Deactivation of visual transduction without guanosine triphosphate hydrolysis by G protein.

G proteins couple receptors to their target enzymes in many signal transduction cascades. It has generally been thought that deactivation of such cascades cannot occur without the hydrolysis of guanosine triphosphate (GTP) by G protein. This requirement has now been reexamined in both vertebrate and invertebrate phototransduction. Results indicate that GTP hydrolysis is not required for deactivation. Evidence is presented for an alternative model in which the target enzyme is deactivated by an inhibitory factor that is available even when GTP hydrolysis is blocked.