A Yeast Acetyl Coenzyme A Carboxylase Mutant Links Very-Long-Chain Fatty Acid Synthesis to the Structure and Function of the Nuclear Membrane-Pore Complex

Abstract

The conditional mRNA transport mutant of Saccharomyces cerevisiae, acc1-7-1 (mtr7-1), displays a unique alteration of the nuclear envelope. Unlike nucleoporin mutants and other RNA transport mutants, the intermembrane space expands, protuberances extend from the inner membrane into the intermembrane space, and vesicles accumulate in the intermembrane space. MTR7 is the same gene as ACC1, encoding acetyl coenzyme A (CoA) carboxylase (Acc1p), the rate-limiting enzyme of de novo fatty acid synthesis. Genetic and biochemical analyses of fatty acid synthesis mutants and acc1-7-1 indicate that the continued synthesis of malonyl-CoA, the enzymatic product of acetyl-CoA carboxylase, is required for an essential pathway which is independent from de novo synthesis of fatty acids. We provide evidence that synthesis of very-long-chain fatty acids (C 26 atoms) is inhibited in acc1-7-1, suggesting that very-long-chain fatty acid synthesis is required to maintain a functional nuclear envelope.The regulation of synthesis and transport of phospholipids has been characterized at length, but the signals which determine the phospholipid and fatty acid composition of particular membranes and membrane-specific lipid requirements are not well understood (7,62,81,83). The present investigation, which is the outgrowth of analysis of a yeast (Saccharomyces cerevisiae) mutant which accumulates poly(A) ϩ RNA in the nucleus at the restrictive temperature (41), documents a critical relationship between fatty acid chain length and the integrity of the nuclear membrane and nuclear pore complex (NPC).The biosynthesis of very-long-chain fatty acids requires four enzyme systems: acetyl coenzyme A (CoA) carboxylase, fatty acid synthase, fatty acid desaturase, and the fatty acyl chain elongation system (Fig. 1). The rate-limiting step of the de novo synthesis of fatty acids is under control of the first of these, acetyl-CoA carboxylase (Acc1p; EC 6.4.1.2). This biotinylated enzyme catalyzes the ATP-dependent carboxylation of acetyl-CoA to yield malonyl-CoA, which serves as the twocarbon-unit donor for the subsequent synthesis of long-chain fatty acids by the fatty acid synthase complex. The chain length of newly synthesized fatty acids appears to depend on the concentration of malonyl-CoA rather than on the activity of the fatty acid synthase complex (35, 43a, 82). Acc1p thus regulates both the overall rate of de novo synthesis and chainlength distribution of long-chain fatty acids. It is perhaps for this reason that its activity is subject to complex regulation (48). Yeast Acc1p/Fas3p has a subunit molecular mass of 250 kDa, is active as a tetramer, and is subject to short-term regulation by phosphorylation (2, 60, 90). Its transcription is positively regulated by Ino2p and Ino4p and negatively regulated by Opi1p; i.e., it is under the general control of phospholipid synthesis (14, 31).Many yeast long-chain fatty acid auxotrophs have been isolated (72). Sixty-one are alleles of acc1 (55, 67), while others bear mutations in fatty acid synthase (19,...

Lipid-dependent Subcellular Relocalization of the Acyl Chain Desaturase in Yeast

“…It is interesting to note that a number of conditional mutants in fatty acid metabolism affect the structure of the ER. For example, a ts mutant in the rate-limiting enzyme of fatty acid synthesis, acetyl-CoA carboxylase, displays a striking separation of inner and outer nuclear membranes (Schneiter et al, 1996). A very similar phenotype is observed in a conditional mutant of the two membrane-bound transcription factors, mga2⌬ spt23 ts .…”

“…Fatty acid composition was determined as previously described (Schneiter et al, 1996). To follow lipid synthesis and to determine the in vivo activity of mutant and wild-type desaturase, strains were cultivated in liquid minimal media to the midlogarithmic growth phase.…”

Yeast Nuclear Envelope Subdomains with Distinct Abilities to Resist Membrane Expansion

“…For example, in mutants with reduced levels of acetyl-CoA carboxylase activity, or mutants in certain early secretory pathway genes, the outer nuclear membrane becomes extended (Schneiter et al, 1996;Kimata et al, 1999;Matynia et al, 2002). Because mutants that block the secretory pathway at later steps do not show nuclear alterations, the latter phenotype is probably due to the accumulation of extra endoplasmic reticulum (ER) membranes in early secretory mutants.…”

HFA1 Encoding an Organelle-specific Acetyl-CoA Carboxylase Controls Mitochondrial Fatty Acid Synthesis in Saccharomyces cerevisiae

“…The ACC-encoding gene has been isolated and was designated as ACC1 and FAS3, respectively (11,12). Unlike fatty-acid synthase mutants, which grow upon fatty acid supplementation, ACC1 disruption is lethal and cannot be compensated by external long-chain fatty acids (12,13). This characteristic is commonly attributed to the malonyl-CoA requirement of cellular very long-chain fatty acid biosynthesis.…”

“…For many years, however, the biochemical function of HFA1 remained elusive because hfa1⌬ disruptants were, other than acc1 mutants, fatty acid-prototrophic and exhibited no obvious ACC deficiency. Accordingly, the fatty acid-requiring phenotype of ACC1 mutants was not compensated by functional HFA1 DNA (12,13). A first clue to the physiological function of HFA1 came from an observation in our laboratory that HFA1 mutants failed to grow on non-fermentable carbon sources.…”