Catalyzes the transfer of a geranyl-geranyl moiety from geranyl-geranyl pyrophosphate to proteins having the C-terminal-XCC or -XCXC, where both cysteines may become modified. geranylgeranyl diphosphate + L-cysteinyl-[protein] = diphosphate + S-geranylgeranyl-L-cysteinyl-[protein] Heterodimer of an alpha and a beta subunit. Belongs to the protein prenyltransferase subunit alpha family.