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1 From the F. M. Kirby Center for Molecular Ophthalmology and the 5 Scheie Eye Institute, University of Pennsylvania, Philadelphia, Pennsylvania; the 2 Human Genetics Center, School of Public Health, and the Department of Ophthalmology and Visual Science, The University of Texas Health Science Center, Houston, Texas; the 4 Jules Stein Eye Institute, University of California School of Medicine, Los Angeles, California; and the 6 Department of Developmental Neurobiology, St. Jude Children’s Research Hospital, Memphis, Tennessee.
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Abstract |
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METHODS. A combination of RT-PCR and rapid amplification of cDNA ends (RACE) was used to isolate the full-length mouse Rp1 cDNA. Antibodies against different regions of the predicted mouse Rp1 protein were generated. Western blot analyses were used to identify the RP1/Rp1 proteins. The subcellular location of RP1 in human and mouse retinas was determined by immunostaining retinal sections.
RESULTS. The full-length mouse Rp1 cDNA is 6944 bp, encoding a predicted protein of 2095 amino acids. Rp1 was found to be a soluble protein of approximately 240 kDa, consistent with predictions based on the cDNA sequence. Immunofluorescence analyses revealed that both the human RP1 and mouse Rp1 proteins are specifically localized in the connecting cilia of rod and cone photoreceptors.
CONCLUSIONS. The presence of RP1/Rp1 in connecting cilia suggests that it may participate in transport of proteins between the inner and outer segments of photoreceptors or in maintenance of cilial structure. This study forms the basis for further investigation of the function of RP1 in retina and the mechanism by which mutations in RP1 lead to photoreceptor cell death.
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Introduction |
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The RP1 gene was the fourth dominant RP gene to be identified,4 5 6 after RHO, RDS, and NRL, which encode rhodopsin, peripherin/RDS, and NRL, respectively.7 8 9 The RP1 gene is located on chromosome 8q12 and consists of four exons with an open reading frame of 6468 bp, encoding a predicted protein of 2156 amino acids, mostly by exon 4 (788–6468 bp). The RP1/Rp1 gene is expressed only in the photoreceptor cells of the retina, as determined by Northern blot analysis4 5 6 and in situ hybridization.4 Analysis of homology between human RP1 and other known proteins demonstrates that the N-terminal portion of RP1 is related to doublecortin (DCX), which is believed to be involved in directing neuronal migration during development of the central nervous system.10
So far, 20 disease-causing mutations have been identified in the RP1 gene.4 5 6 11 12 13 These are either nonsense or frame-shift mutations that cluster within a region extending from codons 658-1053 in exon 4. All these mutant alleles would encode truncated proteins without the carboxy 50% to 70% of RP1. Together these mutations account for approximately 6% to 10% of adRP cases in different ethnically diverse populations.4 6 11 12 13 The most common mutation in RP1, Arg677Ter, is present in approximately 3% of patients with adRP in the United States,4 constituting the third most common adRP mutation, after the Pro23His (9% of cases) and Pro347Leu (4% of cases) mutations in the rhodopsin gene.14 These findings indicate that the RP1 protein plays an important, although as yet unknown, role in photoreceptor function.
To elucidate the function of the RP1 protein and to gain insight into the mechanisms by which mutations in RP1 cause retinal degeneration, we cloned and sequenced the full-length mouse Rp1 cDNA. Based on the amino acid sequence predicted from Rp1 cDNA, we generated antibodies against mouse Rp1 fusion proteins. These antibodies were used to detect the RP1/Rp1 proteins by immunoblotting and to localize the RP1/Rp1 proteins in human and mouse retinas by immunostaining. Our results show that the RP1/Rp1 protein is located in the connecting cilia of rod and cone photoreceptor cells, making it the second protein specifically localized in this important structure of photoreceptors.
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Methods |
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Isolation of Mouse Rp1 cDNA and Sequence
Analysis
A 1.2-kb fragment of the mouse Rp1 cDNA was
originally isolated from a mouse retinal cDNA library (GenBank
accession number AF141021; hosted by the National Center for
Biotechnology Information and available in the public domain at
Http://www.ncbi.nlm.nih.gov/genbank).4
A combination
of RT-PCR using primers designed from the human RP1 cDNA
sequence (GenBank, AF155141), 5' RACE, and 3' RACE was used to extend
this sequence to obtain the full-length Rp1 cDNA sequence.
