SAG Gene: S-Antigen Visual Arrestin – Function, Mutations, and Disease Associations

The SAG gene encodes S-antigen (visual arrestin), a 405-amino acid protein that plays a critical role in the termination of the phototransduction cascade in rod and cone photoreceptor cells. It binds to photoactivated and phosphorylated rhodopsin, quenching signal transduction and preventing overstimulation. Mutations in SAG are associated with autosomal recessive retinitis pigmentosa (RP47) and Oguchi disease, a form of congenital stationary night blindness.

Tissue Expression

Cell Line Expression

Hotspot Mutations

Mutation functional classification

• Phototransduction cascade (KEGG: hsa04744)

• Visual signal transduction (Reactome: R-HSA-2188538)

S-antigen (visual arrestin) is a 45 kDa cytosolic protein predominantly expressed in retinal photoreceptors. It terminates the phototransduction signal by binding to light-activated, phosphorylated rhodopsin, preventing further activation of transducin. The protein contains two major domains: an N-terminal domain that interacts with clathrin and a C-terminal domain that binds phospho-rhodopsin. Defects in SAG lead to prolonged phototransduction and retinal degeneration.