Compartmentalization of two forms of acetyl-CoA carboxylase in plants and the origin of their tolerance toward herbicides.

Abstract

Acetyl-CoA carboxylase (ACCase, EC 6.4.1.2) catalyzes the synthesis of malonyl-CoA, the first intermediate in fatty acid synthesis. We studied the llization of two forms, the prokaryote and the eukaryote forms, of ACCase in pea leaves by comparing the blotin polypeptides of the two ACCases in protein extract from leaves and plastids.We found that the two forms of ACCase were in diferent cell compartments of pea leaves; the prokaryote form was in the plastids, and the eukaryote form was elsewhere, probably in the cytosol. This result suested the existence of two sites of malonyl-CoA thes. The Gramineae, such as rice and wheat, which lack the accD gene encoding one of the subunits ofthe prokaryote form ofACCase in their chloroplast genomes, did not have the prokaryote form of the enzyme but had the eukaryote form. The selectlve grass herbicides of the diphenoxyproponic acid type and the cyclohexanedione type, in vitro, inhibited plastidic ACCase of the eukaryote form from wheat but did not inhibit that of the prokaryote form from pea, suggeting that the origin of the tolerance of intact pea plant toward these herbicides is partly in the insensitivity of the prokaryote form of the enzyme. The origin of the susceptibility of the Gramineae plants toward these herbicides seems to lie in the presence of the herbicide-sensitive eukaryote form and the absence ofthe insenitive prokaryote form due to the lack of the accD gene in plastid.Acetyl-CoA carboxylase (ACCase; EC 6.4.1.2) catalyzes the ATP-dependent carboxylation of acetyl-CoA to form malonyl-CoA. The reaction is the first committed step in the synthesis of fatty acid, providing the essential substrate for fatty acid synthesis. There are two forms of ACCase: a prokaryote form consisting of three protein components biotin carboxylase, carboxyltransferase, and biotin carboxylase carrier protein (1)-and a eukaryote form consisting of three functional domains on a single polypeptide (2-7). It has been believed that, in plants, plastids are the major site of fatty acid synthesis (8) and that only the eukaryote form ofthe enzyme in plastids participates in the synthesis (9, 10). Early studies reported the existence of the prokaryote form in the spinach chloroplast (11), but this finding has been dismissed because the prokaryote form has not yet been purified and the purified ACCases from various plants are all eukaryote form consisting of a subunit size of -200 kDa (5-7), like that of the mammal enzyme (2). ACCase gene encoding the 230-kDa polypeptide has been isolated from a photosynthetic eukaryote alga (12). This large polypeptide is probably nuclearencoded because this sequence is not found in the complete sequence ofchloroplast genomes (13-15). However, recently we have obtained evidence ofthe existence of the prokaryote form in pea chloroplasts by identifying one of the subunits of carboxyltransferase, the chloroplast-encoded accD protein (16). This finding supports the early studies and indicates the existence of two forms of ACCase in pea plants (16,17). ...

Lipid biosynthesis.

“…Coincident with this evolutionary difference, the chloroplast genomes of rice and maize have lost the gene that encodes the putative carboxyltransferase subunit of the prokaryotic-type ACCase. The difference in ACCase organization in the Gramineae has now provided an explanation for the action of the grass-specific herbicides, which inhibit only the eukaryotic form of the enzyme (Konishi and Sasaki, 1994). Although both the cytosolic and plastid eukaryotic ACCases are inhibited by these herbicides, the plastid form in the Gramineae is much more sensitive than is the cytosolic form, and the plastid fatty acid synthesis pathway is more essential to growth than are the secondary pathways dependent upon the cytosolic ACCase.…”

“…However, the Gramineae family of plants is different in that both the plastid and cytosolic ACCase isozymes are large multifunctional polypeptides (Egli et al, 1993;Konishi and Sasaki, 1994). Coincident with this evolutionary difference, the chloroplast genomes of rice and maize have lost the gene that encodes the putative carboxyltransferase subunit of the prokaryotic-type ACCase.…”

A Family of Negative Regulators Targets the Committed Step of de Novo Fatty Acid Biosynthesis

“…Two forms of acetyl-CoA carboxylase are present in nature: a heteromeric (hetACCase) and a homomeric form (homACCase). In prokaryotes, and in plastids of dicots and nongraminaceous monocots, hetACCase is a complex requiring four distinct subunits: biotin carboxylase (BC), biotin carboxyl carrier protein (BCCP), and a-and b-carboxyltransferase (CT) (Guchhait et al, 1974;Konishi and Sasaki, 1994). In contrast, graminaceous monocots possess a homACCase form for plastidial de novo FAS, wherein the catalytic components are fused in tandem as a single polypeptide.…”

Hormonal and Stress Induction of the Gene Encoding Common Bean Acetyl-Coenzyme A Carboxylase

“…Most species have both forms with heteromeric enzymes in plastids and the homodimeric enzyme in the cytosol. In grasses, the homodimeric form is found in both the plastid and cytosol (Konishi and Sasaki, 1994;Konishi et al, 1996) and no heteromeric enzyme exists. Plastids of Brassica napus (Schulte et al, 1997) contain both a homodimeric and the expected heteromeric isoform.…”