Comparative studies on Ureide Permeases in Arabidopsis thaliana and analysis of two alternative splice variants of AtUPS5

Abstract

The recovery of free purine and pyrimidine bases and their degradation products represent alternative pathways in plant cells either to synthesize nucleotides (salvage pathways) by low energy consumption or to reuse organic nitrogen. Such recycling of metabolites often requires their uptake into the cell by specialized transport systems residing in the plasma membrane. In plants, it has been suggested that several protein families are involved in this process, but only a few transporters have so far been characterized. In this work, gene expression, substrate specificities, and transport mechanisms of members of the Ureide Permease family in Arabidopsis (AtUPS) were analyzed and compared. Promoter-GUS studies indicated that the members of the family have distinct and partially overlapping expression patterns with regard to developmental stages or tissue specific localization. In addition, two alternative splice variants of AtUPS5, a novel member of the transporter family, were identified and investigated. The abundance of both alternative mRNAs varied in different organs, while the relative amounts were comparable. AtUPS5l (longer isoform) shares similar structural prediction with AtUPS1 and AtUPS2. In contrast, AtUPS5s (shorter isoform) lacks two transmembrane domains as structural consequence of the additional splice event. When expressed in yeast, AtUPS5l mediates cellular import of cyclic purine degradation products and pyrimidines similarly to AtUPS1 and AtUPS2, but differences in transport efficiencies were observed. AtUPS5s, however, could not be shown to mediate uptake of these compounds into yeast cells and might therefore be defective or have a different function.

Genetic and molecular characterization reveals a unique nucleobase cation symporter 1 in Arabidopsis

“…Characterized Arabidopsis PUP proteins transport adenine, cytosine or secondary compounds such as cytokinins and caffeine [8][9][10]. Two transporters in the Arabidopsis UPS family transport uracil, allantoin and the purines xanthine and hypoxanthine [11][12][13], and an UPS from French bean transports allantoin [14]. The remaining four nucleobase transporter families have orthologs in other prokaryotic and eukaryotic taxa.…”

Heterologous complementation studies reveal the solute transport profiles of a two-member nucleobase cation symporter 1 (NCS1) family in Physcomitrella patens

“…Three of the five Ureide Permease (UPS) in Arabidopsis move allantoin, uracil, uric acid and xanthine (Desimone et al, 2002;Schmidt et al, 2004Schmidt et al, , 2006. Arabidopsis genome contain 12 Nucleobase Ascorbate Transporter (NAT) loci (Maurino et al, 2006).…”

“…Recently, three dimensional structures for the outward-facing, occluded and inward facing states for the NCS1 from Microbacterium liquefaciens the sodium-hydantion transporter MHP1 have been determined, revealing amino acids important for solute and cation interactions (Weyand et al, 2008;Shimamura et al, 2010). NCS1 proteins belong to the LeuT superfamily that is characterized by two five helix inverted repeats that are believed to function as a "rocking-bundle" model (Shimamura et al, 2010).…”

“…NCS1 proteins belong to the LeuT superfamily that is characterized by two five helix inverted repeats that are believed to function as a "rocking-bundle" model (Shimamura et al, 2010). Key amino acids in the presumptive solute and cation binding pocket (including TMI, III, VI & VIII) have been identified -some residues are conserved among NCS1 while others vary (Suppl.…”

The solute specificity profiles of nucleobase cation symporter 1 (NCS1) from Zea mays and Setaria viridis illustrate functional flexibility

“…In Arabidopsis, the genome contains loci that encode for five distinct nucleobase transporter families. Two nucleobase transporter families unique to plants include the twenty onemember purine permeases (PUP) that transport adenine, cytosine, and cytokinins (Gillissen et al 2000;Bürkle et al 2003;Cedzich et al 2008;Jelesko 2012;Szydlowski et al 2013) and the five-member ureide permeases (UPS) facilitating the movement of allantoin, uracil, uric acid, and xanthine (Desimone et al 2002;Schmidt et al 2004Schmidt et al , 2006. The remaining three nucleobase transporter families have orthologues among prokaryotes and eukaryotes.…”