Yeast acetyl-CoA carboxylase is associated with the cytoplasmic surface of the endoplasmic reticulum

Acetyl-CoA carboxylase (ACC1) catalyzes the first and rate limiting step of de novo fatty acid synthesis. Defects in Acc1p were recently correlated with an altered structure/function of the nuclear envelope in yeast. The subcellular distribution of the enzyme was determined in wild-type and mutant cells by cell fractionation and confocal immunofluorescence microscopy. Eben though fatty acid synthesis is generally considered to be a cytosolic reaction, we found that Acc1p cofractionated with nuclei and the ER (endoplasmic reticulum) marker BiP/ Kar2p. Membrane-bound Acc1p was susceptible to proteinase K digestion and was solubilized by mild salt treatment indicating that it is loosely associated with the cytosolic surface of the nuclear ER membrane. Consistent with these observations, immunofluorescence analysis revealed that Acc1p was distributed in a gradient within the cytoplasm that had its highest concentration around the ER. Possible association of Acc1p with the nuclear pore complexes (NPCs) was investigated in strains that display NPC clustering. Results of these experiments suggest that Acc1p localization is independent of NPC distribution. We propose that association of Acc1p with the cytoplasmic surface of the ER membrane is physiologically relevant to 'channel' the enzymatic product of Acc1p, malonyl-CoA, to a putative ER-localized fatty acid chain elongase complex.