Acts as a calcium sensor. CBL proteins interact with CIPK serine-threonine protein kinases. Binds calcium ions. 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. 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. Negatively regulates the enzyme activity of MTN1 in the presence of calcium. 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, CIPK3, CIPK4, CIPK6, CIPK7, CIPK11, CIPK12, CIPK13, CIPK14, CIPK16, CIPK23, and MTN1. Tonoplast localization abolished by 2-bromopalmitate (2-BP) treatment. Ubiquitous. Stronger expression in roots. Expressed in root tip and root hair zone, leaf veins, vascular bundles and vasculature of sepals. Expressed early during germination and increases to a peak level when seedlings are established. The N-terminal 22 amino acids are sufficient for vacuolar membrane targeting. The internal domain (109-199) is sufficient for the interaction with MTN1. S-acylated by PAT10. No visible phenotype when grown under normal conditions; due to partial redundancy with CLB2. Tolerant to low-K(+) stress. Clb2 and cbl3 double mutants show stunted growth, reduced fertility and necrotic lesions at leaf tips. They also have a reduced vacuolar H(+)-ATPase activity, are hypersensitive to excessive metal ions and are more tolerant to low-K(+) conditions. May be due to a competing acceptor splice site. Belongs to the calcineurin regulatory subunit family.