Squid photoreceptor membranes contain several small GTP-binding proteins (22–27 kDa) in addition to heterotrimeric G-proteins. In addition, these membranes contain several proteins which can be ADP-ribosylated by exoenzyme C3 from Clostridium botulinum (C3). These proteins are similar but not identical to C3 substrates from bovine rod outer segments. It is shown that substrates of C3 in squid photoreceptor interact with rhodopsin in a light-dependent manner. These data suggest involvement of small GTP-binding proteins in phototransduction processes in invertebrates.
