Mammalian protein farnesyl transferases are heterodimeric proteins containing two nonidentical alpha and beta subunits that attach farnesyl residues to a cysteine at the fourth position from the COOH terminus of several proteins, including nuclear lamins and p21Ras proteins. The natural substrates contain the Cys-A-A-Xaa recognition sequence, where the A residues are aliphatic and Xaa represents methionine, serine, glutamine or cysteine. The purified farnesyl transferase is an alpha-beta heterodimer. The beta subunit binds the peptide substrate while the alpha subunit is suspected to participate in formation of a stable complex with the substrate farnesyl pyrophosphate. The alpha subunit is shared with a second prenyl transferase, geranyl-geranyl transferase, that attaches 20 carbon geranylgeranyl to Ras related proteins that terminate in a Cys-A-A-Xaa recognition site in which Xaa is leucine.