Pinopsin is a photoreceptive molecule present in the outer segments of chicken pinealocytes. In this paper, the localization of α-subunits of G-proteins, rod transducin (Gt1) and Gq/11, was examined by immunoelectron microscopy to investigate whether these G-proteins colocalize with pinopsin in the outer segments. Ultrathin sections of the chicken pineal gland were double-immunolabeled with antibodies to pinopsin and either Gt1α or Gq/11α. As shown previously, the outer segments around the follicular lumen exhibited divergent morphology with ciliary, bulbous, or lamellate shapes, and most of them displayed pinopsin immunoreactivity. The majority (> 90%) of pinopsin-immunopositive outer segments were labeled by anti-Gt1α and/or anti-Gq/11α antibodies. Application of double-immunolabeling to serial sections demonstrated that a large number of the pinopsin-immunopositive outer segments contained both Gt1α and Gq/11α immunoreactivities. These results suggest that Gt1α and Gq/11α are functionally coupled with light-activated pinopsin within a single outer segment.
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