Synthesis of novel lipids in Saccharomyces cerevisiae by heterologous expression of an unspecific bacterial acyltransferase

Abstract

The bifunctional wax ester synthase/acyl-coenzyme A:diacylglycerol acyltransferase (WS/DGAT) is the key enzyme in storage lipid accumulation in the gram-negative bacterium Acinetobacter calcoaceticus ADP1, mediating wax ester, and to a lesser extent, triacylglycerol (TAG) biosynthesis. Saccharomyces cerevisiae accumulates TAGs and steryl esters as storage lipids. Four genes encoding a DGAT (Dga1p), a phospholipid:diacylglycerol acyltransferase (Lro1p) and two acyl-coenzyme A:sterol acyltransferases (ASATs) (Are1p and Are2p) are involved in the final esterification steps in TAG and steryl ester biosynthesis in this yeast. In the quadruple mutant strain S. cerevisiae H1246, the disruption of DGA1, LRO1, ARE1, and ARE2 leads to an inability to synthesize storage lipids. Heterologous expression of WS/DGAT from A. calcoaceticus ADP1 in S. cerevisiae H1246 restored TAG but not steryl ester biosynthesis, although high levels of ASAT activity could be demonstrated for WS/DGAT expressed in Escherichia coli XL1-Blue in radiometric in vitro assays with cholesterol and ergosterol as substrates. In addition to TAG synthesis, heterologous expression of WS/DGAT in S. cerevisiae H1246 resulted also in the accumulation of fatty acid ethyl esters as well as fatty acid isoamyl esters. In vitro studies confirmed that WS/DGAT is capable of utilizing a broad range of alcohols as substrates comprising long-chain fatty alcohols like hexadecanol as well as short-chain alcohols like ethanol or isoamyl alcohol. This study demonstrated the highly unspecific acyltransferase activity of WS/DGAT from A. calcoaceticus ADP1, indicating the broad biocatalytic potential of this enzyme for biotechnological production of a large variety of lipids in vivo in prokaryotic as well as eukaryotic expression hosts.

FIG. 1.

Storage lipid biosynthesis in recombinant S. cerevisiae. Cells were cultivated for 24 h at 28°C in synthetic minimal dropout medium without uracil and with 2% (wt/vol) galactose (samples 1 to 3) or 2% (wt/vol) galactose plus 0.1% (wt/vol) oleic acid (samples 4 to 6) and analyzed by TLC. Lane A, ergosterol; lane B, triolein; lane C, cholesteryl pamitate; lane D, palmityl palmitate; lanes 1 and 4, S. cerevisiae G175(pESC-URA); lanes 2 and 5, S. cerevisiae H1246(pESC-URA); lanes 3 and 6, S. cerevisiae H1246(pESC-URA::atfA). Total lipid extracts obtained from 1.5 mg of lyophilized cells (each) were applied to lanes 1 to 6.

FIG. 2.

ESI-MS analysis of TAGs purified from S. cerevisiae H1246(pESC-URA::atfA). Cells were cultivated for 24 h at 28°C in synthetic minimal dropout medium without uracil and with 2% (wt/vol) galactose. TAGs were purified from total lipid extracts of lyophilized cells by preparative TLC and then subjected to ESI-MS analysis. All pseudomolecular ions correspond to [M + Na]⁺.

FIG. 3.

GC/MS analysis of total lipid extracts from recombinant yeast strains. Cells were cultivated for 24 h at 28°C in synthetic minimal dropout medium without uracil and with 2% (wt/vol) galactose. Total lipid extracts obtained from 2.5 mg of lyophilized cells were applied. The following strains were used: S. cerevisiae H1246(pESC-URA::atfA) (A), S. cerevisiae H1246(pESC-URA) (B), and S. cerevisiae G175(pESC-URA) (C). Identified substances: 1, ethyl palmitate (m/z = 284 [C₁₈H₃₆O₂]⁺); 2, isoamyl myristate (m/z = 298 [C₁₉H₃₈O₂]⁺); 3, ethyl palmitoleate (m/z = 282 [C₁₈H₃₄O₂]⁺); 4, ethyl stearate (m/z = 312 [C₂₀H₄₀O₂]⁺); 5, isoamyl palmitate (m/z = 326 [C₂₁H₄₂O₂]⁺); 6, isoamyl palmitoleate (m/z = 324 [C₂₁H₄₀O₂]⁺); 7, isoamyl stearate (m/z = 354 [C₂₃H₄₆O₂]⁺); 8, isoamyl oleate (m/z = 352 [C₂₃H₄₄O₂]⁺).