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B.- . Battelle, C. Katti, D. R. Dugger, Jr., A. Legg; A Newly Identified Opsin With a Potential Role in Diurnal Regulation. Invest. Ophthalmol. Vis. Sci. 2008;49(13):147. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
To clone and characterize a newly identified opsin transcript and analyze its distribution and diurnal regulation.
A 701 base pair contig encoding a previously unidentified opsin-like protein was sequenced from an EST collection prepared from the CNS of juvenile Limulus polyphemus by L. Moroz and collaborators. The full-length sequence was amplified by PCR from a ventral eye EST collection. The C-terminus of the predicted protein was expressed in E.coli and used to produce a specific monoclonal antibody. The antiboy was applied in immunocytochemical assays to determine the distribution of the new opsin.
The full-length sequence revealed that this new transcript encodes an opsin with seven predicted transmembrane domains, sequences characteristic for rhabdomeric opsins, two highly conserved cysteines which form a disulfide bond, and a highly conserved lysine at the retinal binding site. We call this protein Limulus polyphemus opsin5 (LpOP5) because four different opsin genes were identified previously in Limulus. LpOP5 aligns most closely with the previously cloned LpOP1 and 2 but is only 47% identical with these proteins at the amino acid level while LpOP1 and 2 are 99% identical to one another. Significantly, LpOP5 has a threonine in the position of the Shiff base counterion instead of the tyrosine typically observed at this position in visible light sensitive opsins of invertebrates including LpOP1 and 2. LpOP5 also has a predicted palmitoylation site in its C-terminus which is absent in LpOP1 and 2. Immunocytochemical studies revealed that LpOP5 is present in the rhabdomers of photoreceptors in lateral and ventral but not median eyes. LpOP1 and 2, on the other hand, are detected in all three eyes. Furthermore, unlike LpOP1, LpOP5 is not removed from lateral eye rhabdomeral membranes by light-driven shedding, a process that involves arrestin.
A new Limulus opsin (LpOP5) is expressed in lateral and ventral eye photoreceptors. The presence of an unusual threonine in the position of the Shiff base counterion indicates its spectral sensitivity may be different from LpOP1 and 2. In contrast to LpOP1, LpOP5 is not significantly internalized by light-driven shedding during the day; therefore the relative levels of LpOP5 and 1 in the lateral eye rhabdom probably change with a diurnal rhythm. The failure of LpOP5 to be internalized by light-driven shedding, a process involving arrestin, also suggests that LpOP5 and 1 interact differently with arrestin.
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