Interphotoreceptor retinoid-binding protein; serine protease family S41.
Interphotoreceptor retinoid-binding protein (IRBP) is a homolog of the S41 protease, C-terminal processing peptidase (CTPase) family. It is thought to facilitate the compartmentalization of the visual cycle that requires poorly soluble and potentially toxic retinoids to cross the aqueous subretinal space between the photoreceptors and the retinal pigment epithelium (RPE). IRBP is secreted by photoreceptors into the interphotoreceptor matrix (IPM) where it is rapidly turned over by a combination of RPE and photoreceptor endocytosis. It is the most abundant soluble protein component of the IPM, consisting of homologous modules, each repeat structure arising through the duplication (as in teleost IRBP) or quadruplication (in tetrapods) of an ancient gene, arisen in the early evolution of the vertebrate eye. IRBP has been shown to promote the release of all-trans retinol from photoreceptors and facilitates its delivery to the RPE. Conversely, IRBP can promote the release of 11-cis-retinal from the RPE, prevent its isomerization in the subretinal space, and transfer it to photoreceptors. In vivo evidence implicates IRBP as a retinoid transporter in the visual cycle, suggesting a critical role for IRBP in cone function essential for human vision. IRBP is a dominant autoimmune antigen in the eye; IRBP proteolysis analysis has proven a useful biomarker for autoimmune uveitis (AU) disorders, a major cause of blindness. This family also includes a chlamydia-secreted protein, designated chlamydia protease-like activity factor (CPAF), known to degrade host proteins, enabling Chlamydia to evade host defenses and replicate.