Identification and Characterization of Sterol Acyltransferases Responsible for Steryl Ester Biosynthesis in Tomato.

Juan A Lara,Albert Ferrer,Rodolfo Lavilla,Albert Boronat,Teresa Altabella,Alma Burciaga-Monge,Monserrat Arró,Angel Chávez,Marc Revés

doi:10.3389/fpls.2018.00588

Abstract

Steryl esters (SEs) serve as a storage pool of sterols that helps to maintain proper levels of free sterols (FSs) in cell membranes throughout plant growth and development, and participates in the recycling of FSs and fatty acids released from cell membranes in aging tissues. SEs are synthesized by sterol acyltransferases, a family of enzymes that catalyze the transfer of fatty acil groups to the hydroxyl group at C-3 position of the sterol backbone. Sterol acyltransferases are categorized into acyl-CoA:sterol acyltransferases (ASAT) and phospholipid:sterol acyltransferases (PSAT) depending on whether the fatty acyl donor substrate is a long-chain acyl-CoA or a phospolipid. Until now, only Arabidopsis ASAT and PSAT enzymes (AtASAT1 and AtPSAT1) have been cloned and characterized in plants. Here we report the identification, cloning, and functional characterization of the tomato (Solanum lycopersicum cv. Micro-Tom) orthologs. SlPSAT1 and SlASAT1 were able to restore SE to wild type levels in the Arabidopsis psat1-2 and asat1-1 knock-out mutants, respectively. Expression of SlPSAT1 in the psat1-2 background also prevented the toxicity caused by an external supply of mevalonate and the early senescence phenotype observed in detached leaves of this mutant, whereas expression of SlASAT1 in the asat1-1 mutant revealed a clear substrate preference of the tomato enzyme for the sterol precursors cycloartenol and 24-methylene cycloartanol. Subcellular localization studies using fluorescently tagged SlPSAT1 and SlASAT1 proteins revealed that SlPSAT1 localize in cytoplasmic lipid droplets (LDs) while, in contrast to the endoplasmic reticulum (ER) localization of AtASAT1, SlASAT1 resides in the plasma membrane (PM). The possibility that PM-localized SlASAT1 may act catalytically in trans on their sterol substrates, which are presumably embedded in the ER membrane, is discussed. The widespread expression of SlPSAT1 and SlASAT1 genes in different tomato organs together with their moderate transcriptional response to several stresses suggests a dual role of SlPSAT1 and SlASAT1 in tomato plant and fruit development and the adaptive responses to stress. Overall, this study contributes to enlarge the current knowledge on plant sterol acyltransferases and set the basis for further studies aimed at understanding the role of SE metabolism in tomato plant growth and development.

Highlights

Sterols are essential eukaryotic cell components that occur in free form (FSs) and conjugated as steryl esters (SEs), steryl glycosides (SGs), and acylated steryl glycosides (ASGs)
The predicted SlPSAT1 consists of 630 amino acid residues with an overall identity of 75% with AtPSAT1 (Figure 1), whereas the predicted SlASAT candidates range in size from 317 to 444 amino acid residues (Table 1) and have overall identity values with AtASAT1 in the range between 33 and 49% (Supplementary Table 2)
This is fully consistent with the fact that SlPSAT1 shares 75% sequence identity and the presence of an N-terminal signal peptide with AtPSAT1, and possesses the six conserved regions (A–F), including the catalytic triad formed by Ser186, Asp458, and His543, FIGURE 10 | Expression of (A) SlPSAT1 and (B) SlASAT1 genes in tomato seedlings exposed to different stresses

Summary

Introduction

Sterols are essential eukaryotic cell components that occur in free form (FSs) and conjugated as steryl esters (SEs), steryl glycosides (SGs), and acylated steryl glycosides (ASGs). In SGs, the hydroxyl group at the C3 position of the sterol backbone is linked through a glycosidic bond to a sugar moiety, usually a single glucose residue, whereas in ASGs, the sugar residue at the C3 position carries a fatty acid esterified to the hydroxyl group at C-6 position. In SEs, the hydroxyl group of the sterol molecule is directly esterified to a fatty acid (Ferrer et al, 2017). Other less abundant sterols or sterol biosynthetic intermediates are found in esterified form, and in some cases, esters of sterol precursors may even predominate in the SE fraction. The fatty acid moiety in SEs shows a wide diversity of species covering a wide range of lengths from C12 to C22, being palmitic, stearic, oleic, linoleic, and linolenic acids the most common ones (Ferrer et al, 2017, and the references cited therein)

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