RT-PCR of the complete coding region from mouse retinal RNA was
performed to confirm the coding sequence. The mouse Rp1 protein
sequence was predicted and analyzed using the ExPASy proteomics server
(hosted by the Swiss Institute of Bioinformatics, Geneva, and available
in the public domain at http://www.expasy.org; Fig. 1
).
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Protein Extracts from Tissues and Western Blot Analysis
Normal adult mouse and rat retinas and additional mouse tissues
were dissected and frozen immediately at -80°C. Total protein
extracts of each tissue were prepared with SDS sample buffer. Protein
samples were boiled and spun at 10,000g for 10 minutes; the
supernatants were then processed for Western blot analysis. One hundred
fifty micrograms of each protein sample was separated by SDS-PAGE on
7.5% gels.15
Proteins were then transferred
electrophoretically to polyvinylidene difluoride (PVDF)
membranes.16
The membranes were blocked for 1 hour in
TBS-T solution (50 mM Tris-HCl [pH 8.0], 150 mM sodium chloride,
0.1% Tween-20), containing 10% nonfat dry milk, 5% normal goat
serum, and 0.05% sodium azide, and incubated either with preimmune or
immune polyclonal antibodies anti-M-Rp1(1 µg/mL) or anti-C'-Rp1 (2.5
µg/mL) in blocking solution for 3 hours. The antibody binding was
detected with alkaline phosphatase–conjugated anti-rabbit (1:10000,
Vector Laboratories, Burlingame, CA) or anti-chicken secondary
antibodies (1:5000, Jackson ImmunoResearch Laboratories, West Grove,
PA) and enhanced chemifluorescent (ECF) substrate (Amersham
Pharmacia). All incubations were done at room temperature. Positive
signals were visualized by fluorometry (Storm 860 Imager; Molecular
Dynamics, Sunnyvale, CA).
Fractionation of Mouse Retina
Adult mouse retinas were first homogenized in a buffer
containing 50 mM Tris-HCl (pH 8.0), 150 mM NaCl, 15 mM EDTA, and 1x
protease inhibitor cocktail (Roche Molecular Biochemicals,
Indianapolis, IN). The homogenate was centrifuged at 20,000g
for 10 minutes at 4°C, and the supernatant containing soluble
proteins was removed. The pellet was resuspended in the same buffer
plus 1% Triton X-100. The resultant suspension was separated by
centrifugation at 14,000g for 10 minutes. The residual
pellet containing insoluble proteins and cell debris was suspended and
extracted with SDS sample buffer.
Preparation of Tissue Sections and Immunostaining Analysis
Eyes of adult mice were dissected after cardiac perfusion with
4% paraformaldehyde in 0.1 M phosphate buffer (pH 7.4) and were fixed
for 6 hours at 4°C. Fixed eye cups were infiltrated overnight with
30% sucrose in the same buffer, embedded in optimal cutting
temperature (OCT) mounting medium, and cryosectioned at 10 µm. Normal
adult human retinas were fixed at 6 hours or less after death in 4%
paraformaldehyde and 0.5% glutaraldehyde or in 4% paraformaldehyde
alone, all in 0.1 M phosphate buffer. The retinas were processed as for
protein extraction and cryosectioned at 12 µm. For immunostaining,
human or mouse retinal sections were pretreated with a blocking
solution (1% normal horse or goat serum, 1% bovine serum albumin, and
0.05% Triton X-100 in PBS [pH 7.4]) for 1 hour at room temperature
and then incubated with primary antibody (two primary antibodies for
double staining) diluted in PBS with 0.3% Triton X-100 overnight at
4°C. After they were rinsed with PBS, the sections were treated with
one or two secondary antibodies in PBS for 1 hour at room temperature.
Slides were then washed in PBS and mounted (Fluoromount-G; Southern
Biotechnology Associates, Birmingham, AL). The primary antibodies used
were chicken polyclonal anti-N'-Rp1 (5 µg/mL) and anti-C'-Rp1 (5
µg/mL), rabbit affinity purified anti-C-Rp1 (1 µg/mL), mouse
monoclonal anti-rhodopsin 4D2 (1:40, from Robert Molday, MD, University
of British Columbia, Vancouver, British Columbia,
Canada),17
and mouse monoclonal human cone-specific
antibody 7G6 (1:40, from Peter MacLeish, MD, Morehouse School of
Medicine, Atlanta, GA).18
The secondary antibodies were
Cy3-conjugated (red) rabbit anti-chicken IgG (1:100), Cy3-conjugated
goat anti-rabbit IgG (1:100), and Cy2-conjugated (green) rabbit
anti-mouse IgG (1:100) from Jackson ImmunoResearch Laboratories (West
Grove, PA). Cell nuclei were counterstained (blue) with 4',
6'-diamidino-2-phenylindole (DAPI, 1 µg/mL; Molecular Probes, Eugene,
OR) added to the mixture of secondary antibodies. Control sections were
treated with preimmune anti-N'-Rp1 or anti-C'-Rp1 at the same dilution.
