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 among the key proteins mediating phototransduction in rod and cone cells of the vertebrate retina. Even though this protein has been extensively characterized in mammals, little is known about its expression patterns in migratory songbirds. Here we characterised Gt expression in the European robin, a night-migratory songbird known for its light-dependent magnetoreception. The mechanism underlying magnetoreception is not fully understood, but one well-supported hypothesis involves a 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. Specifically, we combined single cell RNA sequencing and immunohistochemistry, and tested for protein interaction by pulldown, co-immunoprecipitation, and NanoBiT luminescence assays. We show that genes for G-protein subunits GNB1 and GNB3 (coding for Gt{beta}1 and Gt{beta}3, respectively) are predominantly expressed in rods and cones. Among {gamma}-subunits, GNGT2 (coding for Gt{gamma}T2) was the principal isoform in cones, whereas GNG11 (coding for Gt{gamma}11) was associated with rods. In contrast, we did not detect GNG10 (coding for 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. Furthermore, the absence of GNG10 expression in rods and cones does not support a role of this {gamma}-subunit in photoreceptor-based magnetoreception.