Soluble retinoid carrier essential the proper function of both rod and cone photoreceptors. Participates in the regeneration of active 11-cis-retinol and 11-cis-retinaldehyde, from the inactive 11-trans products of the rhodopsin photocycle and in the de novo synthesis of these retinoids from 11-trans metabolic precursors. The cycling of retinoids between photoreceptor and adjacent pigment epithelium cells is known as the 'visual cycle'. Interacts with DEGS1; the interaction increases synthesis of chromophore-precursors by DEGS1. Retina and pineal gland. Expressed in Mueller cells (at protein level) (PubMed:23143414).