Fourth module of Xenopus interphotoreceptor retinoid-binding protein: activity in retinoid transfer between the retinal pigment epithelium and rod photoreceptors

Purpose. Interphotoreceptor retinoid-binding protein (IRBP), an extracellul ar protein believed to support the exchange of retinoids between the neural retina and retinal pigment epithelium (RPE) in the vertebrate eye, exhibit s a modular, i.e., repeat, structure. The present study was undertaken to d etermine whether an individual module of IRBP has activity in retinoid tran sfer between the RPE and rod photoreceptors. Methods. The retinoid transfer activity of a recombinant protein correspond ing to the fourth module of Xenopus laevis IRBP (X4IRBP) was examined in tw o ways. First, X4IRBP was tested for its ability to support the regeneratio n of porphyropsin in detached/reattached Xenopus retina/RPE-eyecups. Follow ing illumination and removal of native IRBP, Xenopus eyecups supplemented w ith 42 mu M X4IRBP or (as a control) Ringer's solution were incubated in da rkness and then analyzed for regenerated porphyropsin. Second, toad (Bufo m arinus) RPE-eyecup preparations were used to evaluate X4IRBP's ability to p romote the release of 11-cis retinal from the RPE. Results. The regeneration of porphyropsin in X4IRBP-supplemented Xenopus re tina/RPE-eyecups (0.45 +/- 0.04 nmol; mean +/- SEM, n = 11) exceeded that i n controls (0.13 +/- 0.02 nmol, n = 11). For promoting the release of 11-ci s retinal from the toad RPE, 42 mu M X4IRBP was more effective than equimol ar bovine serum albumin although considerably less than that of 26 mu M nat ive bovine IRBP. Conclusions. The results indicate a low but significant activity of IRBP's fourth module in reactions relevant to retinoid exchange.