Disturbing GTP-binding protein function through microinjection into the visual cell of Limulus.

: We have tested the action of three agents microinjected into the ventral nerve photoreceptor of Limulus on the electrical response to dim light. 1. A monoclonal antibody (mAb 4A) against the G alpha subunit of frog transducin reduces the size of the receptor current to 60%, suggesting an interaction with G alpha in the Limulus photoreceptor. 2. Injection of Clostridium botulinum ADP-ribosyltransferase C3 reduces the size to 46%; latency is not affected. The results imply that small GTP-binding proteins play a functional role in photoreception of invertebrates. 3. Injection of GDP-beta-S reduces dose-dependently the size of the receptor current to 15% and prolongs the latency to 200%, presumably by reducing number and rate of G-protein activations.