Protein prenyltransferases are heterodimeric enzymes that catalyze the transfer of the carbon moiety of C[[15]] farnesyl pyrophosphate (FPP) or C[[20]] geranylgeranyl pyrophosphate (GGPP) to a conserved cysteine residue in a CaaX motif of protein and peptide substrates. This postranslational modification is essential for a number of proteins including Ras, Rab, nuclear lamins, trimeric G-protein gamma subunits, protein kinases, and small Ras-related GTP-binding proteins. The addition of a farnesyl group to these proteins, for example, is required to anchor them to the cell membrane, a step required for them to function. (Adapted from PMID:12135472, PMID:12702202 and PMID:26216239.)