Title : Solanum lycopersicum (tomato) possesses multiple lipoyl synthases capable of increasing lipoylation levels in vivo
Abstract:
Lipoic acid (LA) is a functional metabolite with powerful antioxidant capacities present in eukaryotic and prokaryotic organisms. LA is both lipid- and water-soluble, and is the prosthetic group of several key multi-subunit enzyme complexes, including pyruvate dehydrogenase and α-ketoglutarate dehydrogenase of the tricarboxylic acid (TCA) cycle. LA biosynthesis and incorporation into these target proteins (lipoylation) proceeds de novo or via a salvage pathway. During de novo synthesis, octanoyl transferase (LIP2) uses octanoyl groups linked to an acyl carrier protein to transoctanylate target proteins. Subsequently, lipoyl synthase (LIP1) catalyses the final step by inserting two sulphur atoms into the prosthetic group. Whilst a number of the enzymes have been functionally-characterised in Arabidopsis thaliana, the aim of the current work is to identify and evaluate the role of this pathway in a fruit-bearing species. Towards this aim, we identified two proteins in tomato (Solanum lycopersicum) with the molecular characteristics of LIP1. We call these proteins SlLIP1 and SlLIP1p, which possess 78% and 84% amino acid identity with AtLIP1 and AtLIP1p, respectively. Confirming bioinformatic predictions, SlLIP1 has a mitochondrial localisation whereas SlLIP1p is plastidial, as shown by confocal microscopy. Furthermore, both proteins rescue carbon source requirements and lipoylation levels of an Escherichia coli lipoyl synthase mutant (lipA), and thus act as lipoyl synthases in this heterologous system. Additionally, stable over-expression of these genes in tomato produces transcriptional alterations in genes encoding proteins involved in LA metabolism, and target proteins of the TCA cycle, which in turn correlate with developmental differences and increased levels of lipoylation measured in several over-expressing lines
