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 (Gta), which is highly homologous to human retinal rod cell-specific Gt 1 α. Concurrent cDNA cloning of chicken retinal Gt 1 α and Gt 2 α (rod and cone cell-specific α-subunits of transducin, respectively) revealed that the chicken pineal Gta is identical to the retinal Gt 1 α. Double-immunostaining analysis of the chicken pineal sections localized Gt 1 α-immunoreactivity in the rudimentary outer segments of both follicular and parafollicular pinealocytes that were immunopositive to anti-pinopsin antibody. To examine whether pineal Gt 1 α is involved in the pineal phototransduction pathway, trypsin protection assay was applied for detecting the conversion of GDP-bound Gt 1 α into the guanosine 5'-O-(3-thiotriphosphate) (GTPyS)-bound form in the pineal membrane homogenate. It was clearly demonstrated that the pineal Gt 1 α is activated in a light-dependent manner in the presence of GTPyS. These data together suggest strongly that pineal Gt 1 α mediates the phototransduction pathway triggered by pinopsin in the chicken pinealocytes.