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Protein Pages
Cellular Trafficking

Recent update from: 26.06.00

Eucaryotic cells are compartmented units. Compartments are used to separate different functions of the cell like feeding, excretion, protein production, or digestion. Cellular compartments are interlinked by vesicles floating around inside the cell. To coordinate vesicle floating and to direct vesicles to their targets, these are labelled by Rab proteins.
Since cells contain compartments for Information storage (nucleus), protein production (endoplasmatic reticulum (ER)), protein processing (Golgi apparatus), etc. proteins need to be escorted through the compartments. Additionally, transmembrane proteins need chaperons to be inserted into the membrane in the correct direction. Thus to the cell, compartments cause the necessity of transport mediators and guides.
Examples for these guides involved in retinal cytology are Rab proteins , Retinitis Pigmentosa GTPase Regulator (RPGR), and Phosphodiesterase subunit delta (PDE6D).

Rab proteins are small GTP-binding proteins that mediate intracellular vesicular traffic. Their function is important in the control of protein distribution after translation and processing, exocytosis, and endocytosis.
Rab8 and Rab6 are reported to be involved in RHO trafficking through the golgi with Rab6 being a negative regulator of anterograde intra-Golgi transport (5) and coordinator of routing of RHO vesicles to the plasmamembrane (6). Vesicles depleted of Rab 3, 6 or 8 cause retention of RHO in the Golgi apparatus (7)
Further examples of Rab-proteins are Rab5 which has been reported to coordinate early endosome-endosome recognition (18), and Rab27a which is retina specific and is selectively bound by REP-1 for isoprenylation (17).
Rab protein function
RPGR is an ubiquitously expressed protein involved in X-linked retinitis pigmentosa (RP3), showing homology to RCC1 guanine nucleotide exchange factor ( GEF ). Investigations on bovine and mouse RPGR revealed several splice variants in different tissues (16).
The mouse homologue has been localized by immunochemical studies to the Golgi apparatus. This localization emphasizes an RPGR function as a GEF in trans Golgi transport. This notion is supported by in-vitro studies using a polypeptide of the 254 C-terminal amino acids of mRPGR as a bait for interacting proteins (8). Interaction with FKBP38, a member of a subfamily of immunophillins, was found. This protein has been localized to the Golgi fraction of fractionated retinal cell from frogs (3) and can be localized in the PIS by immunohistology. An involvement of FKBP38 - and therefore RPGR - in folding and transport of nascent RHO (8) as well as RPGR on the catalytic PDE6A and PDE6B subunits is proposed (1).
Several studies of proteins interacting with RPGR have been processed. These proteins have been described as RPGR-interacting motor-like proteins (RIMLP) indicating a function of RPGR in the Golgi as a component necessary for the transport of proteins (2), (15)
RPGR is isoprenylated. Lack of isoprenylation abolishes the localization of RPGR to the Golgi apparatus (21) . Lack of isoprenylation at least does not seem to be essential for its retinal function (9).

PDE6D has been first recognized as a chaperone for the catalytic PDE6A and PDE6B subunits (1). Prenylated PDE6A and PDE6B had been found to be bound by PDE6D to be solubilized from membranes possibly as regulatory function in the visual cascade (10), (10).
PDE6D interacts with RPGR in a thermosensitive fashion. Interaction is abolished by mutations in the RCC1-domain of RPGR (12).
Latest results indicate a function on small GTP-binding proteins by stabilizing the GTP-bound state (11).

