Heterotrimeric GTP-binding proteins (G proteins) play an important role in phototransduction. The presence of G-protein subclasses has been reported in photoreceptive membranes, e.g., the Gi subgroup (transducin) in vertebrate rods, and the Gq subgroup in the eyes of the Arthropoda and the Mollusca. We examined the immunoreactivity and distribution of a Gq homologue in the cerebral ocelli of Perinereis brevicirris (Polychaeta, Annelida) using an anti-GqC antibody raised against a conserved sequence at the C-terminal of the alpha-subunit of Gq (Gq-alpha). The anti-GqC antibody labeled a 48-kDa band on the Western blot of proteins from the Perinereis ocelli. The anti-GtC antibody, which is raised against the C-terminal sequence of bovine transducin alpha-subunit (Gt-alpha), did not cross-react to the ocellar proteins of Perinereis. The rhabdomeric layers of the anterior and posterior ocelli were strongly labeled by anti-GqC on light-microscopic immunohistology. Immunoelectron microscopy showed that the Gq molecules were specifically localized in the photoreceptive membrane of the rhabdomeric microvilli. These results suggest that the Gq protein plays a role in the phototransduction of the Perinereis ocelli.

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Functional ultrastructural changes in the rhabdomeric photoreceptors of the cerebral ocelli are described for normal and sexually mature (epitoke) Perinereis brevicirris (Polychaeta, Annelida). With sexual maturation, the cerebral ocelli hypertrophied, increasing in volume to 5.5 times that of ocelli in the normal state, and the thickness of the retinal layer increased up to 10 times. Perinereis ocelli have a pigmented retinal layer consisting of at least two cell types: photoreceptor cell (PR) and pigmented supporting cells (PS). In epitoke ocelli, PR bear well-developed rhabdomeric microvilli, multilamellar bodies, and numerous cytoplasmic membranous structures, including vesicles, smooth endoplasmic reticulum, and secondary lysosomes. Localization of a visual Gq protein in the ocelli was studied with anti-GqC antibody. The antibody strongly labeled not only microvilli and multilamellar bodies throughout the retinal layer, but also secondary lysosomes and vesicles in the cytoplasm of the PR in the epitoke ocelli, although labeling was observed only in the microvilli and multilamellar bodies in normal ocelli. Reverse transcription/polymerase chain reaction analysis revealed that the amount of G protein alpha subunit mRNA in the epitoke head increased by roughly twice that of the normal head. Since Gq protein is essential for phototransduction in Perinereis ocelli, these results suggest that the sites are involved in photoreceptive membrane turnover, which occurs much more extensively in epitoke ocelli. Thus, epitoke ocelli may represent a model system for studying rhabdomeric photoreceptive membrane turnover.

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