Abstract
Rab proteins, key players in vesicular transport in all eukaryotic cells, are post-translationally modified by lipid moieties. Two geranylgeranyl groups are attached to the Rab protein by the heterodimeric enzyme Rab geranylgeranyl transferase (RGT) αβ. Partial impairment in this enzyme activity in Arabidopsis, by disruption of the AtRGTB1 gene, is known to influence plant stature and disturb gravitropic and light responses. Here it is shown that mutations in each of the RGTB genes cause a tip growth defect, visible as root hair and pollen tube deformations. Moreover, FM 1-43 styryl dye endocytosis and recycling are affected in the mutant root hairs. Finally, it is demonstrated that the double mutant, with both AtRGTB genes disrupted, is non-viable due to absolute male sterility. Doubly mutated pollen is shrunken, has an abnormal exine structure, and shows strong disorganization of internal membranes, particularly of the endoplasmic reticulum system.
Highlights
Eukaryotic cells are dependent on vesicular transport, employing protein machinery that is conserved throughout all Eukarya
The AtRGTB2 gene is located on the third chromosome, comprises nine exons, and its structure is very similar to that of its paralogue, AtRGTB1, except that the sequence corresponding to the first exon of AtRGTB2 is split into two shorter exons in the AtRGTB1 gene (Supplementary Fig. S1 available at JXB online)
The expression of the AtRGTB2 gene was down-regulated in the Atrgtb2-1 line, on contrast to the expression of the possibly truncated AtRGTB2 mRNA detectable in the Atrgtb2-2 line
Summary
Eukaryotic cells are dependent on vesicular transport, employing protein machinery that is conserved throughout all Eukarya. Crucial players in this process are small GTPases, especially Rab proteins. At the very end of the C-terminal tail lies a double cysteine motif, which serves as the target for geranylgeranylation by Rab geranylgeranyl transferase (RGT) (Rak et al, 2004; Wu et al, 2009). This modification with two 20-carbon isoprenoid chains enables anchoring of Rab proteins to membranes. A lack of this modification (due to mutations of these particular cysteines) has a disastrous effect on Rab function, since unprenylated Rabs are soluble and cannot play their roles in vesicle formation, transport, and delivery (Gomes et al, 2003)
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