Preabsorption tests were performed by preincubating the diluted
affinity purified anti-C-Rp1 with a 20-fold molar excess of fusion
protein GST-C'-Rp1 for 1 hour before the immunostaining procedures.
Sections were viewed with a microscope equipped for epifluorescence
(Leica, Deerfield, IL) or with a laser scanning confocal microscope
(Radiance 2000-MP; Bio-Rad). Confocal image files were processed on
computer (Confocal Assistant software; Bio-Rad).
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Specificity of Anti-Rp1 Antibodies
Two rabbit polyclonal antibodies, anti-M-Rp1 and anti-C-Rp1, were
generated against His-tagged fusion proteins His-M-Rp1 and His-C-Rp1
(Fig. 2A)
. Two chicken polyclonal antibodies, anti-N'-Rp1 and
anti-C'-Rp1, were made against GST fusion proteins GST-N'-Rp1 and
GST-C'-Rp1, respectively (Fig. 2B)
. Before using these antibodies to
study Rp1, we first tested their specificity by Western blot analysis.
Because the Rp1 portion of the GST fusion proteins were contained
within the Rp1 portion of the His-tagged fusion proteins, they were
used as the Rp1 antigen providers to test the antibodies raised against
His-tagged fusion proteins and vice versa. For example, GST-M'-Rp1
fusion protein was used to test antibody anti-M-Rp1 (made against
His-M-Rp1). As shown in Figure 2C
, anti-M-Rp1 specifically recognized
the corresponding GST fusion protein GST-M'-Rp1, but not GST-N'-Rp1 and
GST-C'-Rp1, which were made from different regions of Rp1. Anti-C-Rp1
reacted with GST-C'-Rp1, but not GST-N'-Rp1 or GST-M'-Rp1. In a similar
fashion, the His-tagged fusion proteins were also used to test the
antibodies raised against GST fusion proteins. Anti-N'-Rp1 detected
His-N-Rp1, but not His-M-Rp1 or His-C-Rp1. Anti-C'-Rp1 only detected
C-terminal His-tagged fusion protein (Fig. 2D)
. Therefore, these
antibodies were confirmed to have anti-Rp1 specificity and recognized
the appropriate domains of the Rp1 protein.
Identification of the Rp1 Protein
Two polyclonal antibodies against distinct regions of predicted
mouse Rp1 protein, anti-M-Rp1 and anti-C'-Rp1, were found to
specifically detect a protein of approximately 240 kDa on Western blot
analysis of mouse retinal extracts, which is in agreement with the
predicted size of the Rp1 protein. In addition to the major band at 240
kDa, two weak bands of approximately 220 and 105 kDa were also detected
by anti-M-Rp1, and a 90-kDa band was detected by anti-C'-Rp1 (Fig. 3A) . These lower molecular weight species may be due to the degradation of
Rp1 protein, or nonspecific binding. In the negative control
experiments, preimmune antibodies did not detect any specific bands in
mouse retinal extracts.
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The expression of the mouse Rp1 gene was previously shown to be specific to the retina by Northern blot analysis.4 6 To determine the tissue distribution of Rp1 protein, anti-Rp1 antibodies were used to detect Rp1 in total protein extracted from several mouse tissues, including heart, liver, lung, spleen, brain, and skeletal muscle. The 240-kDa band corresponding to the expected Rp1 protein was limited to the retina, using both anti-M-Rp1 (Fig. 4A) and anti-C'-Rp1 (Fig. 4B) antibodies. This observation further confirms that Rp1 is a retina-specific protein. Because the mRNA for Rp1 is not produced in muscle, lung, and spleen, it is likely that the 220-kDa protein detected by the anti-M-Rp1 antibody in these tissues is due to nonspecific binding (Fig. 4A) .
