A pre-genetic study of the isoforms of malic enzyme associated with lipid accumulation in Mucor circinelloides

The oleaginous fungus Mucor circinelloides possesses at least six isoforms of malic enzyme (EC 1.1.1.40), a key lipogenic enzyme in filamentous fungi. These isoforms were detected using a specific stain for activity after native PAGE of cell extracts. Only one isoform (isoform IV) was associated with lipid accumulation, appearing only after N-exhaustion from the medium (which is a pre-requisite for lipid accumulation) in glucose-growing cells. Isoforms I, II, V and VI were involved in anaerobic growth and only appeared under O(2)-limited conditions. Isoform III appeared to be constitutive and was formed under conditions of active (balanced) growth and is therefore thought to play a crucial role in basic metabolism. Growth on acetate increased the amount of cell lipid (from 25-27% in glucose-grown cells to 37-38% in acetate-grown cells) accumulated by M. circinelloides and this was associated with the appearance of isoform IV of malic enzyme prior to N-exhaustion in these cultures. Amino acid sequence analysis of isoforms III and IV suggests that these two malic enzymes may be encoded by a single gene and that isoform IV is formed from isoform III by post-translational modification initiated by either N-limitation (when glucose was the carbon source) or growth on acetate as the sole carbon source.