The subcellular localization of acetyl-CoA carboxylase 2

Abstract

Animals, including humans, express two isoforms of acetyl-CoA carboxylase (EC 6.4.1.2), ACC1 (Mr ‫؍‬ 265 kDa) and ACC2 (Mr ‫؍‬ 280 kDa). The predicted amino acid sequence of ACC2 contains an additional 136 aa relative to ACC1, 114 of which constitute the unique N-terminal sequence of ACC2. The hydropathic profiles of the two ACC isoforms generally are comparable, except for the unique N-terminal sequence in ACC2. The sequence of amino acid residues 1-20 of ACC2 is highly hydrophobic, suggesting that it is a leader sequence that targets ACC2 for insertion into membranes. The subcellular localization of ACC2 in mammalian cells was determined by performing immunofluorescence microscopic analysis using affinity-purified anti-ACC2-specific antibodies and transient expression of the green fluorescent protein fused to the C terminus of the N-terminal sequences of ACC1 and ACC2. These analyses demonstrated that ACC1 is a cytosolic protein and that ACC2 was associated with the mitochondria, a finding that was confirmed further by the immunocolocalization of a known human mitochondria-specific protein and the carnitine palmitoyltransferase 1. Based on analyses of the fusion proteins of ACC-green fluorescent protein, we concluded that the N-terminal sequences of ACC2 are responsible for mitochondrial targeting of ACC2. The association of ACC2 with the mitochondria is consistent with the hypothesis that ACC2 is involved in the regulation of mitochondrial fatty acid oxidation through the inhibition of carnitine palmitoyltransferase 1 by its product malonyl-CoA.A cetyl-CoA carboxylase (ACC) catalyzes the carboxylation of acetyl-CoA to form malonyl-CoA, an intermediate substrate that plays a pivotal role in the regulation of fatty acid metabolism. Besides its role in the biosynthesis of long-chain fatty acids (1-3), malonyl-CoA has been implicated in the regulation of the carnitine palmitoyl-CoA shuttle system that is involved in the mitochondrial ␤-oxidation of long-chain fatty acids. In animals, two isoforms of the carboxylase have been identified, ACC1 (M r ϭ 265,000) and ACC2 (M r ϭ 280,000) (4, 5). The two enzymes are encoded by separate genes and display distinct tissue distribution and regulation (6-9). The ACC1 carboxylases are highly expressed in lipogenic tissues, such as liver and adipose, and their levels are regulated transcriptionally while their activities are regulated posttranslationally by phosphorylation͞dephosphorylation of selected serine residues and by allosteric regulation through the action of citrate and palmitoyl-CoA (10-18). Dietary and hormonal states of the animal affect the level and activities of the ACC1 enzymes. A carbohydrate-rich, low-fat diet stimulates the expression and activities of ACC1, whereas starvation and diabetes reduce the ACC1 activities by repressing the expression of the ACC1 gene or by increasing the phosphorylation levels of the ACC1 protein (or both). Treating diabetic animals with insulin increases the activity of the enzyme either by dephosphorylation of the protein or by...

Glucose and fat metabolism in adipose tissue of acetyl-CoA carboxylase 2 knockout mice

“…Malonyl-CoA is also considered a key regulator of fatty acid oxidation and energy metabolism, because it inhibits carnitine palmitoyltransferase I (CPT I) and hence mitochondrial fatty acid oxidation (7). Our studies provided the evidence that ACC2 is localized at the mitochondrial membranes of the heart, muscle, and liver; hence, the malonyl-CoA it synthesizes controls the transfer of the cytosolic long-chain fatty acyl-CoA to the mitochondria by an allosteric regulation of the membrane-bound CPT I (6,7).…”

Substrate cycling between de novo lipogenesis and lipid oxidation: a thermogenic mechanism against skeletal muscle lipotoxicity and glucolipotoxicity

“…More recent advances in the mechanism by which malonyl-CoA is regulated by ACC point to the subcellular localization of the ACC isoforms and to the compartmentalization of malonyl-CoA in the cell. 37 ACC-1 is thought to reside in the cytoplasm, where it synthesizes the pool of malonyl-CoA that is used for de novo lipogenesis, whereas ACC-2, by contrast, is thought to control the pool of malonyl-CoA that regulates fatty acid oxidation. This isoform-specific role results from its postulated capacity to associate with mitochondria, which juxtaposes the ACC-2 enzyme with mitochondrial CPT-1.…”

OPINIONS Acetyl-coenzyme A carboxylase – an attractive enzyme for biotechnology

“…Proteins synthesized in cytosol and targeted to organelles are presented in immature form, including targeting peptide marked with green for plastid targeting peptide, and with black for peptide responsible for directing protein to the outer mitochondrial membrane. Dicotyledonous accA is a plastid genome gene for CT-beta subunit whereas all other peptides are coded on nuclear genome (Tong 2005) form has an N-terminal mitochondria targeting peptide which docks the enzyme on the outer mitochondrial membrane exposed to the cytosol (Abu-Elheiga et al, 2000). The two forms of ACCases play different roles in human cellular homeostasis, which results from differences in genes regulation, proteins activity and product allocation.…”