Arrestins are signal transduction modulators that quench the activated State of receptors. X-arrestin (ARRX) is specifically expressed in the red-, green-, and blue-sensitive cone photoreceptors, and is most likely a modulator of cone phototransduction. The human gene for X-arrestin at Xcen- Xq22 has been shown to be ~20 kb in size and to consist of 17 exons and 16 introns. The exons are generally small, including exon 16 of 10 bp, and are clustered into three groups, separated by the two largest introns. This gene structure is generally similar to that of S-antigen, the rod photoreceptor arrestin. There is remarkable similarity, however, among the individual exons between the two genes in that 10 of the exons are identical in size. The 5' upstream region of the X-arrestin gene contains TATA and CAAT boxes, typical of genes expressed in a tissue-specific manner, in contrast to the S-antigen gene, which lacks these promoter sequences. The promoter elements, common to both the X-arrestin and S-antigen genes, include the Ret-1/PCE-1 (PCE-1-like in X-arrestin), CRX, and the thyroid hormone/retinoic acid-responsive sequences, the former two being present in a number of photoreceptor- expressed genes. Three CRX-binding elements, 15 bp apart, are present in a cluster. The common promoter elements between the cone-expressed genes, X- arrestin and color opsins, include the TATA box, PCE-1, and CRX-binding sequences, the combination of which might be important for directing cone- specific expression.
- Cone photoreceptor
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