In planta function of compatible solute transporters of the AtProT family

The three proline transporters of Arabidopsis thaliana (AtProTs) transport the compatible solutes proline and glycine betaine and the stress-induced compound γ-aminobutyric acid when expressed in heterologous systems. The aim of the present study was to show transport and physiological relevance of these three AtProTs in planta. Using single, double, and triple knockout mutants and AtProT-overexpressing lines, proline content, growth on proline, transport of radiolabelled betaine, and expression of AtProT genes and enzymes of proline metabolism were analysed. AtProT2 was shown to facilitate uptake of L- and D-proline as well as [14C]glycine betaine in planta, indicating a role in the import of compatible solutes into the root. Toxic concentrations of L- and D-proline resulted in a drastic growth retardation of AtProT-overexpressing plants, demonstrating the need for a precise regulation of proline uptake and/or distribution. Furthermore evidence is provided that AtProT genes are highly expressed in tissues with elevated proline content—that is, pollen and leaf epidermis

AtGAT1, a high affinity transporter for -aminobutyric acid in Arabidopsis thaliana

Functional characterization of Arabidopsis thaliana GAT1 in heterologous expression systems, i.e. Saccharomyces cerevisiae and Xenopus laevis oocytes, revealed that AtGAT1 (At1g08230) codes for an H(+)-driven, high affinity γ-aminobutyric acid (GABA) transporter. In addition to GABA, other ω-aminofatty acids and butylamine are recognized. In contrast to the most closely related proteins of the proline transporter family, proline and glycine betaine are not transported by AtGAT1. AtGAT1 does not share sequence similarity with any of the non-plant GABA transporters described so far, and analyses of substrate selectivity and kinetic properties showed that AtGAT1-mediated transport is similar but distinct from that of mammalian, bacterial, and S. cerevisiae GABA transporters. Consistent with a role in GABA uptake into cells, transient expression of AtGAT1/green fluorescent protein fusion proteins in tobacco protoplasts revealed localization at the plasma membrane. In planta, AtGAT1 expression was highest in flowers and under conditions of elevated GABA concentrations such as wounding or senescence

The AtProT family. Compatible solute transporters with similar substrate specificity but differential expression patterns

Proline transporters (ProTs) mediate transport of the compatible solutes Pro, glycine betaine, and the stress-induced compound gamma-aminobutyric acid. A new member of this gene family, AtProT3, was isolated from Arabidopsis (Arabidopsis thaliana), and its properties were compared to AtProT1 and AtProT2. Transient expression of fusions of AtProT and the green fluorescent protein in tobacco (Nicotiana tabacum) protoplasts revealed that all three AtProTs were localized at the plasma membrane. Expression in a yeast (Saccharomyces cerevisiae) mutant demonstrated that the affinity of all three AtProTs was highest for glycine betaine (K-m = 0.1-0.3 mM), lower for Pro (K-m = 0.4-1 mM), and lowest for gamma-aminobutyric acid (K-m = 4-5 mM). Relative quantification of the mRNA level using real-time PCR and analyses of transgenic plants expressing the beta-glucuronidase (uidA) gene under control of individual AtProT promoters showed that the expression pattern of AtProTs are complementary. AtProT1 expression was found in the phloem or phloem parenchyma cells throughout the whole plant, indicative of a role in long-distance transport of compatible solutes. beta-Glucuronidase activity under the control of the AtProT2 promoter was restricted to the epidermis and the cortex cells in roots, whereas in leaves, staining could be demonstrated only after wounding. In contrast, AtProT3 expression was restricted to the above-ground parts of the plant and could be localized to the epidermal cells in leaves. These results showed that, although intracellular localization, substrate specificity, and affinity are very similar, the transporters fulfill different roles in planta

L25 functions as a conserved ribosomal docking site shared by nascent chain-associated complex and signal-recognition particle

The nascent chain-associated complex (NAC) is a dimeric protein complex of archaea and eukarya that interacts with ribosomes and translating polypeptide chains. We show that, in yeast, NAC and the signal-recognition particle (SRP) share the universally conserved ribosomal protein L25 as a docking site, which is in close proximity to the ribosomal exit tunnel. The amino-terminal segment of β-NAC was found to be required for L25 binding. Purified NAC can prevent protein aggregation in vitro and thus shows certain properties of a molecular chaperone. Interestingly, the α-subunit of NAC interacts with the 54 kDa subunit of SRP. Consistent with a regulatory role of NAC in protein translocation into the endoplasmic reticulum (ER), we find that deletion of NAC results in an induction of the ER stress-response pathway. These results identify L25 as a conserved interaction platform for specific cytosolic factors that guide nascent polypeptides to their proper cellular destination