Rod-type transducin α-subunit mediates a phototransduction pathway in the chicken pineal gland

The chicken pineal gland is a photosensitive neuroendocrine organ producing melatonin in circadian clock-regulated and light-sensitive manners. To understand the relationship between the photoreceptive molecule pinopsin and the light-dependent melatonin suppression that is sensitive to pertussis toxin treatment, we have searched for pertussis toxin-sensitive G protein α- subunits expressed in the chicken pineal gland. Here we report the cDNA cloning of the pineal transducin α-subunit (Gtα), which is highly homologous to human retinal rod cell-specific Gt₁α. Concurrent cDNA cloning of chicken retinal Gt₁α and Gt₂α (rod and cone cell-specific α-subunits of transducin, respectively) revealed that the chicken pineal Gtα is identical to the retinal Gt₁α. Double-immunostaining analysis of the chicken pineal sections localized Gt₁α-immunoreactivity in the rudimentary outer segments of both follicular and parafollicular pinealocytes that were immunopositive to anti-pinopsin antibody. To examine whether pineal Gt₁α is involved in the pineal phototransduction pathway, trypsin protection assay was applied for detecting the conversion of GDP-bound Gt₁α into the guanosine 5'-O-(3-thiotriphosphate) (GTPγS)-bound form in the pineal membrane homogenate. It was clearly demonstrated that the pineal Gt₁α is activated in a light-dependent manner in the presence of GTPγS. These data together suggest strongly that pineal Gt₁α mediates the phototransduction pathway triggered by pinopsin in the chicken pinealocytes.