Abstract
Acyl carrier protein (ACP) is a highly conserved cofactor protein that is required by Type II fatty acid synthases (FASs). Here, we demonstrate that up to three mitochondrial ACP (mtACP) isoforms support the Arabidopsis (Arabidopsis thaliana) mitochondrially localized Type II FAS. The physiological importance of the three mtACPs was evaluated by characterizing the single, double, and triple mutants. The mtACP1 (At2g44620), mtACP2 (At1g65290), and mtACP3 (At5g47630) single mutants showed no discernible morphological growth phenotype. Functional redundancy among the three mtACPs was indicated by the embryo-lethal phenotype associated with simultaneous loss of all three mtACP genes. Characterization of all double mutant combinations revealed that although the mtacp1 mtacp3 and mtacp2 mtacp3 double mutant combinations showed no observable growth defect, the mtacp1 mtacp2 double mutant was viable but displayed delayed growth, reduced levels of posttranslationally lipoylated mitochondrial proteins, hyperaccumulation of photorespiratory Gly, and reduced accumulation of many intermediates in central metabolism. These alterations were partially reversed when the mtacp1 mtacp2 double mutant plants were grown in a nonphotorespiratory condition (i.e. 1% CO2 atmosphere) or in the presence of 2% Suc. In summary, mtACP, as a key component of mitochondrial fatty acid biosynthesis, is important in generating the fatty acid precursor of lipoic acid biosynthesis. Thus, the incomplete lipoylation of mitochondrial proteins in mtacp mutants, particularly Gly decarboxylase, affects the recovery of photorespiratory carbon, and this appears to be critical during embryogenesis.
Highlights
At least three enzyme systems, each localized in distinct subcellular compartments, generate fatty acids in plants
System, which is primarily present in the cytosol of animal and yeast cells, acyl carrier protein (ACP) exists as a domain within a multi-domain enzyme that integrates all catalytic components needed for fatty acid biosynthesis (Leibundgut et al, 2008)
Is found in the mitochondria of animal and yeast cells (Hiltunen et al, 2009), a discrete soluble ACP is used to tether the fatty acyl intermediates to the phosphopantetheine cofactor, as they are shuttled among individual mono-functional enzymes for fatty acid biosynthesis (White et al, 2005)
Summary
At least three enzyme systems, each localized in distinct subcellular compartments, generate fatty acids in plants The majority of these fatty acids are used as building blocks for cellular lipids (e.g., glycerolipids of membranes and triacylglycerol in seeds). The mitochondrially-localized Type II fatty acid synthase system (mtFAS) appears to primarily contribute the fatty acid precursor of lipoic acid, an essential cofactor of several key enzymes in metabolism (Wada et al, 1997; Guan et al, 2015; Guan and Nikolau, 2016; Guan et al, 2017) Both ptFAS and mtFAS systems utilize acyl carrier protein (ACP) as a cofactor protein that shuttles acyl intermediates between active sites of the catalytic components of each FAS system (White et al, 2005). The terminal thiol of the ACP-carried phosphopantetheine arm covalently binds to the acyl intermediates via a high-energy thioester bond (Lambalot et al, 1996)
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