Acts as a calcium sensor. CBL proteins interact with CIPK serine-threonine protein kinases. Binding of a CBL protein to the regulatory NAF domain of a CIPK protein lead to the activation of the kinase in a calcium-dependent manner. Binds four calcium ions per subunit. Mediates the activation of AKT1 by CIPK proteins (CIPK6, CIPK16, and CIPK23) in response to low potassium conditions and in the context of stomatal movement. Mediates the inactivation of the proton pump AHA2 by CIPK11. Probably involved in regulating signaling responses to abscisic acid. Homodimer (By similarity). Part of a K(+)-channel calcium-sensing kinase/phosphatase complex composed by a calcium sensor CBL (CBL1, CBL2, CBL3 or CBL9), a kinase CIPK (CIPK6, CIPK16 or CIPK23), a phosphatase PP2C (AIP1) and a K(+)-channel (AKT1). Interacts with PP2CA, CIPK1, CIPK2, CIPK4, CIPK6, CIPK7, CIPK9, CIPK11, CIPK12, CIPK13, CIPK14/SR1, CIPK16, CIPK23, and CIPK24. The tonoplast localization is S-acylation dependent and is abolished by 2-bromopalmitate (2-BP) treatment. Ubiquitous. Stronger expression in aerial parts. Expressed in guard cells, meristems and elongation zones of roots, mesophyll cells of leaves, cortex and pith of inflorescence stems and anthers and stamen filaments in flowers. Expressed early during germination and increases to a peak level when seedlings are established. By light. EF-hands 1 and 4 have been shown to bind calcium. It is not known if EF-hands 2 and 3 are capable of calcium-binding. The N-terminal 22 amino acids are necessary and sufficient for vacuolar membrane targeting. S-acylated in vivo by PAT10. The ratio of acylation by either palmitate or stearate between Cys-4, Cys-12 and Cys-18 is unknown. No visible phenotype when grown under normal conditions; due to partial redundancy with CLB3. Chlorotic symptoms under low-K(+) stress. Clb2 and cbl3 double mutants show stunted growth, reduced fertility and necrotic lesions at leaf tips. They have also a reduced vacuolar H(+)-ATPase activity, are hypersensitive to excessive metal ions and are more tolerant to low-K(+) conditions. Blockage of vesicle trafficking by Brefeldin-A does not affect S-acylation and vacuolar membrane targeting of CBL2. Belongs to the calcineurin regulatory subunit family.