in Ca2+-regulated kinases

The Arabidopsis calcineurin B-like calcium sensor proteins (AtCBLs) interac t with a group of serine-threonine protein kinases (AtCIPKs) in a calcium-d ependent manner. Here we identify a 24 amino acid domain (NAF domain) uniqu e to these kinases as being required and sufficient for interaction with al l known AtCBLs. Mutation of conserved residues either abolished or signific antly diminished the affinity of AtCIPK1 for AtCBL2. Comprehensive two-hybr id screens with various AtCBLs identified 15 CIPKs as potential targets of CBL proteins. Database analyses revealed additional kinases from Arabidopsi s and other plant species harbouring the NAF interaction module. Several of these kinases have been implicated in various signalling pathways mediatin g responses to stress, hormones and environmental cues. Full-length CIPKs s how preferential interaction with distinct CBLs in yeast and irt vitro assa ys. Our findings suggest differential interaction affinity as one of the me chanisms generating the temporal and spatial specificity of calcium signals within plant cells and that different combinations of CBL-CIPK proteins co ntribute to the complex network that connects various extracellular signals to defined cellular responses.