is one of the three enzymes
in the prenyltransferase
group. Farnesyltransferase (FTase) adds a 15-carbon isoprenoid
called a farnesyl group
bearing a CaaX motif
: a four-amino acid
sequence at the carboxyl terminus
of a protein. Farnesyltransferase's targets include members of the Ras superfamily of small GTP-binding proteins
critical to cell cycle
progression. For this reason, several FTase inhibitors
are undergoing testing as anti-cancer agents. FTase inhibitors have shown efficacy as anti-parasitic agents, as well. FTase is also believed to play an important role in development of progeria
and various forms of cancers
proteins by adding an isoprenoid lipid
called a farnesyl group
to the -SH of the cysteine near of the end of the target protein to form a thioether
linkage. This process, called farnesylation (the more general term prenylation
), causes farnesylated proteins to become membrane
-associated due to the hydrophobic
nature of the farnesyl group. Most farnesylated proteins are involved in cellular signaling
wherein membrane association is critical for function.
Farnesyltransferase structure and function
Farnesyltransferase has two subunits
: a 48kDa alpha subunit and a 46kDa beta subunit. Both subunits are primarily composed of alpha helices
. The α subunit is made of a double layer of paired alpha helices stacked in parallel, which wraps partly around the beta subunit like a blanket. The alpha helices of the β subunit form a barrel. The active site is formed by the center of the β subunit flanked by part of the α subunit. Farnesyltransferase coordinates a zinc cation
on its β subunit at the lip of the active site. Farnesyltransferase has a hydrophobic binding pocket for farnesyl diphosphate
, the lipid donor molecule. All farnesyltransferase substrates have a cysteine
as their fourth-to-last residue. This cysteine engages in an SN2
type attack, coordinated by the zinc and a transient stabilizing magnesium
ion on the farnesyl diphosphate, displacing the diphosphate. The product remains bound to farnesyltransferase until displaced by new substrates. The last three amino acids of the CaaX motif are removed later.
There are four binding pockets in FTase, which accommodate the last four amino acids on the carboxyl-terminus of a protein. Only those with a suitable CaaX motif can bind (C is Cysine, A is an aliphatic
amino acid, and X is variable). The carboxyl-terminal amino acid (X) discriminates FTase’s targets from those of the other prenyltransferases, allowing only six different amino acids to bind with any affinity. It has been shown that geranylgeranyltransferase
one of the other prenyltransferases can prenylate some of the substrates of Farnesyltransferase and vice versa.
- Reid, T. Scott, Terry, Kimberly L., Casey, Patrick J., Beese, Lorena S., (2004) Crystallographic Analysis of CaaX prenyltransferases Complexed with Substrates Defines Rules of Protein Substrate Selectivity, J. Mol. Bio, 343, 417-433.
- Eastman, Richard T., Buckner, Frederick S., et al., (2006) Fighting parasitic disease by Blocking Protein Farnesylation, Journal of Lipid Research, 47, 233-240.
- Beese, Lorena, S., Lane, Kimberly T. Structural biology of protein farnesyltransferase and geranylgeranyltransferase type 1. Journal of Lipid Research, 47, 68 –698.
- Long, Stephen B., Casey, Patrick J., Beese, Lorena S., Reaction path of protein farnesyltransferase at atomic resolution. Nature, 419, 645-650.