Arabidopsis thaliana plants possess a specific farnesylcysteine lyase that is involved in detoxification and recycling of farnesylcysteine.

SummaryIn plants, prenylated proteins are involved in actin organization, calcium-mediated signal transduction, andmany other biological processes. Arabidopsis thaliana mutants lacking functional protein prenyltransferasegeneshavealsorevealedrolesforprenylatedproteinsinphytohormonesignalingandmeristemdevelopment.However,todate,theturnoverofprenylatedplantproteinsandthefateoftheprenylcysteine(PC)residuehavenot been described. We have detected an enzyme activity in Arabidopsis plants that metabolizes farnesylcy-steine(FC)tofarnesal,whichissubsequentlyreducedtofarnesol.Unlikeits mammalianortholog,ArabidopsisFC lyase exhibits specificity for FC over geranylgeranylcysteine (GGC), and recognizes N-acetyl-FC (AFC). FClyase is encoded by a gene on chromosome 5 of the Arabidopsis genome (FCLY, At5g63910) and isubiquitously expressed in Arabidopsis tissues and organs. Furthermore, T-DNA insertions into the FCLY genecause significant decreases in FC lyase activity and an enhanced response to abscisic acid (ABA) in seedgermination assays. The effects of FCLY mutations on ABA sensitivity are even greater in the presence ofexogenous FC. These data suggest that plants possess a specific FC detoxification and recycling pathway.Keywords: ABA, Arabidopsis, farnesylcysteine, farnesylcysteine lyase, isoprenoid recycling.IntroductionMost prenylated proteins possess a farnesylcysteine (FC) orgeranylgeranylcysteine (GGC) methyl ester at the C-termi-nus(Clarke,1992;ZhangandCasey,1996).Thismodificationis generated by the successive actions of protein prenyl-transferases, prenylprotein proteases and prenylcysteine(PC) methyltransferases, which have been described inplants and other eukaryotes (Clarke, 1992; Crowell, 2000;Zhang and Casey, 1996). Collectively, these modificationspromote the association of prenylated proteins with mem-branes and/or other proteins.Whereas much is known about the biosynthesis, modi-fication and functions of prenylated plant proteins, less isknown about the turnover of these proteins and the fate ofthe PC residue. Proteolytic degradation of prenylatedproteins generates PC, which retains the thioether linkagebetween the amino acid and the prenyl group. In mamma-lian cells, PC is metabolized to cysteine and a prenylaldehyde (i.e. farnesal or geranylgeranial) by an unusualthioether oxidase called PC lyase (Digits et al., 2002;Tschantz et al., 1999, 2001; Zhang et al., 1997; Figure 1).However, PCs and PC methyl esters can also be convertedto the corresponding sulfoxides by flavin-dependent mono-oxygenases (FMOs) or to prenylthiols by cysteine b-lyases,and are potential substrates for cytochrome P450s (Cash-man et al., 1990; Park et al., 1992, 1994; Sausen and Elfarra,1990). PC lyase is a lysosomal, membrane-associatedflavoprotein that catalyzes C–S bond cleavage by a uniquemechanism involving stereospecific removal of the pro-Shydrogen at the C1 position of the prenyl moiety, with non-covalently bound FAD functioning as an electron acceptor,followed by hydrolysis of the resulting thiocarbenium ion toa hemithioacetal intermediate (Digits et al., 2002; Tschantz