-!- This enzyme, along with EC 2.5.1.58 and EC 2.5.1.60, constitutes the protein prenyltransferase family of enzymes. -!- Catalyzes 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. -!- These protein acceptors have the C-terminal sequence CA(1)A(2)X, where the terminal residue, X, is preferably leucine; serine, methionine, alanine or glutamine makes the protein a substrate for EC 2.5.1.58. -!- The enzymes are relaxed in specificity for A(1), but cannot act if A(2) is aromatic. -!- Known targets of this enzyme include most gamma-subunits of heterotrimeric G proteins and Ras-related GTPases such as members of the Ras and Rac/Rho families. -!- The Zn(2+) is required for peptide, but not for isoprenoid, substrate binding.