REP1 Function
Modification of Rab-proteins

For transfer of geranylgeranyl modifications to Rab proteins two Rab escort proteins (REP1 and REP2) are required. These escort proteins act as substrate carriers for Rab-Geranyl-Geranyl-transferase (GGTase II) which catalyses the attachment of geranylgeranyl modifications from geranylgeranyl-diphosphate (GG-PP) to the Rab-protein.
REP1 and REP2 are coexpressed throughout the body. Due to substrate specificities of REP1 for Rab3A, and Rab3D, Rab proteins abundant to neural-tissues, deficiencies in REP1 lead to phenotypes affecting retinal tissues. Further studies revealed a further REP1 specific Rab protein termed Rab27a due to its size of 27 kDa. Rab27a is slightly bigger than the usual size of Rab proteins. Substrate specificities of Rab27A, Rab3A and Rab3D result in considerably lowered affinity of REP2 to these REP1 specific substrates(4), (17).. Rab27a is selectively unprenylated in CHM patients (19) indicating an involvement in the pathogenesis of CHM. In contrast mutations in Rab27A do not cause CHM or other retina associated disturbances. This is supported by studies on transgenic mice expressing dominant-negative Rab27a mutations causing slowed preferential GDP-binding, reduced GTPase-activity, and inhibition of GTP/GDP exchange. In these studies mice did not show any signs in the retina but expressed extensive cataracts and corneal and stromal ring opacities. (14)
Human Rab27a mutations cause Griscelli syndrome a rare autosomal recessive disorder expressing pigmentary dilution of the skin and hair, large clumps of pigment in hair shafts and accumulation of melanosomes in melanocytes (13). This indicates an involvement of Rab27A in the intracellular routing of melanosomes.
The relationship of Rab GGTase, Rab proteins, and REP summarises as follows:
The REP-Rab complex attaches to the catalytic dimer of Rab GGTase which transfers a geranylgeranyl group from GG-PP to the Rab protein. Geranylated-Rab stays attached to REP and is escorted to the Rab-specific protein target (4).
The target proteins are currently unknown. Investigations on REP1 deficient mice showed a possible effect on embryonic membranes which causes Rep1 null mutations to be lethal in mice (20).

