Summarize the steps of the G protein cycle.
1. An activated receptor interact with the G protein, GTP replaces GDP on the alpha subunit, the G protein dissociates into alpha-GTP and a By dimer, the G protein is now on.
2. Activated G proteins interact with their target enzymes or ion channels.
3. An intrinsic GTPase activity of the alpha subunit hydrolyzes GTP to GDP, alpha-GDP and the By dimer reassociate inactivating the G protein.