Multiple isoforms of malic enzyme in the oleaginous fungus, Mortierella alpina

Abstract

Malic enzyme (ME; E.C. 1.1.1.40) is the only enzyme that can provide NADPH for fatty acid biosynthesis in oleaginous micro-organisms. However, it can simultaneously fulfil other roles and may thus exist in different forms, possibly coded for by different genes. At least seven isoforms (A–G) of ME were identified in the oleaginous fungus, Mortierella alpina, using a specific activity stain following non-denaturing polyacrylamide gel electrophoresis (PAGE) of extracts of cells grown under different conditions. Only isoform E, which arises from isoform D, was associated with lipid accumulation, becoming evident after nitrogen depletion from the medium and, under which conditions, lipid accumulation occurs. Isoforms A, B, C, F, and G were associated with oxygen-limited growth. Isoforms D and E occurred under both anaerobic and aerobic growth conditions. During the storage of the whole cells at −20 °C, isoform E was gradually converted to isoform G suggesting that a further post-transcriptional modification of the protein was occurring.