Prenylation is a class of lipid modification involving covalent addition of either farnesyl (15-carbon) or geranylgeranyl (20-carbon) isoprenoids to conserved cysteine residues at or near the C-terminus of proteins. Known prenylated proteins include fungal mating factors, nuclear lamins, Ras and Ras-related GTP-binding proteins (G proteins), the subunits of trimeric G proteins, protein kinases, and at least one viral protein. Prenylation promotes membrane interactions of most of these proteins, which is not surprising given the hydrophobicity of the lipids involved. In addition, however, prenylation appears to play a major role in several protein-protein interactions involving these species. The emphasis in this review is on the enzymology of prenyl protein processing and the functional significance of prenylation in cellular events. Several other recent reviews provide more detailed coverage of aspects of prenylation that receive limited attention here owing to length restrictions (1-4).

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Association of oncogenic ras proteins with cellular membranes appears to be a crucial step in transformation. ras is synthesized as a cytosolic precursor, which is processed to a mature form that localizes to the plasma membrane. This processing involves, in part, a conserved sequence, Cys-Ali-Ali-Xaa (in which Ali is an amino acid with an aliphatic side chain and Xaa is any amino acid), at the COOH terminus of ras proteins. Yeast a-factor mating hormone precursor also possesses a COOH-terminal Cys-Ali-AliXaa sequence. However, while the COOH-terminal cysteine has been implicated as a site of palmitoylation of ras proteins, in mature a-type mating factor this residue is modified by an

The ability of cells to communicate with and respond to their external environment is critical for their continued existence. A universal feature of this communication is that the external signal must in some way penetrate the lipid bilayer surrounding the cell. In most cases of such signal acquisition, the signaling entity itself does not directly enter the cell but rather transmits its information to specific proteins present on the surface of the cell membrane. These proteins then communicate with additional proteins associated with the intracellular face of the membrane. Membrane localization and function of many of these proteins are dependent on their covalent modification by specific lipids, and it is the processes involved that form the focus of this article.