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R. Wright, D. Hong; Identification and Characterization of a Novel Whirlin Isoform as a RPGR ORF15 Binding Protein. Invest. Ophthalmol. Vis. Sci. 2010;51(13):4065.
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© ARVO (1962-2015); The Authors (2016-present)
Mutations in the retinitis pigmentosa GTPase regulator (RPGR) are a frequent cause of inherited retinal degeneration. The RPGR gene undergoes complex alternative splicing and encodes constitutive and ORF15 variants. The constitutive transcripts are widely expressed and contain nineteen exons (RPGRex1-19), while the RPGRORF15 variants are preferentially expressed in the retina and contain exons 1-14 plus a large, alternatively spliced exon 15. A large number of disease causing mutations in the ORF15 exon suggest that this isoform is essential for the viability of photoreceptors. This study was designed to further elucidate the function of the RPGR ORF15 isoform by identifying potential interacting partners.
Mouse retinal cDNA was used in a yeast two-hybrid screen with the C-terminus of RPGR ORF15. RT-PCR and immunoblot analysis were used to confirm the presence of selected candidate partners in the retina. Results obtained in the yeast two-hybrid screen were confirmed by pull-down assays using recombinant protein and transfected cell lines, and immunohistochemistry was used to determine the cellular localization.
Our yeast two-hybrid screen identified a novel variant of whirlin, a PDZ scaffold protein expressed in cochlear hair cells and retinal photoreceptors, as a protein interacting with RPGR ORF15. This short, N-terminal whirlin isoform contains 322 amino acid residues resulting in a full PDZ1 domain and truncated PDZ2 domain. Polyclonal antibodies against the N-terminus of whirlin confirmed expression of this novel isoform in both the retina and cochlea, and immunohistochemistry using antibodies against the N-terminal and C-terminal ends of full-length whirlin showed isoform specific localization within photoreceptors. Whirlin isoforms detected by the N-terminal antibody localized to the root and connecting cilium while those detected by the C-terminal antibody localized in the outer segment and connecting cilium.
These findings indicate that whirlin binds RPGR ORF15 and the N-terminal isoform may be required for photoreceptor function. Mutations in the N-terminus of whirlin disrupt the structure of the PDZ domain and result in Usher syndrome type 2 (USH2D), a hereditary disorder characterized by severe hearing loss and retinitis pigmentosa. The interaction between RPGR ORF15 and whirlin provides a potential mechanism for the retinal degeneration observed in USH2D.
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