Conserved Alternative Splicing of Arabidopsis Transthyretin-Like Determines Protein Localization and S-Allantoin Synthesis in Peroxisomes  

Abstract

S-allantoin, a major ureide compound, is produced in plant peroxisomes from oxidized purines. Sequence evidence suggested that the Transthyretin-like (TTL) protein, which interacts with brassinosteroid receptors, may act as a bifunctional enzyme in the synthesis of S-allantoin. Here, we show that recombinant TTL from Arabidopsis thaliana catalyzes two enzymatic reactions leading to the stereoselective formation of S-allantoin, hydrolysis of hydroxyisourate through a C-terminal Urah domain, and decarboxylation of 2-oxo-4-hydroxy-4-carboxy-5-ureidoimidazoline through an N-terminal Urad domain. We found that two different mRNAs are produced from the TTL gene through alternative use of two splice acceptor sites. The corresponding proteins differ in the presence (TTL 12 ) and the absence (TTL 22 ) of a rare internal peroxisomal targeting signal (PTS2). The two proteins have similar catalytic activity in vitro but different in vivo localization: TTL 12 localizes in peroxisomes, whereas TTL 22 localizes in the cytosol. Similar splice variants are present in monocots and dicots. TTL originated in green algae through a Urad-Urah fusion, which entrapped an N-terminal PTS2 between the two domains. The presence of this gene in all Viridiplantae indicates that S-allantoin biosynthesis has general significance in plant nitrogen metabolism, while conservation of alternative splicing suggests that this mechanism has general implications in the regulation of the ureide pathway in flowering plants.

Metabolic Control of Redox and Redox Control of Metabolism in Plants

“…Those from spinach leaves and castor beans are anion selective and enable diffusion of small carboxylic acids such as intermediates of photorespiration, b-oxidation, and the glyoxylate cycle (285)(286)(287)(288). It is likely that further transport functions will be uncovered given that our understanding of peroxisomal metabolism has had to be expanded to include enzymes of the OPP (218), purine catabolism (93,191), and polyamine metabolism (320).…”

Plant Peroxisomes: Biogenesis and Function

“…In addition to these processes, plant peroxisomes also participate or are implicated in a plethora of other metabolic and signaling pathways, such as the glyoxylate cycle, detoxification, generation of signaling molecules, biosynthesis of salicylic acid, and the metabolism of urate, polyamines, sulfite, and branched-chain amino acids (reviewed in . Recent studies have also revealed roles for peroxisomes in plant immune response (Lipka et al, 2005;Coca and San Segundo, 2010;Rojas et al, 2012) and the biosynthesis of biotin (Tanabe et al, 2011), S-allantoin (Lamberto et al, 2010), phylloquinone (Widhalm et al, 2012), and isoprenoids (Sapir-Mir et al, 2008;Tholl and Lee, 2011).…”

“…The physiological functions of a number of enzymes and metabolic pathways indicated by proteomic data have been verified. Examples include the oxidative pentose phosphate pathway (Meyer et al, 2011), betaine aldehyde dehydrogenase (Missihoun et al, 2011), SDRa (Wiszniewski et al, 2009), and the bifunctional transthyretin-like protein involved in purine catabolism and S-allantoin biosynthesis (Lamberto et al, 2010), which have significantly broadened our knowledge of peroxisome metabolism.…”

Complexity of the Alternative Splicing Landscape in Plants

“…It either generates distinct protein isoforms with altered (related, distinct, or opposing) functions or determines the fate of the RNA transcript by regulation of mRNA stability through NMD-and microRNA (miRNA)-mediated mechanisms, or by influencing mRNA transport, localization, and translational efficiency. In plants, there are several examples where protein isoforms from alternatively spliced pre-mRNAs are targeted to different cellular locations (Dixon et al, 2009;Lamberto et al, 2010;Remy et al, 2013). mRNA isoforms may differ in the untranslated regions (UTRs) with functional implications in transcript localization, stability, or translation.…”