May play a role in an as yet undefined retina-specific signal transduction. Could bind to photoactivated-phosphorylated red/green opsins. Homodimer; disulfide-linked in response to retinal illumination (By similarity). Interacts with CXCR4; the interaction is dependent on the C-terminal phosphorylation of CXCR4 and modulates the calcium ion mobilization activity of CXCR4 (By similarity). Retina and pineal gland. Belongs to the arrestin family.