Heterologous expression of limulus rhodopsin

Area of Science:

• Biochemistry
• Molecular Biology
• Vision Science

Background:

• Invertebrate visual pigments, like rhodopsin, are assembled in rhabdomeric membranes of photoreceptors.
• Phototransduction, the process of converting light into electrical signals, occurs in these specialized membranes.

Purpose of the Study:

• To investigate the biosynthesis and functional expression of Limulus (horseshoe crab) rhodopsin in heterologous systems.
• To determine if Limulus opsin can correctly fold and bind its chromophore (11-cis-retinal) in non-native cellular environments.

Main Methods:

• Limulus opsin was expressed using cDNA and cRNA in three cell culture systems: mammalian COS1 cells, insect Sf9 cells, and Xenopus oocytes.
• Epitope-tagged opsin was affinity-purified and analyzed for protein folding, localization, and 11-cis-retinal binding.
• Functional assays, including light-sensitive current measurements and spectral analysis, were performed.

Main Results:

• All tested cell systems synthesized Limulus opsin polypeptide efficiently.
• None of the systems resulted in stable 11-cis-retinal binding; opsin in COS1 and Sf9 cells showed misfolding and degradation.
• Xenopus oocytes expressing Limulus opsin mRNA, but not cRNA, showed light-dependent currents.
• Transgenic Drosophila expressing Limulus opsin formed functional visual pigment with normal spectral properties.

Conclusions:

• Limulus opsin requires photoreceptor-specific factors for correct folding and chromophore binding.
• This requirement may be a general characteristic of invertebrate opsins.
• These findings may explain functional differences between invertebrate and vertebrate visual pigments.