The phototransduction cascade in invertebrates involves the coupling of rhodopsin activation to the action of the enzyme phospholipase C. This step is performed by G-proteins. An antibody against the α-subunit of a mouse Gq type G-protein recognized protein bands in Western blots of lateral eye and ventral nerve photoreceptors of Limulus. The protein bands had an apparent molecular mass of about 42 kDa. The antibody also recognized protein bands of a similar molecular mass in immunoblots of brain and intestine tissue. Immunoreactivity was found in lateral eye frozen sections where it was confined to the rhabdom region. When the antibody was applied to ultrathin sections of ventral nerve photoreceptors, the highest density of labeling was found on the rhabdomeral microvilli, but gold particles were also scattered in the cytoplasm. We conclude that a G-protein of the type Gq participates in the phototransduction of Limulus.
