CIPK serine-threonine protein kinases interact with CBL proteins. Binding of a CBL protein to the regulatory NAF domain of CIPK protein lead to the activation of the kinase in a calcium-dependent manner. Involved in K(+) homeostasis under low-K(+) stress. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interacts with CBL2 and CBL3. Targeted to the tonoplast when interacting with CBL2 or CBL3. Expressed at low levels in roots and shoots. Detected in root vascular bundles and in the leaf vascular tissue and hydathode, but not in root tips. The activation loop within the kinase domain is the target of phosphorylation/activation by upstream protein kinases. The PPI motif mediates the interaction with the ABI (abscisic acid-insensitive) phosphatases (By similarity). May be due to a competing donor splice site. May be due to a competing donor splice site. May be due to a competing donor splice site. Belongs to the protein kinase superfamily. CAMK Ser/Thr protein kinase family. SNF1 subfamily.