Distribution of interstitial retinol‐binding protein (IRBP) in the vertebrates

Abstract

Immunoblots of interphotoreceptor matrix preparations from 20 species belonging to six vertebrate classes were probed with antibodies against bovine interstitial retinol-binding protein (b-IRBP). Each preparation displayed an immunoreactive protein band. In the Osteichthyes, the apparent Mr of this band was 67,600 +/- 2,700 (mean +/- SD, n = 8). In two of the Osteichthyes, the band was resolved into a closely spaced doublet. Including previously published data for five mammals and one amphibian, species from the other classes (Chondrichthyes, one species; Amphibia, four species; Reptilia, one species; Aves, one species; Mammalia, nine species) had IRBPs with Mr that averaged 2.0 times that of the Osteichthyes, namely 134,200 +/- 8,600 (mean +/- SD, n = 17). Frog IRBP was very similar to mammalian IRBP in terms of its immunohistochemical distribution (determined with rabbit anti-frog IRBP antibodies), its molecular weight (sodium dodecyl sulfate polyacrylamide gel electrophoresis and gel-filtration chromatography), retinol- and concanavalin A-binding ability, and because it was synthesized and secreted in vitro by the isolated retina but not by the pigmented layers of eye. Goldfish IRBP apparently binds exogenous (3H)-retinol but does not bind concanavalin A and has about half the Mr of frog IRBP. The occurrence of IRBP-like proteins cross-reacting with anti b-IRBP antibodies in the interphotoreceptor matrix of all six major vertebrate classes is consistent with the hypothesis that IRBP is an important element in the vertebrate visual cycle.

Comparison of Proteins in the Interphotoreceptor Matrix of Vertebrates

“…Large mammals (human and cattle) have exceptionally large percentages of IRBP. The MWs for IRBP (obtained from gels stained for protein) agree well with those found by Bridges et al [4] (measured on immunoblots) for those species examined by both studies. The mean MW (and standard deviation) is 135 ± 5 kD for IRBP from all species.…”

“…Attention was focused on the proteins in the interphotoreceptor matrix (IPM), the thin layer of extracellular material between the neural retina and the retinal pigment epi thelium (RPE), by Adler and Severin in 1981 [1], Since that time, interphotoreceptor reti noid-binding protein (IRBP) was discovered [2,3], and many studies have dealt with its occurrence [4,5], ontogeny [6][7][8][9] and disap pearance during retinal degeneration [6-8, 10, 11 ] in several species.…”

Interphotoreceptor retinoid-binding protein (IRBP): Expression in the adult and developingXenopus retina

“…The size of the immunoreactive protein corresponds to the apparent molecular weight of native Xenopus IRBP (Gonzalez-Fernandez et al, 1993a). Xenopus IRBP is therefore similar in size to that of higher vertebrates and elasmobranchii (Bridges et al, 1986) and approximately twice the size of teleost IRBP (Wagenhorst et al, 1995). Figure 4 shows the immunohistochemical localization of IRBP in the adult Xenopus retina.…”

Retinoid-binding proteins: mediators of retinoid action

“…In contrast with the mammalian and amphibian proteins, IRBP in teleosts is composed of only two homologous repeats, corresponding to the Nterminal and C-terminal repeats of the mammalian protein [27]. Proteins that share sequence identity with IRBP have also been identified in bacteria and plants.…”