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. May function as a positive regulator of salt or drought responses. Homodimer (By similarity). Interacts with PP2CA, CIPK2, CIPK11, CIPK23 and CIPK24. Targeted to the cell membrane when interacting with CIPK24. Expressed in green tissues, but not in the roots. The N-terminal 12 amino acids are sufficient for cell membrane targeting of a heterologous protein. Both N-myristoylation and calcium-mediated conformational changes are essential for its function. Belongs to the calcineurin regulatory subunit family.