protein geranylgeranyltransferase activity CAAX-protein geranylgeranyltransferase activity https://github.com/geneontology/go-ontology/issues/25798 https://github.com/geneontology/go-ontology/issues/30193 protein geranylgeranyltransferase type I GO:0004662 geranylgeranyltransferase type I activity GGTaseI activity MetaCyc:RXN-3701 EC:2.5.1.59 Reactome:R-HSA-9954195 type I protein geranyl-geranyltransferase activity GGTase-I activity Catalysis of the reaction: geranylgeranyl diphosphate + protein-cysteine = S-geranylgeranyl-protein + diphosphate. This reaction is the formation of a thioether linkage between the C-1 atom of the geranylgeranyl group and a cysteine residue fourth from the C-terminus of the protein. The protein substrates have the C-terminal sequence CA1A2X, where the terminal residue, X, is preferably leucine and A2 should not be aromatic. Known substrates include most g-subunits of heterotrimeric G proteins and Ras-related GTPases such as members of the Ras and Rac/Rho families. Reactome:R-HSA-9947999 geranylgeranyl-diphosphate:protein-cysteine geranyltransferase activity molecular_function