Prenylation (also known as isoprenylation or lipidation) is the addition of hydrophobic molecules to a protein or a biomolecule. It is usually assumed that prenyl groups (3-methylbut-2-en-1-yl) facilitate attachment to cell membranes, similar to lipid anchors like the GPI anchor, though direct evidence of this has not been observed. Prenyl groups (also called isoprenyl groups, having one hydrogen atom more than isoprene) have been shown to be important for protein–protein binding through specialized prenyl-binding domains. Protein prenylation Protein prenylation involves the transfer of either a farnesyl or a geranylgeranyl moiety to C-terminal cysteine(s) of the target protein. There are three enzymes that carry out prenylation in the cell, farnesyl transferase, Caax protease and geranylgeranyl transferase I. Farnesylation is a type of prenylation, a post-translational modification of proteins by which an isoprenyl group is added to a cysteine residue. It is an important process to mediate protein–protein interactions and protein–membrane interactions. Prenylation sites There are at least 3 types of sites that are recognized by prenylation enzymes. The CaaX motif is found at the COOH-terminus of proteins, such as lamins or Ras. The motif consists of a cysteine (C), two aliphatic amino acids ("aa") and some other terminal amino acid ("X"). If the X position is serine, alanine, or methionine, the protein is farnesylated. For instance, in rhodopsin kinase the sequence is CVLS. If X is leucine, the protein is geranylgeranylated. The second motif for prenylation is CXC, which, in the Ras-related protein Rab3A, leads to geranylgeranylation on both cysteine residues and methyl esterification. The third motif, CC, is also found in Rab proteins, where it appears to direct only geranylgeranylation but not carboxyl methylation. Carboxyl methylation only occurs on prenylated proteins. Farnesyltransferase and geranylgeranyltransferase I Farnesyltransferase and geranylgeranyltransferase I are very similar proteins. They consist of two subunits, the α-subunit, which is common to both enzymes, and the β-subunit, whose sequence identity is just 25%. These enzymes recognise the CaaX box at the C-terminus of the target protein. C is the cysteine that is prenylated, a is any aliphatic amino acid, and the identity of X determines which enzyme acts on the protein. Farnesyltransferase recognizes CaaX boxes where X = M, S, Q, A, or C, whereas geranylgeranyltransferase I recognizes CaaX boxes with X = L or E. Rab geranylgeranyl transferase Rab geranylgeranyltransferase, or geranylgeranyltransferase II, transfers (usually) two geranylgeranyl groups to the cysteine(s) at the C-terminus of Rab proteins.