When inactive, these heterotrimeric proteins are associated with transmembrane receptors.  
When inactive, these heterotrimeric proteins are associated with transmembrane receptors.  When active, a subunit's dissociation activates an enzyme, typically adenylyl cyclase, triggering the formation of a second messenger.  The subunit's ability to hydrolyze GTP, causing the trimer to reassemble, regulates the activity of, for ten points, what protein class, whose malfunctions are linked to cholera and diabetes, and which won Alfred Gilman and Martin Rodbell a 1994 Nobel Prize.
When inactive, these heterotrimeric proteins are associated with transmembrane receptors.  
G proteins