Deficient Geranylgeranylation of Ram/Rab27 in Choroideremia (*)
- From the Department of Molecular Genetics, University of Texas Southwestern Medical Center, Dallas, Texas 75235
- § To whom correspondence should be addressed: Dept. of Molecular Genetics, University of Texas Southwestern Medical Center, Dallas, TX 75235. Tel.: 214-648-8643; Fax: 214-648-8804.
Abstract
Choroideremia, an X-linked form of retinal degeneration, results from defects in the Rab escort protein-1 (REP-1) gene. REP-1 and REP-2 assist in the attachment of geranylgeranyl groups to Rab GTPases, a modification essential for their action as molecular switches regulating intracellular vesicular transport. If Rabs that depend preferentially on REP-1 for prenylation exist, they will accumulate unprenylated in choroideremia cells. Using recombinant Rab geranylgeranyl transferase and REPs to label unprenylated cytosolic proteins, we identified one unprenylated protein in choroideremia lymphoblasts that was prenylated in vitro more efficiently by REP-1 than by REP-2. This protein was purified and identified as Ram (renamed Rab27), a previously cloned Rab of unknown function. Immunohistochemistry of rat retina showed that Ram/Rab27 is expressed in the pigment epithelium and choriocapillaris, the two retinal cell layers that degenerate earliest in choroideremia. These results raise the possibility that the retinal degeneration in choroideremia results from the deficient geranylgeranylation of Ram/Rab27 or a closely related protein.
Footnotes
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↵* This research was supported by grants from The Foundation Fighting Blindness (National Retinitis Pigmentosa Foundation, Inc.), National Institutes of Health Grant HL20948, and the Perot Family Foundation. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore by hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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↵1 The abbreviations used are:
- CHM
-
choroideremia
- RPE
-
retinal pigment epithelium, Rab GDI, Rab GDP dissociation inhibitor
- GG
-
geranylgeranyl
- GGTase
-
geranylgeranyl transferase
- REP
-
Rab escort protein
- DTT
-
dithiothreitol
- PAGE
-
polyacrylamide gel electrophoresis
- CHAPS
-
cholamidopropyl-dimethylammonio-propanesulfonate
- PBS
-
phosphate-buffered saline.
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- Received June 27, 1995.
- Revision received August 2, 1995.
- © 1995 by The American Society for Biochemistry and Molecular Biology, Inc.
