Substrate Specificity and Expression Profile of Amino Acid Transporters (AAPs) in Arabidopsis

Abstract

Three amino acid transporter genes (AAP3-5) were isolated from Arabidopsis by complementation of a yeast mutant defective in histidine uptake. Transport is driven against a concentration gradient and sensitive to protonophores. Analysis of the substrate specificity demonstrates that the carriers have a broad substrate specificity covering the major transport forms of reduced nitrogen, i.e. glutamine and glutamate. The transporters have similar affinities for glutamate, glutamine, and alanine but differ with respect to valine, phenylalanine, histidine, arginine, and lysine. AAP3 and AAP5 efficiently transport arginine and lysine and are involved in basic amino acid transport. The predicted polypeptides of 53 kDa are highly hydrophobic with 12 putative membrane-spanning regions and show significant homologies to Arabidopsis amino acid transporters AAP1 and AAP2. Each of the genes has a different organ-specific expression in the plant. AAP3 is exclusively expressed in roots and AAP4 mainly in source leaves, stems, and flowers, whereas AAP5 is found in all tissues. The specific distribution in the plant and the different substrate specificities of AAP transporters may indicate that tissues differ both qualitatively and quantitatively regarding import or export of amino acids.

Functional Characterization and Expression Analysis of the Amino Acid Permease RcAAP3 from Castor Bean1

“…This is in contrast to the expression patterns seen for RcAAP1 and RcAAP2, which are predominantly expressed in the cotyledons, to a lesser extent in the roots, and at low levels in endosperm, hypocotyl, and the source and sink tissues of mature plants . In addition, AtAAP3, the most closely related gene in Arabidopsis, is almost exclusively found in roots, where it has been suggested to function in uptake and retrieval of amino acids from the soil (Fischer et al, 1995). This suggests a different biological role for RcAAP3, possibly functioning more generally in the accumulation of amino acids for protein synthesis.…”

“…During the past few years, the use of yeast transport mutants and their complementation with plant cDNA libraries has resulted in the isolation and characterization of different families of amino acid permeases from Arabidopsis Kwart et al, 1993;Fischer et al, 1995;Rentsch et al, 1996). The use of sequence homology also enabled the identification of transporters from ESTs and genomic clones (Chen and Bush, 1997).…”

“…Direct transport measurements would be required with individual radiolabeled amino acids in order to differentiate between inhibition due to competition for transport and that for binding alone. Fischer et al (1995) have grouped the AAPs into two subfamilies based on their recognition of basic amino acids: AAP1, AAP2, and AAP4 recognize acidic and neutral amino acids and ureides, whereas AAP3 and AAP5 are general transporters that also recognize basic amino acids. The inhibition studies indicate that RcAAP3 is similar to the latter subfamily, and in the phylogenetic analysis RcAAP3 appears to be the most closely related to AtAAP3 in the Arabidopsis AAP family.…”

“…Based on sequence similarities, the plant transporters can be classified into two major superfamilies (Fischer et al, 1998). ATF (amino acid transport family), the largest of these families, includes the AAP gene family (Fischer et al, 1995), transporters that prefer Pro (ProT) (Rentsch et al, 1996), Lys/ His transporters (LHT) (Chen and Bush, 1997), and AUX1-related proteins (Bennett et al, 1996). The second superfamily, APC (amino acid-polyamine-choline facilitator), consists of two families: CAT (cationic amino acid transporter), which contains AtCAT1, a high-affinity uptake system for basic amino acids , and the GABA permease-related transporter family for which a homolog has recently been identified (Fischer et al, 1998).…”

“…This results largely from the isolation of the genes encoding amino acid permeases using complementation of yeast transport mutants (Fischer et al, 1998). The biochemical properties of the Arabidopsis amino acid permeases have been described in a number of cases (Fischer et al, 1995(Fischer et al, , 1998. However, almost nothing is known about amino acid permeases in other plant species.…”