Modulates calcium-dependent activity of inositol 1,4,5-triphosphate receptors (ITPRs). Inhibits agonist-induced intracellular calcium signaling. Enhances inactivation and does not support calcium-dependent facilitation of voltage-dependent P/Q-type calcium channels. Causes calcium-dependent facilitation and inhibits inactivation of L-type calcium channels by binding to the same sites as calmodulin in the C-terminal domain of CACNA1C, but has an opposite effect on channel function. Suppresses the calcium-dependent inactivation of CACNA1D. Inhibits TRPC5 channels. Prevents NMDA receptor-induced cellular degeneration. Required for the normal transfer of light signals through the retina. Homodimer. Interacts (via C-terminus) with ITPR1, ITPR2 and ITPR3. This binding is calcium dependent and the interaction correlates with calcium concentration. An additional calcium-independent interaction with the N-terminus of ITPR1 results in a decreased InsP(3) binding to the receptor (By similarity). Interacts with CACNA1A (via C-terminal CDB motif) in the pre- and postsynaptic membranes (By similarity). Interacts with CACNA1C (via C-terminal C and IQ motifs). Interacts with CACNA1D (By similarity). The binding to the C motif is calcium independent whereas the binding to IQ requires the presence of calcium and is mutually exclusive with calmodulin binding (By similarity). Interacts with TRPC5 (via C-terminus) (By similarity). Interacts (via EF-hands 1 and 2) at microtubules with MAP1LC3B (By similarity). Interacts with MYO1C (By similarity). Interacts (via EF-hands 1 and 2) with NSMF (via the central NLS-containing motif region), the interaction occurs in a calcium dependent manner after synaptic NMDA receptor stimulation and prevents nuclear import of NSMF. Interacts with SPACA9 (By similarity). EF-1 binds magnesium constitutively under physiological conditions, EF-3 and EF-4 bind calcium cooperatively and EF-2 binds neither calcium nor magnesium. Phosphorylated. The phosphorylation regulates the activity (By similarity).