Chimeric Nature of Pinopsin between Rod and Cone Visual Pigments

Abstract

Chicken pineal pinopsin is the first example of extra-retinal opsins, but little is known about its molecular properties as compared with retinal rod and cone opsins. For characterization of extra-retinal photon signaling, we have developed an overexpression system providing a sufficient amount of purified pinopsin. The recombinant pinopsin, together with similarly prepared chicken rhodopsin and green-sensitive cone pigment, was subjected to photochemical and biochemical analyses by using low-temperature spectroscopy and the transducin activation assay. At liquid nitrogen temperature (-196 degrees C), we detected two kinds of photoproducts, bathopinopsin and isopinopsin, having their absorption maxima (lambda(max)) at 527 and approximately 440 nm, respectively, and we observed complete photoreversibility among pinopsin, bathopinopsin, and isopinopsin. A close parallel of the photoreversibility to the rhodopsin system strongly suggests that light absorbed by pinopsin triggers the initial event of cis-trans isomerization of the 11-cis-retinylidene chromophore. Upon warming, bathopinopsin decayed through a series of photobleaching intermediates: lumipinopsin (lambda(max) 461 nm), metapinopsin I (460 nm), metapinopsin II (385 nm), and metapinopsin III (460 nm). Biochemical and kinetic analyses showed that metapinopsin II is a physiologically important photoproduct activating transducin. Detailed kinetic analyses revealed that the formation of metapinopsin II is as fast as that of a chicken cone pigment, green, but that the decay process of metapinopsin II is as slow as that of the rod pigment, rhodopsin. These results indicate that pinopsin is a new type of pigment with a chimeric nature between rod and cone visual pigments in terms of the thermal behaviors of the meta II intermediate. Such a long-lived active state of pinopsin may play a role in the pineal-specific phototransduction process.

An extended family of novel vertebrate photopigments is widely expressed and displays a diversity of function

“…Typically, nonvisual pigments can be monostable (e.g., pinopsin [Nakamura et al 1999] and va opsin [Sato et al 2011]) or bistable (e.g., neuropsin [Yamashita et al 2010] and parapinopsin [Koyanagi et al 2004]), as they form stable opsin and chromophore complexes with multiple retinal isomers (11-cis and all-trans retinoids). To explore whether the new opsins are monostable or bistable, UV-visible (UV-vis) spectrophotometry was used to determine their particular absorbance characteristics.…”

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“…For example, pinopsin is involved in photoreception in the pineal organs of birds [17,18] and lizards [19]. It is suggested to activate both transducin and the G-protein G 11 and therefore to drive two different phototransduction cascades [53,54]. The parapinopsin recently found in the pineal organ of the lamprey [21] is a UV-sensitive and bistable opsin with stable dark and light-activated states.…”

Opsins and mammalian photoentrainment

“…This opsin possesses the characteristic features of a visual opsin (equivalents of Lys-296, Glu-113, Cys-110 and Cys-187, and Asn-15), but its expression is restricted to the pineal and thus was named pinopsin. In vitro expression of chicken pinopsin and its reconstitution with 11-cis-retinal reveals that this opsin possesses a maximal absorbance (λ max ) of 470 nm (Nakamura et al 1999;Okano et al 1994) and 462 nm (Max et al 1998). Max et al (1995) confirmed the localisation of chicken pinopsin to the pineal and determined the structure of the gene.…”