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Expression of the Rp1 Protein during Development
Retinas from mice aged 4 to 12 postnatal (P) days (P4–P12) were
examined by Western blot analysis (Fig. 8A)
and confocal microscopy (Fig. 8B)
to determine the pattern of
expression of the Rp1 protein during photoreceptor development. For
Western blot analysis, total protein from one retina at each time point
was used to allow for comparison of the level of Rp1 protein through
time course. The Rp1 protein was not detectable at P4. A very low level
of Rp1 was detected at P8. The levels of Rp1 protein gradually
increased over time, reaching a maximum in adult retinas (6–8 weeks).
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Discussion |
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The connecting cilium is a slender structure physically connecting the outer segments and inner segments of retinal photoreceptor cells, with a total length of approximately 200 to 500 nm and a diameter of 170 nm in most mammal photoreceptors.21 23 24 In connecting cilia of photoreceptors, the most obvious cytoskeletal elements are axonemal microtubule doublets (9 x 2 + 0), which arise from the basal body at the distal end of the inner segments, run through the connecting cilia, and extend into the proximal outer segments. The doublets become singlets (9 x 1 + 0) in the outer segments and finally disappear distal to the connecting cilia.21 23 25 The RP1 labeling we observed was longitudinally oriented, extending from the apical inner segment through the connecting cilium into the proximal outer segment. The location of RP1 was consistent with the distribution of the axonemal microtubules in the connecting cilia and outer segments. Accordingly, we hypothesize that the RP1 protein may interact with microtubules in the connecting cilia of photoreceptor cells. This hypothesis is further strengthened by the presence of a potential DCX domain at the N-terminal end of predicted RP1 proteins (as will be discussed later).
In addition to the intense labeling of RP1 in the connecting cilia, the inner segments were also faintly positive for RP1 in both human and mouse retinas. This may reflect newly synthesized RP1 in transit to the connecting cilia or may indicate an additional function of RP1 in the inner segments. For example, it is possible that RP1 interacts with cytoplasmic microtubules in the inner segments. The inner plexiform layer and amacrine cells in human retina and neonatal mouse retina were also labeled by anti-C-Rp1, although this pattern was not found in adult mouse retina. Labeling of the inner retinal cells may be due to cross-reactions of the antibody with non-RP1 proteins, in that the expression of Rp1 mRNA was detected only in the photoreceptor cells of retina by in situ hybridization.4
Antibodies raised against two distinct regions of mouse Rp1 protein both recognized the same protein of approximately 240 kDa, confirming the predicted size of Rp1. The approximately same size proteins of 240 kDa detected in human, rat, and bovine retinal extracts by anti-Rp1 antibodies indicate that the RP1 proteins in these species also have a similar size. The smaller band of 220 kDa detected by anti-M-Rp1 in various mouse tissues, such as retina, skeletal muscle, lung, and spleen, is likely to be due to nonspecific binding, because this 220-kDa band was not detected by anti-C'-Rp1 antibody and because the mRNA for RP1/Rp1 was detected only in retina by Northern blot analysis.4 5 6 The two lower molecular weight bands detected by anti-M-Rp1 or anti-C'-Rp1 may result from partial degradation or posttranslational modification of the protein in photoreceptors. The absence of a predicted hydrophobic transmembrane sequence is consistent with our results that the Rp1 is a soluble protein.
The Development of Rp1 Protein
The Rp1 protein is detectable at P6, and the level of Rp1 protein
shows a gradual increase in postnatal mouse retinas, consistent with
prior data regarding mouse Rp1 mRNA.4
The
appearance of Rp1 protein is concomitant with the morphologic
development of the outer segments, which start forming at approximately
day 5 after birth.22
Expression of proteins known to be
required for the formation of outer segments, such as rhodopsin and
rds/peripherin, also begins at approximately P5.26
27
28
This implies that, together with other photoreceptor proteins, RP1 may
be involved in the formation of outer segments.
Possible Functional Domains of the RP1 Protein
Alignment of the predicted human RP1 and mouse Rp1 protein
sequences shows 80.1% similarity and 60.4% identity between the two
proteins. Although several regions in the N- and C-terminal regions
show complete identity between the human and mouse RP1 proteins, the
level of identity between human RP1 and mouse Rp1 is lower than that
observed for other photoreceptor proteins. For example, the first two
identified adRP genes, rhodopsin and peripherin/RDS, have 99% and 98%
identity between human and mouse, respectively.29
This
divergence in the RP1 protein sequence may explain why the
disease-causing mutations found so far involve large disruptions of
this protein and suggests that some amino acid substitutions may not be
detrimental to protein function.
