Between the pigment epithelium and the outer limiting membrane of the retina is an extracellular compartment filled with the interphotoreceptor matrix (IPM). A prominent component of the IPM is a glycoprotein known as interstitial retinol-binding protein (IRBP). Using in vitro techniques, we compared the ability of the cells that border this compartment to internalize colloidal gold (CG) coated with either IRBP or ovalbumin, a glycoprotein not found in the IPM. Neither IRBP-CG nor ovalbumin-CG was internalized by the Muller's cells. Both rod and cone photoreceptors take up IRBP-CG, which is observed in small vesicles and multivesicular bodies. Neither photoreceptor type takes up ovalbumin-CG. Acid phosphatase cytochemistry indicates that acid phosphatase reaction product in the multivesicular bodies co-localizes with IRBP-CG, which suggests that this molecule is degraded by rod and cone photoreceptors and is not recycled. The pigment epithelium internalizes IRBP-CG and ovalbumin-CG, both of which remain in small cytoplasmic vesicles near the apical plasma membrane. There is no indication that vesicles that contain either IRBP-CG or ovalbumin-CG fuse with the lysosomal system in the pigment epithelial cells during the incubation.
Endocytosis and degradation of interstitial retinol-binding protein: differential capabilities of cells that border the interphotoreceptor matrix.
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J G Hollyfield, H H Varner, M E Rayborn, G I Liou, C D Bridges; Endocytosis and degradation of interstitial retinol-binding protein: differential capabilities of cells that border the interphotoreceptor matrix.. J Cell Biol 1 May 1985; 100 (5): 1676–1681. doi: https://doi.org/10.1083/jcb.100.5.1676
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