Protein Farnesylation Takes Part in Arabidopsis Seed Development

Methyl-Jasmonate Functions as a Molecular Switch Promoting Cross-Talk between Pathways for the Biosynthesis of Isoprenoid Backbones Used to Modify Proteins in Plants

In plants, the plastidial mevalonate (MVA)-independent pathway is required for the modification with geranylgeranyl groups of CaaL-motif proteins, which are substrates of protein geranylgeranyltransferase type-I (PGGT-I). As a consequence, fosmidomycin, a specific inhibitor of 1-deoxy-d-xylulose (DX)-5 phosphate reductoisomerase/DXR, the second enzyme in this so-called methylerythritol phosphate (MEP) pathway, also acts as an effective inhibitor of protein prenylation. This can be visualized in plant cells by confocal microscopy by expressing GFP-CaM-CVIL, a prenylation sensor protein. After treatment with fosmidomycin, the plasma membrane localization of this GFP-based sensor is altered, and a nuclear distribution of fluorescence is observed instead. In tobacco cells, a visual screen of conditions allowing membrane localization in the presence of fosmidomycin identified jasmonic acid methyl esther (MeJA) as a chemical capable of gradually overcoming inhibition. Using Arabidopsis protein prenyltransferase loss-of-function mutant lines expressing GFP-CaM-CVIL proteins, we demonstrated that in the presence of MeJA, protein farnesyltransferase (PFT) can modify the GFP-CaM-CVIL sensor, a substrate the enzyme does not recognize under standard conditions. Similar to MeJA, farnesol and MVA also alter the protein substrate specificity of PFT, whereas DX and geranylgeraniol have limited or no effect. Our data suggest that MeJA adjusts the protein substrate specificity of PFT by promoting a metabolic cross-talk directing the origin of the prenyl group used to modify the protein. MVA, or an MVA-derived metabolite, appears to be a key metabolic intermediate for this change in substrate specificity.

A plant protein farnesylation system in prokaryotic cells reveals Arabidopsis AtJ3 produced and farnesylated in E. coli maintains its function of protecting proteins from heat inactivation

BACKGROUND

Protein farnesylation involves the addition of a 15-carbon polyunsaturated farnesyl group to proteins whose C-terminus ends with a CaaX motif. This post-translational protein modification is catalyzed by a heterodimeric protein, i.e., farnesyltransferase (PFT), which is composed of an α and a β subunit. Protein farnesylation in plants is of great interest because of its important roles in the regulation of plant development, responses to environmental stresses, and defense against pathogens. The methods traditionally used to verify whether a protein is farnesylated often require a specific antibody and involve isotope labeling, a tedious and time-consuming process that poses hazardous risks.

RESULTS

Since protein farnesylation does not occur in prokaryotic cells, we co-expressed a known PFT substrate (i.e., AtJ3) and both the α and β subunits of Arabidopsis PFT in E. coli in this study. Farnesylation of AtJ3 was detected using electrophoretic mobility using SDS-PAGE and confirmed using mass spectrometry. AtJ3 is a member of the heat shock protein 40 family and interacts with Arabidopsis HSP70 to protect plant proteins from heat-stress-induced denaturation. A luciferase-based protein denaturation assay demonstrated that farnesylated AtJ3 isolated from E. coli maintained this ability. Interestingly, farnesylated AtJ3 interacted with E. coli HSP70 as well and enhanced the thermotolerance of E. coli. Meanwhile, AtFP3, another known PFT substrate, was farnesylated when co-expressed with AtPFTα and AtPFTβ in E. coli. Moreover, using the same strategy to co-express rice PFT α and β subunit and a potential PFT target, it was confirmed that OsDjA4, a homolog of AtJ3, was farnesylated.

CONCLUSION

We developed a protein farnesylation system for E. coli and demonstrated its applicability and practicality in producing functional farnesylated proteins from both mono- and dicotyledonous plants.

The Arabidopsis DUF239 gene family encodes Neprosin-like proteins that are widely expressed in seed endosperm

Domain of unknown function 239 (DUF239) is a conserved sequence found in the catalytic site of Neprosins which are specific secreted prolyl endopeptidases found in the Nepenthes genus. Neprosins participate in the nitrogen cycle by digesting preys trapped in the pitcher of these carnivorous plants. Apart from that, DUF239s have been poorly documented in plants. We have identified 50 genes containing DUF239-coding sequences in the Arabidopsis genome that are distributed across six distinct phylogenetic clusters. The chromosomal distribution suggests that several genes are the result of recent duplication events, with up to eight genes found in a strict tandem distribution. In Arabidopsis, most of DUF239-containing sequences are also associated to a Neprosin-activating domain (DUF4409) and an amino-terminal α-helix which corresponds to the typical domain organization of the Neprosins described in the Nepenthes genus. Analysis of Arabidopsis transcriptomic datasets reveals that 39 genes are exclusively expressed in reproductive organs, mainly during seed development and more specifically in the endosperm (23 genes). The peculiar expression pattern of the DUF239 gene family in Arabidopsis suggests new functions of Neprosin-like proteins in plants during seed development.