Searches for homology between Rp1 and other known protein sequences detected one region of homology at the N-terminal end of Rp1, which is related to DCX, a brain-specific protein implicated in X-linked lissencephaly and double cortex syndrome.10 DCX has strong homology to the human KIAA0369 protein (or DCAMKL1), a central nervous system protein coexpressed in migrating neurons with DCX.30 It has been recently shown that DCX, as well as DCAMKL1, are members of a new family of microtubule-associated proteins (MAPs), based on their colocalization with microtubules, coassembly with microtubules, and dramatic effect on microtubule polymerization.30 31 32 DCX and DCAMKL1 interact with microtubules through two tandemly repeated DCX domains (DC).33 It is these two repeated DC domains that represent the homology of RP1 protein with DCX (Fig. 1) . Based on the similar distribution of the RP1 protein and axonemal microtubules in the connecting cilia, the DC domains in the RP1 protein sequence provide further evidence that RP1 may interact with microtubules.
Possible Function of the RP1 Protein in Photoreceptor Cells
Photoreceptor cells are highly polarized, and their outer
segments have no biosynthetic machinery.34
All components
of the outer segment, including the proteins for phototransduction and
the lipids of the disc membranes must first be synthesized in the inner
segment and transported to the outer segments. Moreover, the outer
segment is continually renewed at a rapid rate
(
107 rhodopsin molecules per day per cell) as
the distal discs are shed and newly synthesized membrane is added at
the base.35
In addition, the concentration of
phototransduction proteins changes in response to the light and dark
cycle.36
37
An active system(s) in the photoreceptor must
exist to transport materials between the inner segments and the outer
segments. As the only continuous structural link between the inner
segment and outer segment, the connecting cilium is thought to be the
critical channel through which the proteins are directionally
transported between these compartments. This model for protein
transport through the connecting cilia is well supported by
experimental evidence.38
39
For example, a recent study
demonstrated that rhodopsin was present in and presumably transported
through the ciliary membrane.39
However, the exact
molecular motors for transport of proteins through the connecting cilia
remain to be defined.
Several other proteins also have been localized to the connecting cilia, including a kinesin family member, myosin family members, retinitis pigmentosa guanosine triphosphatase (GTPase) regulator (RPGR), and RPGR interacting protein (RPGRIP).40 41 42 43 44 KIF3A, a component of kinesin-II, has been localized to the connecting cilium and synaptic ribbon in fish and vertebrate photoreceptors.40 45 KIF3A shares a conserved microtubule-based motor domain with kinesin superfamily proteins46 and is required to transport opsin and arrestin from the inner to the outer segment.47
Conventional myosin (myosin II) has been found in the connecting cilium with the same distribution as actin. It has been proposed that the actin-myosin system of the connecting cilium may function to initiate the morphogenesis of a disc membrane.48 Mutations in myosin VIIa were found in patients with Usher syndrome type 1B, an autosomal recessive form of RP and congenital deafness.42 There is a conserved actin-based motor domain in the N-terminal region of myosin VIIa.49 Myosin VIIa has been localized in the retinal pigment epithelium and connecting cilia of photoreceptors.39 50 51 Myosin VIIa is thus a candidate motor for photoreceptor protein transport.
Mutations in the RPGR gene cause RP3, a form of X-linked RP.52 RPGR and its interacting protein, RPGRIP, are colocalized in the outer segment and/or connecting cilia of photoreceptor cells.43 44 53 RPGR is thought to mediate vesicular transport or maintain the polarized protein distribution across the connecting cilium.43 44 RPGRIP is thought to be a structural component of the ciliary axoneme, and one of its functions is to anchor RPGR within the cilium.43
Although localization of a microtubule-associated kinesin family member, an actin-associated myosin family member, and RPGR in the connecting cilia is intriguing, they are all ubiquitously expressed proteins. It remains unknown whether these molecules indeed mediate protein transport between the inner segment and outer segment.39 47 RPGRIP, and now RP1, are the only two proteins found to be uniquely localized in the connecting cilia of photoreceptors.
The location of RP1 in the connecting cilia, and its homology with DCX make RP1 an attractive candidate to participate in transport of newly synthesized outer segment proteins from the inner segments to the site of disc membrane assembly through the connecting cilia. It is possible, for example, that RP1 interacts with microtubules through its N-terminal DC domain, whereas the C-terminal portion of RP1 binds a protein that is headed for the outer segment. Alternative functions for RP1 can also be envisioned, such as regulation of microtubule dynamics through its DC domain, maintenance of the structure and orientation of connecting cilia, or blockage of diffusion between the inner segments and outer segments.
