Geranylgeranylation is a form of prenylation, which is a post-translational modification of proteins that involves the attachment of one or two 20-carbon lipophilic geranylgeranyl isoprenoid units from geranylgeranyl diphosphate to one or two cysteine residue(s) at the C-terminus of specific proteins.
Prenylation (including geranylgeranylation) is thought to function, at least in part, as a membrane anchor for proteins.
GGTase I catalyzes the addition of one geranylgeranyl group onto the C-terminal consensus sequence CAAL (somewhat similar to farnesyltransferase reactions), where C=cysteine, A=any aliphatic amino acid, and L=leucine.
Rab GGTase adds a total of two geranylgeranyl groups onto two cysteine residues at the C-terminal consensus sequence CXC or XXCC.
[2] An example of this can be seen in the lipid anchoring of the Rho GTPase family of signaling molecules and the gamma subunit of heterotrimeric G proteins.