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. Homodimer. Tonoplast localization abolished by 2-bromopalmitate (2-BP) treatment. S-acylated by PAT10. Belongs to the calcineurin regulatory subunit family.