RESUMO
A long-standing concept in vision science has held that a single photoreceptor expresses a single type of opsin, the protein component of visual pigment. However, the number of examples in the literature of photoreceptors from vertebrates and invertebrates that break this rule is increasing. Here, we describe a newly discovered Limulus opsin, Limulus opsin5, which is significantly different from previously characterized Limulus opsins, opsins1 and 2. We show that opsin5 is co-expressed with opsins1 and 2 in Limulus lateral and ventral eye photoreceptors and provide the first evidence that the expression of co-expressed opsins can be differentially regulated. We show that the relative levels of opsin5 and opsin1 and 2 in the rhabdom change with a diurnal rhythm and that their relative levels are also influenced by the animal's central circadian clock. An analysis of the sequence of opsin5 suggests it is sensitive to visible light (400-700 nm) but that its spectral properties may be different from that of opsins1 and 2. Changes in the relative levels of these opsins may underlie some of the dramatic day-night changes in Limulus photoreceptor function and may produce a diurnal change in their spectral sensitivity.
Assuntos
Relógios Biológicos/efeitos da radiação , Ritmo Circadiano/efeitos da radiação , Caranguejos Ferradura/metabolismo , Caranguejos Ferradura/efeitos da radiação , Luz , Opsinas/metabolismo , Células Fotorreceptoras de Invertebrados/metabolismo , Sequência de Aminoácidos , Animais , Anticorpos , Relógios Biológicos/genética , Membrana Celular/metabolismo , Membrana Celular/efeitos da radiação , Ritmo Circadiano/genética , Etídio/metabolismo , Fluorescência , Secções Congeladas , Regulação da Expressão Gênica/efeitos da radiação , Caranguejos Ferradura/genética , Medições Luminescentes , Dados de Sequência Molecular , Opsinas/química , Opsinas/genética , Opsinas/imunologia , Células Fotorreceptoras de Invertebrados/citologia , Células Fotorreceptoras de Invertebrados/efeitos da radiação , Filogenia , Transporte de RNA/efeitos da radiação , RNA Mensageiro/genética , RNA Mensageiro/metabolismo , Alinhamento de Sequência , Homologia de Sequência de Aminoácidos , Transdução de Sinais/efeitos da radiaçãoRESUMO
Arrestins participate in the termination of phototransduction in both vertebrates and invertebrates. However, the visual arrestins of invertebrates and vertebrates differ significantly from one another in that the invertebrate visual arrestins become phosphorylated rapidly in response to light while those in the photoreceptors of vertebrates do not. In an effort to understand the functional relevance of arrestin phosphorylation, we examined this process in the photoreceptors of the horseshoe crab Limulus polyphemus. We report that Limulus visual arrestin can be phosphorylated at three sites near its C-terminus and show that arrestin molecules phosphorylated on one, two, and three sites are normally present in both light- and dark-adapted photoreceptors. Light adaptation increases the amount of arrestin phosphorylated at three sites.
