Plays important roles in both rhabdomere development and in photoreceptor cell survival. Might function as a calcium-sequestering 'sponge' to regulate the amount of free cytoplasmic calcium. It binds 0.3 mole of Ca(2+) per mole of protein. Homodimer. Hypodense compartment. Soma and axons of photoreceptor cells of compound eyes and ocelli. Expressed early in photoreceptor cell development. Partially edited. Target of Adar.