Properties of the ipRGC Photopigment: Melanopsin
IpRGC’s are unique in that they contain their own photopigment, melanopsin (source). Melanopsin is categorized in the opsin family because of its ability to receive a photon of light and translate it into an electrochemical signal. This ability has been observed and is well understood in the photoreceptor rods and cones. The photoreceptors are key players in translating visual stimuli to a language the brain can understand. Their ability to interpret visual cues on their own depends on the opsins that are integrated into their structure (SOURCE). Comparing photoreceptor opsins and the opsin expressed in ipRGC’s is helpful because they both act to relay signals to the brain. The pathway however, that transfers signals from each is quite different and will be discussed in the following section.
Provencio and colleagues isolated an unknown type of opsin from photosensitive dermal melanophores of Xenopus laevis, also known as the African clawed frog WHAT YEAR?(pickers). Due to this discovery they named this new opsin, melanopsin (pickers)(hankins). Despite the fact the african clawed frog is a vertebrate, tests showed this opsin identified more closely with invertebrate opsins (39%), than with vertebrates (~27%) (hankins). ...? More recent studies have revealed there are actually two types of melanopsin: mammalian-like Opn4m and Xenopus-like Opn4x (hankins).
All opsins are G-protein coupled receptors (GPCRs) and their most distinctive feature is located on their seventh transmembrane domain (source). This feature is the presence of a retinal attachment site which binds to the chromophore through a schiff base linkage (davies). The chromophore is a region on the opsin where two orbitals fall in the range of the visible light spectrum. Once light hits the chromophore it excites an electron and the light is absorbed. Retinal then converts the absorbed light into energy by initiating a conformational change. The positively charged schiff base linkage is balanced by the negative charge in the third transmembrane domaine (davies).
The third cytoplasmic loop of melanopsin shows poor conservation when compared to other opsins through an amino acid alignment (davies). This suggests that melanopsin can activate the phototransduction cascade without high amino acid conservation. Melanopsin also has a distinct genomic structure and forms its own branch in the GPCR family (davies sources?). The poor amino acid conservation and distinct structure indicate melanopsin behaves differently than other opsins and is also evolutionarily unique. ... if we know more about them we can help fix them
Structural and Functional Properties of ipRGCs Involved in the Phototransduction Process
Light detection in the eye begins when 11-cis retinal absorbs a photon and photoisomerises? to the all-trans state. This conformational change of the chromophore then changes the configuration of the opsin which...