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Published on July 18, 2008; 10.1104/pp.108.120477

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Received April 3, 2008
Accepted July 16, 2008

Functional analysis of Arabidopsis post-prenylation CaaX processing enzymes and their function in subcellular protein targeting

Keren Bracha-Drori , Keren Shichrur , Tsofnat Cohen Lubetzky , and Shaul Yalovsky *

Department of Plant Sciences, Tel Aviv University, Tel Aviv 69978, ISRAEL

* Corresponding author; email: shauly{at}tauex.tau.ac.il.

Prenylation is a posttranslational protein modification essential for developmental processes and response to abscicic acid. Following prenylation, the three C-terminal residues are proteoliticaly removed and in turn the free carboxyl group of the isoprenyl cysteine is methylated. The proteolysis and methylation, collectively referred to as CaaX processing, are catalyzed by Ste24 or Rce1 endoproteases and by an isoprenyl cysteine methyltrasnferase, ICMT. Arabidopsis contains single STE24 and RCE1 and two ICMT homologues. Here we show that in yeast AtRCE1 promoted a-mating factor secretion and membrane localization of a ROP GTPase. Furthermore, GFP fusion proteins of AtSTE24, AtRCE1, AtICMTA and AtICMTB are co-localized in the ER, indicating that prenylated proteins reach this compartment and that CaaX processing is likely required for subcellular targeting. AtICMTB can process yeast a-factor more efficiently than AtICMTA. Sequence and mutational analyses revealed that the higher activity AtICMTB is conferred by five residues, which are conserved between yeast Ste14p, human ICMT and AtICMTB but not in AtICMTA. Quantitative real-time RT-PCR and microarray data show that AtICMTA expression is significantly lower compared to AtICMTB. AtICMTA null mutants have a wild type phenotype indicating that its function is redundant. However, AtICMT RNAi lines had fasciated inflorescence stems, altered phylotaxis and developed multiple buds without stem elongation. The phenotype of the ICMT RNAi lines is similar to farnesyltrasnferase {beta}-subunit mutant era1 but is more subtle. Collectively, the data suggest that AtICMTB is likely the major isoprenyl carboxy methyltansferase and that methylation modulates activity of prenylated proteins.






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