OPSIN 3; OPN3

Alternative titles; symbols

• PANOPSIN
• ENCEPHALOPSIN; ECPN

HGNC Approved Gene Symbol: OPN3

Cytogenetic location: 1q43 Genomic coordinates (GRCh38): 1:241,593,123-241,640,368 (from NCBI)

▼ Description
Opsins, including OPN3, are members of the superfamily of guanine nucleotide-binding protein (G protein)-coupled receptors, which function through the activation of a G protein and an effector enzyme. Opsin proteins consist of a single polypeptide chain of 340 to 500 amino acids that form 7 alpha-helical transmembrane regions connected by cytoplasmic and extracellular loops.

▼ Cloning and Expression
By screening an EST database with a putative Xenopus extraretinal opsin, followed by RACE, Blackshaw and Snyder (1999) obtained full-length cDNAs encoding mouse and human OPN3, which they called encephalopsin. The predicted 403-amino acid human OPN3 protein shares 85.5% identity with the mouse protein. Northern blot analysis of mouse tissues revealed strong expression of Opn3 in cortex, cerebellum, and testes only, with faint expression in heart, liver, and kidney. In situ hybridization localized Opn3 to pachytene spermatids in testes. Blackshaw and Snyder (1999) also detected high expression of Opn3 in the preoptic area and paraventricular nucleus of the hypothalamus. Opn3 showed developmentally regulated, highly patterned expression in other regions of the brain, being enriched in selected regions of the cerebral cortex, cerebellar Purkinje cells, a subset of striatal neurons, selected thalamic nuclei, and a subset of interneurons in the ventral horn of the spinal cord. The authors observed rostrocaudal gradients of Opn3 expression present in cortex, cerebellum, and striatum, as well as radial stripes of Opn3 expression in cerebellum. Blackshaw and Snyder (1999) concluded that Opn3 may play a role in encephalic photoreception.

By searching an EST database for sequences related to mammalian nonvisual opsins, followed by further database searches and RACE, Halford et al. (2001) assembled a cDNA sequence encoding OPN3, which they called panopsin. The predicted protein differed slightly from the sequence predicted by Blackshaw and Snyder (1999). OPN3 contains features characteristic of functional opsins, including a C-terminal region enriched in serine and threonine residues, an N-linked glycosylation site, a C-linked palmitoylation site, 2 conserved cysteine residues, and a conserved ERY tripeptide. Northern blot analysis of human tissues detected 2 major OPN3 transcripts of 2.1 and 2.5 kb resulting from the use of 2 alternate polyadenylation signals. OPN3 expression was observed in all tissues examined, including brain, liver, placenta, heart, lung, skeletal muscle, kidney, and pancreas. PCR analysis detected OPN3 in retina cDNA. Halford et al. (2001) noted that they detected a wider tissue distribution of OPN3 expression compared with Blackshaw and Snyder (1999), who did not detect mouse Opn3 in retina.

▼ Gene Structure
By comparing genomic and cDNA sequences, Halford et al. (2001) determined that the OPN3 gene contains 4 exons spanning approximately 47 kb.

▼ Mapping
Using a somatic cell hybrid panel and FISH, Halford et al. (2001) mapped the OPN3 gene to chromosome 1q43. They concluded that the OPN3 gene overlaps with the KMO gene (603538) and that the 2 genes are transcribed from opposite strands. Halford et al. (2001) also noted that the CHML gene (118825) lies within intron 1 of the OPN3 gene.