THE SMALL G-PROTEIN ARF1(GDP) BINDS TO THE G(T)BETA-GAMMA SUBUNIT OF TRANSDUCIN, BUT NOT TO G(T)ALPHA(GDP)-G(T)BETA-GAMMA

AlF4- activates heterotrimeric G-proteins G alpha subunits but not small GDP/GTP-binding proteins like ARF1. On retinal membranes containing holotransducin (G(t) alpha(GDP)-G(t) beta gamma) and incubated with ARF(GDP), AlF4- induced Ct alpha(GDP-AlF4) lease and ARF(GDP) binding, probably to the remaining membrane-attached G(t) beta gamma. On phospholipid vesicles reconstituted with G(t) beta gamma, ARF(GDP) bound in proportion to G(t) beta gamma, and was released upon subsequent G(t) alpha(GDP) addition. Thus ARF(GDP) competes with G(y) alpha(GDP) for binding to G(t) beta gamma, probably through a conserved motif in the 'alpha 2 helix' of G(t) alpha and ARF. This motif is found in the C-terminal helix of PH domains that bind to G beta gamma.