Immunoelectron-microscopic study of G-protein distribution in photoreceptor cells of the cephalopod Sepia officinalis.

Previous studies suggest that more than one single light-stimulated transduction pathway seems to be present in photoreceptor cells of invertebrates. Accordingly, more than one light-dependent G-protein has been detected in squid photoreceptor cells. Two different antibodies were used to locate the distribution of G-proteins in Sepia photoreceptors. One antiserum (anti-G alpha-common) has been raised against the peptide CGAGESGKSTIVKQMK. This sequence is found in most G alpha-protein subunits, it is also present in transducin of vertebrates. This sequence however, is conserved only partially in G alpha q from photosensory membranes of the squid. The second antiserum (anti-G alpha q) has been raised against the peptide QLNLKEYNLV. This sequence is present at the C-terminus of e.g. G alpha q from mouse brain and is identical also in squid. Anti-G alpha q very strongly interacted with the rhabdomeres of the photoreceptor cells from Sepia. In the non-rhabdomeric cytoplasm of the photoreceptor cells gold granules (bound to the second antibody) were almost absent. With anti-G alpha-common, only a small number of gold particles could be found at the rhabdomeres. The non-rhabdomeric cytoplasm was not labelled. Thus, further G-proteins that might be involved in a light dependent phototransduction mechanism besides the phosphoinositide pathway can hardly be expected to belong to the group of 'common' G-proteins.