Nonidentical subunits of p21^H-ras farnesyltransferase: Peptide binding and farnesyl pyrophosphate carrier functions

The protein farnesyltransferase purified from rat brain contains two nonidentical subunits, α and β. The holoenzyme forms a stable complex with [³H]farnesyl pyrophosphate (FPP) that can be isolated by gel filtration. The [³H]FPP is not covalently bound to the enzyme; it is released unaltered when the enzyme is denatured. When incubated with an acceptor such as p21^H-ras, the complex transfers [³H]farnesyl from the bound [³H]FPP to the ras protein. This transfer is not sensitive to dilution by unbound FPP, suggesting that the [³H]FPP is bound at a site that leads to direct transfer to the p21^H-ras acceptor. Cross-linking studies show that the p21^H-ras binds to the lower molecular weight subunit (β-subunit), raising the possibility that the [³H]FPP binds to the α-subunit. If this suggestion can be confirmed, it would invoke a reaction mechanism in which the α-subunit acts as a prenyl pyrophosphate carrier that delivers FPP to p21^H-ras which is bound to the β-subunit.