AlF4 − activates heterotrimeric G-proteins Gα subunits but not small GDP/GTP-binding proteins like ARF1. On retinal membranes containing holotransducin (Gt α GDP-Gt βγ and incubated with ARFGDP, AlF4 − induced Gt α GDP-AlF4 release and ARFGDP binding, probably to the remaining membrane-attached Gt βγ. On phospholipid vesicles reconstitued with Gt βγ, ARFGDP bound in proportion to Gt βγ, and was released upon subsequent GtαGDP addition. Thus ARFGDP competes with GtαGDP for binding to Gtβγ, probably through a conserved motif in the ‘α2 helix’ of Gtα and ARF. This motif is found in the C-terminal helix of PH domains that bind to Gβγ.
