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R. Maddala, J. Qiu, A. M. Pendergast, P. V. Rao; Down-Regulation of Expression of the Lens-Specific Intermediate Filament Protein Phakinin, and the c-cbl-Associated Protein CAP/Ponsin, in Lenses From Transgenic Mice Overexpressing Rho GDI and Abi-1/Abi-2 Knock-Out Mice. Invest. Ophthalmol. Vis. Sci. 2007;48(13):2009.
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The Rho GTPases and c-Abl-interactor proteins (Abi-1 and Abi-2), which primarily target the actin cytoskeleton, play important role(s) in lens growth and function. To understand the pleiotropic role(s) of Rho GTPases and Abi proteins, we evaluated differential gene expression profiles in lenses from transgenic mice overexpressing Rho GDI and from Abi-1/Abi-2 knockout mice.
Total RNA extracted from 7 day-old lenses of control, Rho GDI and Abi-1/Abi-2 het/null mice was subjected to cDNA microarray analysis using arrays representing ≈ 34,000 gene transcripts. Expression and distribution of CAP was characterized in detail by RT-PCR, immunoblot and immunofluorescence analysis in mouse lens epithelial cells and tissue.
Lenses overexpressing Rho GDI and those from Abi-1/Abi-2 het/null mice revealed consistent downregulation (5-10 folds) of CAP splice variants and phakinin compared to littermate control lenses. The mouse lens expresses 4 different splice variants of CAP and expression of one of the splice variants (U58883) was noted to be abundant. Immunoblot analysis of CAP in mouse lens fractions revealed that this protein is distributed predominantly in the Triton X100 soluble and insoluble fractions of fiber cells. In mouse lens primary epithelial cells, CAP is distributed along the intermediate filaments and focal adhesions exhibiting perfect colocalization with vimentin and vinculin.
This study identifies the CAP gene as a highly expressed gene in the lens. CAP is a cytoskeletal interacting protein possessing a conserved sorbin peptide homology domain and three tandem SH3 domains. Further, while some of the variants of this protein localize to intermediate filament proteins, others are distributed to the focal adhesions in lens epithelial cells. Importantly, this study demonstrates a role for Rho GTPases and Abi proteins in expression of CAP splice variants and phakinin.
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