CRALBP

CRALBP, or cellular retinaldehyde-binding protein, is a soluble retinal protein expressed primarily in the retinal pigment epithelium and Müller glial cells. It is encoded by the RLBP1 gene and is also known as RLBP1. The protein binds 11-cis-retinal and 11-cis-retinol with high affinity, helping to store and stabilize these retinoids in retinal cells as part of the visual cycle.

Functionally, CRALBP participates in the vertebrate visual cycle by shuttling 11-cis-retinoids between photoreceptors and supporting retinal cells, aiding in the regeneration of the visual pigment after light exposure. In Müller cells, it contributes to a cone-specific visual cycle, supplying 11-cis-retinal to cones. CRALBP cooperates with key enzymes of the cycle, such as RPE65 and various retinol dehydrogenases, to maintain a steady supply of 11-cis-retinal for phototransduction.

Structurally, CRALBP belongs to the CRAL-TRIO superfamily and features a ligand-binding pocket well suited for hydrophobic retinoids. The protein functions as a soluble cytosolic binder within retinal cells, enabling efficient presentation and transfer of retinoids in the visual cycle.

Clinical significance, mutations in RLBP1 are associated with autosomal recessive inherited retinal dystrophies, including retinitis pigmentosa and macular dystrophy. Affected individuals typically present with night blindness in childhood and progressive vision loss, with possible macular involvement and, in some cases, cystoid macular edema. Animal models lacking functional CRALBP show disrupted retinoid cycling and impaired visual pigment regeneration.