1. Becker,J., Linari,M., Manson,F., Wright,A.F., Meindl,A., and Meitinger,T. The Delta Subunit Of Rod Phosphodiesterase Interacts With The RCC1 Homologous Domain Of RPGR. 1998; Invest.Ophthalmol.Vis.Sci. 39: S953
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2. Bernoud-Hubac,N., Roepman,R., Cremers,F.P., and Ferreira 1,P.A. Immunolocalization Of RPGR And RPGR-Substrate Isoforms In The Bovine And Human Retina. 2000; Invest.Ophthalm.Vis.Sci. 41: S876
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3. Breuer,D., Schmerl,S., Deretic,D., and Swaroop,A. Cellular localization and biochemical analysis of the retinal Retitinis Pigmentosa GTPase Regulator (Rpgr) which is mutated in X-linked Retinitis Pigmentosa. 1998; Am.J.Hum.Genet. 63: A353
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4. Cremers,F.P., Armstrong,S.A., Seabra,M.C., Brown,M.S., and Goldstein,J.L. REP-2, a Rab escort protein encoded by the choroideremia-like gene. 1994; J.Biol.Chem. 269: 2111-2117.
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5. Deretic,D. Rab proteins and post-Golgi trafficking of rhodopsin in photoreceptor cells. 1997; Electrophoresis. 18: 2537-2541.
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6. Deretic,D. and Papermaster,D.S. Rab6 is associated with a compartment that transports rhodopsin from the trans-Golgi to the site of rod outer segment disk formation in frog retinal photoreceptors. 1993; Journal.of.Cell Science. 106: 803-813.
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7. Deretic,D., Puleo Schepke,B., and Trippe,C. Formation of rhodopsin-bearing post-golgi vesicles in a cell free system. 1995; Invest.Ophthalmol.Vis.Sci. 36: S513
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8. Hong,D., Yue,G., and Li,T. Physikal interaction between RPGR and FKBP38, a homolog of the FK-506 binding proteins. 2000; Invest.Ophthalm.Vis.Sci. 41: S330
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9. Kirschner,R., Rosenberg,T., Schultz-Heienbrok,R., Lenzner,S., Feil,S., Roepman,R., Cremers,F.P., Ropers,H.H., and Berger,W. RPGR transcription studies in mouse and human tissues reveal a retina-specific isoform that is disrupted in a patient with X- linked retinitis pigmentosa. 1999; Hum.Mol.Genet. 8: 1571-1578.
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10. Li,G.Y., Liu,O.L., Laabich,A., and Cooper,N.G.F. Dark And Light-Mediated Regulation Of Calcium/Calmodulin- Dependent Protein Kinase Ii In The Adult Rat Retina. 1998; Invest.Ophthalmol.Vis.Sci. 39: S1051
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11. Linari,M., Hanzal-Bayer,M., and Becker,J. The delta subunit of rod specific cyclic GMP phosphodiesterase, PDE delta, interacts with the Arf-like protein Arl3 in a GTP specific manner. 1999; FEBS Lett. 458: 55-59.
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12. Linari,M., Ueffing,M., Manson,F., Wright,A., Meitinger,T., and Becker,J. The retinitis pigmentosa GTPase regulator, RPGR, interacts with the delta subunit of rod cyclic GMP phosphodiesterase. 1999; Proc.Natl.Acad.Sci.U.S.A. 96: 1315-1320.
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13. Menasche,G., Pastural,E., Feldmann,J., Certain,S., Ersoy,F., Dupuis,S., Wulffraat,N., Bianchi,D., Fischer,A., Le Deist,F., and de Saint Basile,G. Mutations in RAB27A cause Griscelli syndrome associated with haemophagocytic syndrome. 2000; Nat.Genet. 25: 173-176.
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14. Ramalho,J.S., Huxley,C., and Seabra,M.C. The Role Of RAB27A In The Pathogenesis Of Choroideremia: Generation Of Transgenic Mice Expressing Dominant-Negative Mutants Of RAB27A. 1999; Invest.Ophthalmol.Vis.Sci. 40: S473
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15. Roepman,R., Bernoud-Hubac,N., Maugeri,A., Ropers,H.H., Berger,W., Cremers,F., and Ferreira ,P. Isolation of a novel motor-like protein that interacts with Retinitis Pigmentosa GTPase Regulator (RPGR) in the rod outer segments of the retina. 1999; Am.J.Hum.Genet. 65: A111
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16. Roepman,R., Schick,D., Berger,W., Cremers,F., and Ferreira,P. Characterization Of Retina-Specific Substrate Isoforms For Retinitis Pigmentosa GTPase Regulator. 1999; Invest.Ophthalmol.Vis.Sci. 40: S590
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17. Seabra,M.C., Ho,Y.K., and Anant,J.S. Deficient geranylgeranylation of Ram/Rab27 in choroideremia. 1995; J.Biol.Chem. 270: 24420-24427.
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18. Steele Mortimer,O., Clague,M.J., Huber,L.A., Chavrier,P., Gruenberg,J., and Gorvel,J.P. The N-terminal domain of a rab protein is involved in membrane- membrane recognition and/or fusion. 1994; EMBO Journal. 13: 34-41.
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19. Tolmachova,T., Ramalho,J.S., Anant,J.S., Schultz,R.A., Huxley,C.M., and Seabra,M.C. Cloning, mapping and characterization of the human RAB27A gene. 1999; Gene. 239: 109-116.
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20. van den Hurk,J., Oerlemans,F., van denPol,D., Jaissle,G., R?ther,K., Ropers,H.-H., Wierings,B., Cremers,F., and Hendriks,W. Maternal transmission of a chorioideremia mutation in mice is embryonic lethal. 1995; Am.J.Hum.Genet. 57: A53
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21. Yan,D., Swain,P.K., Breuer,D., Tucker,R.M., Wu,W., Fujita,R., Rehemtulla,A., Burke,D., and Swaroop,A. Biochemical characterization and subcellular localization of the mouse retinitis pigmentosa GTPase regulator (mRpgr). 1998; J.Biol.Chem. 273: 19656-19663.
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