UNCOUPLING OF GTP-BINDING FROM TARGET STIMULATION BY A SINGLE MUTATION IN THE TRANSDUCIN-ALPHA SUBUNIT

Glutamic acid-203 of the alpha subunit of transducin (alpha(T)) reside s within a domain that undergoes a guanosine triphosphate (GTP)-induce d conformational change that is essential for effector recognition. Ch anging the glutamic acid to an alanine in bovine alpha(T) yielded an a lpha subunit (alpha(T)E203A) that was fully dependent on rhodopsin for GTP-guanosine diphosphate (GDP) exchange and showed GTP hydrolytic ac tivity similar to that measured for wild-type alpha(T). However, unlik e the wild-type protein, the GDP-bound form of alpha(T)E203A was const itutively active toward the effector of transducin, the cyclic guanosi ne monophosphate phosphodiesterase. Thus, the alpha(T)E203A mutant rep resents a short-circuited protein switch that no longer requires GTP f or the activation of the effector target phosphodiesterase.