Expression patterns and interaction profiles of heterotrimeric transducin subunits in the retina of the European robin (Erithacus rubecula)

The heterotrimeric G-protein transducin (Gt) is mediating phototransduction in rod and cone cells of the vertebrate retina, but its expression patterns in migratory songbirds is unknown. We characterised Gt expression in the European robin, a night-migratory songbird known for its light-dependent magnetoreception. One well-supported hypothesis of magnetoreception involves radical-pair formation in the blue light receptor cryptochrome type 4a. The - and {gamma}-subunits of cone specific transducin have been identified as possible interaction partners of cryptochrome 4a. Therefore, we analysed the expression patterns of various G-protein subunits in bird photoreceptors by combining single cell RNA sequencing and immunohistochemistry. Protein-protein interaction was tested by pulldown, co-immunoprecipitation, and NanoBiT luminescence assays. G-protein subunits Gt{beta}1 and Gt{beta}3 are predominantly expressed in rods and cones, and Gt{gamma}T2 was the principal isoform in cones, whereas Gt{gamma}11 was associated with rods. In contrast, we did not detect Gt{gamma}10 expression in either photoreceptor type. Interaction assays demonstrated that all three {beta}{gamma} combinations; {beta}{gamma}T2, {beta}{gamma}10, and {beta}{gamma}11, can associate in vitro. These findings indicate that {beta}{gamma} dimer formation in vivo is likely constrained by the photoreceptor-specific expression of the respective subunits. The absence of Gt{gamma}10 expression in rods and cones does not support a role in photoreceptor-based magnetoreception.