All the disease-causing mutations found in RP1 so far cluster in the beginning of exon 4, downstream from the DC domain (exons 2 and 3). These mutations are either nonsense mutations or frame-shift mutations that lead to premature termination of translation. Because these mutations occur after the final intron–exon junction in RP1, it is likely that the mutant RP1 mRNAs are not destroyed by nonsense-mediated decay, but that truncated RP1 proteins are produced.54 It is thus possible that mutations in RP1 disrupt protein transport through the connecting cilium by separating the N-terminal DC domain from the C-terminal domain(s) of the RP1 protein. Defective protein transport would disrupt outer segment formation and ultimately lead to photoreceptor cell death. Additional studies are needed to test this hypothesis and help elucidate the mechanism by which mutations in RP1 cause retinal degeneration.
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Acknowledgements |
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Footnotes |
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Supported by National Institutes of Health Grants EY12910 and EY12950; Research to Prevent Blindness; the Foundation Fighting Blindness; the Rosanne Silbermann Foundation, Livingston, New Jersey; the Mackall Foundation Trust, New York, New York; and the March of Dimes Birth Defects Foundation, White Plains, New York.
Submitted for publication July 9, 2001; accepted September 14, 2001.
Commercial relationships policy: N.
The publication costs of this article were defrayed in part by page charge payment. This article must therefore be marked "advertisement" in accordance with 18 U.S.C. 
1734 solely to indicate this fact.
Corresponding author: Eric A. Pierce, F. M. Kirby Center for Molecular Ophthalmology, University of Pennsylvania, 305 Stellar-Chance Laboratories, 422 Curie Boulevard, Philadelphia, PA 19104; epierce{at}mail.med.upenn.edu.
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Q. Liu, A. Lyubarsky, J. H. Skalet, E. N. Pugh Jr, and E. A. Pierce RP1 Is Required for the Correct Stacking of Outer Segment Discs Invest. Ophthalmol. Vis. Sci., October 1, 2003; 44(10): 4171 - 4183. [Abstract] [Full Text] [PDF]
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P. Hahn, T. Lindsten, G.-S. Ying, J. Bennett, A. H. Milam, C. B. Thompson, and J. L. Dunaief Proapoptotic Bcl-2 Family Members, Bax and Bak, Are Essential for Developmental Photoreceptor Apoptosis Invest. Ophthalmol. Vis. Sci., August 1, 2003; 44(8): 3598 - 3605. [Abstract] [Full Text] [PDF]
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D.-H. Hong, B. Pawlyk, M. Sokolov, K. J. Strissel, J. Yang, B. Tulloch, A. F. Wright, V. Y. Arshavsky, and T. Li RPGR Isoforms in Photoreceptor Connecting Cilia and the Transitional Zone of Motile Cilia Invest. Ophthalmol. Vis. Sci., June 1, 2003; 44(6): 2413 - 2421. [Abstract] [Full Text] [PDF]
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Y. Zhao, D.-H. Hong, B. Pawlyk, G. Yue, M. Adamian, M. Grynberg, A. Godzik, and T. Li The retinitis pigmentosa GTPase regulator (RPGR)- interacting protein: Subserving RPGR function and participating in disk morphogenesis PNAS, April 1, 2003; 100(7): 3965 - 3970. [Abstract] [Full Text] [PDF]
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C. Grayson, F. Bartolini, J. P. Chapple, K. R. Willison, A. Bhamidipati, S. A. Lewis, P. J. Luthert, A. J. Hardcastle, N. J. Cowan, and M. E. Cheetham Localization in the human retina of the X-linked retinitis pigmentosa protein RP2, its homologue cofactor C and the RP2 interacting protein Arl3 Hum. Mol. Genet., November 15, 2002; 11(24): 3065 - 3074. [Abstract] [Full Text] [PDF]
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J. Gao, K. Cheon, S. Nusinowitz, Q. Liu, D. Bei, K. Atkins, A. Azimi, S. P. Daiger, D. B. Farber, J. R. Heckenlively, et al. Progressive photoreceptor degeneration, outer segment dysplasia, and rhodopsin mislocalization in mice with targeted disruption of the retinitis pigmentosa-1 (Rp1) gene PNAS, April 16, 2002; 99(8): 5698 - 5703. [Abstract] [Full Text] [PDF]
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