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 (By similarity). Homodimer. Expressed at low levels in roots, shoots, culms, leaves and young spikelets. By salt, drought and cold stresses, and abscisic acid (ABA). Belongs to the calcineurin regulatory subunit family.